Selection Statements switch

C/C++ has a built-in multiple-branch selection statement, called switch, which
successively tests the value of an expression against a list of integer or character
constants. When a match is found, the statements associated with that constant are
executed. The general form of the switch statement is
switch (expression) {
case constant1:
statement sequence
break;
case constant2:
statement sequence
break;
case constant3:
statement sequence
break;
.
..
default
statement sequence
}
The expression must evaluate to a character or integer value. Floating-point expressions,
for example, are not allowed. The value of expression is tested, in order, against the
values of the constants specified in the case statements. When a match is found, the
statement sequence associated with that case is executed until the break statement or
the end of the switch statement is reached. The default statement is executed if no
matches are found. The default is optional and, if it is not present, no action takes place
if all matches fail.
Standard C specifies that a switch can have at least 257 case statements. Standard
C++ recommends that at least 16,384 case statements be supported! In practice, you
will want to limit the number of case statements to a smaller amount for efficiency.
Although case is a label statement, it cannot exist by itself, outside of a switch.

The break statement is one of C/C++'s jump statements. You can use it in loops as
well as in the switch statement (see the section "Iteration Statements"). When break is
encountered in a switch, program execution "jumps" to the line of code following the
switch statement.
There are three important things to know about the switch statement:
The switch differs from the if in that switch can only test for equality,whereas
if can evaluate any type of relational or logical expression.
No two case constants in the same switch can have identical values. Of course,
a switch statement enclosed by an outer switch may have case constants that
are the same.
If character constants are used in the switch statement, they are automatically
converted to integers.
The switch statement is often used to process keyboard commands, such as menu
selection. As shown here, the function menu() displays a menu for a spelling-checker
program and calls the proper procedures:
void menu(void)
{
char ch;
printf("1. Check Spelling\n");
printf("2. Correct Spelling Errors\n");
printf("3. Display Spelling Errors\n");
printf("Strike Any Other Key to Skip\n");
printf(" Enter your choice: ");
ch = getchar(); /* read the selection from
the keyboard */
switch(ch) {
case '1':
check_spelling();
break;
case '2':
correct_errors();
break;
case '3':
display_errors();
break;
default :

printf("No option selected");
}
}
Technically, the break statements inside the switch statement are optional. They
terminate the statement sequence associated with each constant. If the break statement
is omitted, execution will continue on into the next case's statements until either a
break or the end of the switch is reached. For example, the following function uses the
"drop through" nature of the cases to simplify the code for a device-driver input
handler:
/* Process a value */
void inp_handler(int i)
{
int flag;
flag = -1;
switch(i) {
case 1: /* These cases have common */
case 2: /* statement sequences. */
case 3:
flag = 0;
break;
case 4:
flag = 1;
case 5:
error(flag);
break;
default:
process(i);
}
}
This example illustrates two aspects of switch. First, you can have case statements
that have no statement sequence associated with them. When this occurs, execution
simply drops through to the next case. In this example, the first three cases all execute
the same statements, which are
flag = 0;
break;

break statement is present. If i matches 4, flag is set to 1 and, because there is no break
statement at the end of that case, execution continues and the call to error(flag) is
executed. If i had matched 5, error(flag) would have been called with a flag value of −1
(rather than 1).
The fact that cases can run together when no break is present prevents the
unnecessary duplication of statements, resulting in more efficient code.

Selection Statements if and switch

C/C++ supports two types of selection statements: if and switch. In addition, the ?
operator is an alternative to if in certain circumstances.
if
The general form of the if statement is
if (expression) statement;
else statement;
where a statement may consist of a single statement, a block of statements, or nothing
(in the case of empty statements). The else clause is optional.
If expression evaluates to true (anything other than 0), the statement or block that
forms the target of if is executed; otherwise, the statement or block that is the target of
else will be executed, if it exists. Remember, only the code associated with if or the
code associated with else executes, never both.
In C, the conditional statement controlling if must produce a scalar result. A scalar
is either an integer, character, pointer, or floating-point type. In C++, it may also be of
type bool. It is rare to use a floating-point number to control a conditional statement
because this slows execution time considerably. (It takes several instructions to perform
a floating-point operation. It takes relatively few instructions to perform an integer or
character operation.)
The following program contains an example of if. The program plays a very simple
version of the "guess the magic number" game. It prints the message ** Right ** when
the player guesses the magic number. It generates the magic number using the
standard random number generator rand() , which returns an arbitrary number
between 0 and RAND_MAX (which defines an integer value that is 32,767 or larger).
rand() requires the header file stdlib.h. (A C++ program may also use the new-style
header <cstdlib>.)
/* Magic number program #1. */
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int magic; /* magic number */

int guess; /* user's guess */
magic = rand(); /* generate the magic number */
printf("Guess the magic number: ");
scanf("%d", &guess);
if(guess == magic) printf("** Right **");
return 0;
}
Taking the magic number program further, the next version illustrates the use of
the else statement to print a message in response to the wrong number.
/* Magic number program #2. */
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int magic; /* magic number */
int guess; /* user's guess */
magic = rand(); /* generate the magic number */
printf("Guess the magic number: ");
scanf("%d", &guess);
if(guess == magic) printf("** Right **");
else printf("Wrong");
return 0;
}

Nested ifs
A nested if is an if that is the target of another if or else. Nested ifs are very common
in programming. In a nested if, an else statement always refers to the nearest if
statement that is within the same block as the else and that is not already associated
with an else. For example,

if(i)
{
if(j) statement 1;
if(k) statement 2; /* this if */
else statement 3; /* is associated with this else */
}
else statement 4; /* associated with if(i) */
As noted, the final else is not associated with if(j) because it is not in the same block.
Rather, the final else is associated with if(i). Also, the inner else is associated with if(k),
which is the nearest if.
Standard C specifies that at least 15 levels of nesting must be supported. In practice,
most compilers allow substantially more. More importantly, Standard C++ suggests
that at least 256 levels of nested ifs be allowed in a C++ program. However, nesting
beyond a few levels is seldom necessary, and excessive nesting can quickly confuse the
meaning of an algorithm.
You can use a nested if to further improve the magic number program by
providing the player with feedback about a wrong guess.
/* Magic number program #3. */
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int magic; /* magic number */
int guess; /* user's guess */
magic = rand(); /* get a random number */
printf("Guess the magic number: ");
scanf("%d", &guess);
if (guess == magic) {
printf("** Right **");
printf(" %d is the magic number\n", magic);
}
else {
printf("Wrong, ");
if(guess > magic) printf("too high\n");
else printf("too low\n");
}

return 0;
}
The if-else-if Ladder
A common programming construct is the if-else-if ladder, sometimes called the if-else-if
staircase because of its appearance. Its general form is
if (expression) statement;
else
if (expression) statement;
else
if (expression) statement;
..
.
else statement;
The conditions are evaluated from the top downward. As soon as a true condition is
found, the statement associated with it is executed and the rest of the ladder is
bypassed. If none of the conditions are true, the final else is executed. That is, if all
other conditional tests fail, the last else statement is performed. If the final else is not
present, no action takes place if all other conditions are false.
Although the indentation of the preceding if-else-if ladder is technically correct, it
can lead to overly deep indentation. For this reason, the if-else-if ladder is generally
indented like this:
if (expression)
statement;
else if (expression)
statement;
else if (expression)
statement;
.
..
else
statement;
Using an if-else-if ladder, the magic number program becomes
/* Magic number program #4. */

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int magic; /* magic number */
int guess; /* user's guess */
magic = rand(); /* generate the magic number */
printf("Guess the magic number: ");
scanf("%d", &guess);
if(guess == magic) {
printf("** Right ** ");
printf("%d is the magic number", magic);
}
else if(guess > magic)
printf("Wrong, too high");
else printf("Wrong, too low");
return 0;
}
The ? Alternative
You can use the ? operator to replace if-else statements of the general form:
if(condition) expression;
else expression;
However, the target of both if and else must be a single expression—not another
statement.
The ? is called a ternary operator because it requires three operands. It takes the
general form
Exp1 ? Exp2 : Exp3
where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.
The value of a ? expression is determined as follows: Exp1 is evaluated. If it is true,
Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then
Exp3 is evaluated and its value becomes the value of the expression. For example,
consider x = 10;
y = x>9 ? 100 : 200;
In this example, y is assigned the value 100. If x had been less than 9, y would have
received the value 200. The same code written with the if-else statement would be
x = 10;
if(x>9) y = 100;
else y = 200;
The following program uses the ? operator to square an integer value entered by
the user. However, this program preserves the sign (10 squared is 100 and −10 squared
is −100).
#include <stdio.h>
int main(void)
{
int isqrd, i;
printf("Enter a number: ");
scanf("%d", &i);
isqrd = i>0 ? i*i : -(i*i);
printf("%d squared is %d", i, isqrd);
return 0;
}
The use of the ? operator to replace if-else statements is not restricted to
assignments only. Remember, all functions (except those declared as void) may return
a value. Thus, you can use one or more function calls in a ? expression. When the
function's name is encountered, the function is executed so that its return value may be
determined. Therefore, you can execute one or more function calls using the ? operator
by placing the calls in the expressions that form the ?'s operands. Here is an example.
#include <stdio.h>
int f1(int n);
int f2(void);

int main(void)
{
int t;
printf("Enter a number: ");
scanf("%d", &t);
/* print proper message */
t ? f1(t) + f2() : printf("zero entered.\n");
return 0;
}
int f1(int n)
{
printf("%d ", n);
return 0;
}
int f2(void)
{
printf("entered.\n");
return 0;
}
Entering a 0 in this example calls the printf() function and displays the message zero
entered. If you enter any other number, both f1() and f2() execute. Note that the value
of the ? expression is discarded in this example. You don't need to assign it to anything.
A word of warning: Some C++ compilers rearrange the order of evaluation of an
expression in an attempt to optimize the object code. This could cause functions that
form the operands of the ? operator to execute in an unintended sequence.
Using the ? operator, you can rewrite the magic number program yet again.
/* Magic number program #5. */
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int magic;
int guess;

magic = rand(); /* generate the magic number */
printf("Guess the magic number: ");
scanf("%d", &guess);
if(guess == magic) {
printf("** Right ** ");
printf("%d is the magic number", magic);
}
else
guess > magic ? printf("High") : printf("Low");
return 0;
}
Here, the ? operator displays the proper message based on the outcome of the test
guess > magic.

True and False in C and C++

course of action is to be taken. A conditional expression evaluates to either a true or
false value. In C, a true value is any nonzero value, including negative numbers. A
false value is 0. This approach to true and false allows a wide range of routines to be
coded extremely efficiently.
C++ fully supports the zero/nonzero definition of true and false just described. But
C++ also defines a Boolean data type called bool, which can have only the values true
and false. As explained in Chapter 2, in C++, a 0 value is automatically converted into
false and a nonzero value is automatically converted into true. The reverse also
applies: true converts to 1 and false converts to 0. In C++, the expression that controls a
conditional statement is technically of type bool. But since any nonzero value converts to true and any zero value converts to false, there is no practical difference between C
and C++ on this point.

Shorthand Assignments

There is a variation on the assignment statement, sometimes referred to as a shorthand

assignment, that simplifies the coding of a certain type of assignment operation. For

example,

x = x+10;

can be written as

x += 10;

The operator += tells the compiler to assign to x the value of x plus 10.

This shorthand works for all the binary operators (those that require two

operands). In general, statements like:

var = var operator expression

can be rewritten as

var operator = expression

For another example,

x = x-100;

is the same as

x -= 100;

Shorthand notation is widely used in professionally written C/C++ programs; you

should become familiar with it.

Type Casts in C C++

You can force an expression to be of a specific type by using a cast. The general form of
a cast is
(type) expression
where type is a valid data type. For example, to make sure that the expression x/2
evaluates to type float, write
(float) x/2

Casts are technically operators. As an operator, a cast is unary and has the same
precedence as any other unary operator.
Although casts are not usually used a great deal in programming, they can be very
useful when needed. For example, suppose you wish to use an integer for loop control,
yet to perform computation on it requires a fractional part, as in the following
program:
#include <stdio.h>
int main(void) /* print i and i/2 with fractions */
{
int i;
for(i=1; i<=100; ++i)
printf("%d / 2 is: %f\n", i, (float) i /2);
return 0;
}
Without the cast (float), only an integer division would have been performed. The cast
ensures that the fractional part of the answer is displayed.

Expressions in C C++

Operators, constants, and variables are the constituents of expressions. An expression in
C/C++ is any valid combination of these elements. Because most expressions tend to
follow the general rules of algebra, they are often taken for granted. However, a few
aspects of expressions relate specifically to C and C++.

Order of Evaluation
Neither C nor C++ specifies the order in which the subexpressions of an expression are
evaluated. This leaves the compiler free to rearrange an expression to produce more
optimal code. However, it also means that your code should never rely upon the order
in which subexpressions are evaluated. For example, the expression
x = f1() + f2();
does not ensure that f1() will be called before f2() .

Type Conversion in Expressions
When constants and variables of different types are mixed in an expression, they are
all converted to the same type. The compiler converts all operands up to the type of
the largest operand, which is called type promotion. First, all char and short int values
are automatically elevated to int. (This process is called integral promotion.) Once this
step has been completed, all other conversions are done operation by operation, as
described in the following type conversion algorithm:
IF an operand is a long double
THEN the second is converted to long double
ELSE IF an operand is a double
THEN the second is converted to double
ELSE IF an operand is a float
THEN the second is converted to float
ELSE IF an operand is an unsigned long
THEN the second is converted to unsigned long
ELSE IF an operand is long
THEN the second is converted to long
ELSE IF an operand is unsigned int
THEN the second is converted to unsigned int

Precedence Summary

Table 2-8 lists the precedence of all operators defined by C. Note that all operators,
except the unary operators and ?, associate from left to right. The unary operators
(*, &, −) and ? associate from right to left.

Highest              ( ) [ ] −> .
                      ! ~ ++ – – (type) * & sizeof
                           * / %
                            + −
                          << >>
                           < <= > >=
                          == !=
                              &
                          ^
                            |
                         &&
                          ||
                               ?:
                    = += −=*= /= etc.
Lowest ,

Table 2-8. The Precedence of C Operators

The [ ] and ( ) Operators

Parentheses are operators that increase the precedence of the operations inside them.
Square brackets perform array indexing (arrays are discussed fully in Chapter 4).
Given an array, the expression within square brackets provides an index into that
array. For example,
#include <stdio.h>
char s[80];
int main(void)
{
s[3] = 'X';
printf("%c", s[3]);
return 0;
}
first assigns the value 'X' to the fourth element (remember, all arrays begin at 0) of
array s, and then prints that element.

The Compile-Time Operator sizeof

sizeof is a unary compile-time operator that returns the length, in bytes, of the variable
or parenthesized type-specifier that it precedes. For example, assuming that integers
are 4 bytes and doubles are 8 bytes,
double f;
printf("%d ", sizeof f);
printf("%d", sizeof(int));
will display 8 4.
Remember, to compute the size of a type, you must enclose the type name in
parentheses. This is not necessary for variable names, although there is no harm done
if you do so.

C/C++ defines (using typedef) a special type called size_t, which corresponds
loosely to an unsigned integer. Technically, the value returned by sizeof is of type
size_t. For all practical purposes, however, you can think of it (and use it) as if it were
an unsigned integer value.
sizeof primarily helps to generate portable code that depends upon the size of the
built-in data types. For example, imagine a database program that needs to store six
integer values per record. If you want to port the database program to a variety of
computers, you must not assume the size of an integer, but must determine its actual
length using sizeof. This being the case, you could use the following routine to write a
record to a disk file:
/* Write 6 integers to a disk file. */
void put_rec(int rec[6], FILE *fp)
{
int len;
len = fwrite(rec, sizeof(int)*6, 1, fp);
if(len != 1) printf("Write Error");
}
Coded as shown, put_rec() compiles and runs correctly in any environment, including
those that use 16- and 32-bit integers.
One final point: sizeof is evaluated at compile time, and the value it produces is
treated as a constant within your program.

The ? Operator Conditional or ternary operator

C/C++ contains a very powerful and convenient operator that replaces certain
statements of the if-then-else form. The ternary operator ? takes the general form
Exp1 ? Exp2 : Exp3;
where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.
The ? operator works like this: Exp1 is evaluated. If it is true, Exp2 is evaluated
and becomes the value of the expression. If Exp1 is false, Exp3 is evaluated and its
value becomes the value of the expression. For example, in
x = 10;
y = x>9 ? 100 : 200;
y is assigned the value 100. If x had been less than 9, y would have received the value
200. The same code written using the if-else statement is
x = 10;
if(x>9) y = 100;
else y = 200;
The ? operator will be discussed more fully in Chapter 3 in relationship to the other
conditional statements.

Bitwise Operators

Unlike many other languages, C/C++ supports a full complement of bitwise
operators. Since C was designed to take the place of assembly language for most
programming tasks, it needed to be able to support many operations that can be done
in assembler, including operations on bits. Bitwise operation refers to testing, setting, or
shifting the actual bits in a byte or word, which correspond to the char and int data
types and variants. You cannot use bitwise operations on float, double, long double,
void, bool, or other, more complex types. Table 2-6 lists the operators that apply to
bitwise operations. These operations are applied to the individual bits of the
operands.

Operator          Action
&                      AND
|                        OR
^                Exclusive OR (XOR)
~                 One's complement (NOT)

>>                  Shift right
<<                    Shift left

Table 2-6. Bitwise Operators

The bitwise AND, OR, and NOT (one's complement) are governed by the same
truth table as their logical equivalents, except that they work bit by bit. The exclusive
OR has the truth table shown here:
p   q    p^q
0   0     0
1   0     1
1   1     0
0   1     1
As the table indicates, the outcome of an XOR is true only if exactly one of the
operands is true; otherwise, it is false.
Bitwise operations most often find application in device drivers—such as modem
programs, disk file routines, and printer routines —because the bitwise operations
can be used to mask off certain bits, such as parity. (The parity bit confirms that the
rest of the bits in the byte are unchanged. It is usually the high-order bit in each byte.)
Think of the bitwise AND as a way to clear a bit. That is, any bit that is 0 in either
operand causes the corresponding bit in the outcome to be set to 0. For example, the
following function reads a character from the modem port and resets the parity bit to 0:
char get_char_from_modem(void)
{
char ch;
ch = read_modem(); /* get a character from the
modem port */
return(ch & 127);
}

Parity is often indicated by the eighth bit, which is set to 0 by ANDing it with a
byte that has bits 1 through 7 set to 1 and bit 8 set to 0. The expression ch & 127 means
to AND together the bits in ch with the bits that make up the number 127. The net
result is that the eighth bit of ch is set to 0. In the following example, assume that ch
had received the character "A" and had the parity bit set:
The bitwise OR, as the reverse of AND, can be used to set a bit. Any bit that is set
to 1 in either operand causes the corresponding bit in the outcome to be set to 1. For
example, the following is 128 | 3:
Ill 2-2
An exclusive OR, usually abbreviated XOR, will set a bit on if and only if the bits
being compared are different. For example, 127 ^120 is
Remember, relational and logical operators always produce a result that is either
true or false, whereas the similar bitwise operations may produce any arbitrary value
in accordance with the specific operation. In other words, bitwise operations may
produce values other than 0 or 1, while logical operators will always evaluate to 0 or 1.
The bit-shift operators, >> and <<, move all bits in a variable to the right or left as
specified. The general form of the shift-right statement is
 

1 0 0 0 0 0 0 0 128 in binary
0 0 0 0 0 0 1 1 3 in binary
¦___________ bitwise OR
1 0 0 0 0 0 1 1 result
0 1 1 1 1 1 1 1 127 in binary
0 1 1 1 1 0 0 0 120 in binary
^___________ bitwise XOR
0 0 0 0 0 1 1 1 result
Parity bit
1 1 0 0 0 0 0 1 ch containing an "A" with parity set
0 1 1 1 1 1 1 1 127 in binary
&___________ bitwise AND
0 1 0 0 0 0 0 1 "A"without parity

The bitwise OR, as the reverse of AND, can be used to set a bit. Any bit that is set
to 1 in either operand causes the corresponding bit in the outcome to be set to 1. For
example, the following is 128 | 3:
Ill 2-2
An exclusive OR, usually abbreviated XOR, will set a bit on if and only if the bits
being compared are different. For example, 127 ^120 is
Remember, relational and logical operators always produce a result that is either
true or false, whereas the similar bitwise operations may produce any arbitrary value
in accordance with the specific operation. In other words, bitwise operations may
produce values other than 0 or 1, while logical operators will always evaluate to 0 or 1.
The bit-shift operators, >> and <<, move all bits in a variable to the right or left as
specified. The general form of the shift-right statement isvariable >> number of bit positions
The general form of the shift-left statement is
variable << number of bit positions
As bits are shifted off one end, 0's are brought in the other end. (In the case of a
signed, negative integer, a right shift will cause a 1 to be brought in so that the sign bit
is preserved.) Remember, a shift is not a rotate. That is, the bits shifted off one end do
not come back around to the other. The bits shifted off are lost.
Bit-shift operations can be very useful when you are decoding input from an
external device, like a D/A converter, and reading status information. The bitwise shift
operators can also quickly multiply and divide integers. A shift right effectively divides
a number by 2 and a shift left multiplies it by 2, as shown in Table 2-7. The following
program illustrates the shift operators:
/* A bit shift example. */
#include <stdio.h>
int main(void)
{
unsigned int i;
int j;
i = 1;
/* left shifts */
for(j=0; j<4; j++) {
i = i << 1; /* left shift i by 1, which
is same as a multiply by 2 */
printf("Left shift %d: %d\n", j, i);
}
/* right shifts */
for(j=0; j<4; j++) {
i = i >> 1; /* right shift i by 1, which
is same as a division by 2 */
printf("Right shift %d: %d\n", j, i);
}
return 0;
}

unsigned char x;                     x as each statement executes                 value of x
x = 7;                                                   0 0 0 0 0 1 1 1                           7
x = x<<1;                                            0 0 0 0 1 1 1 0                           14
x = x<<3;                                            0 1 1 1 0 0 0 0                          112
x = x<<2;                                           1 1 0 0 0 0 0 0                           192
x = x>>1;                                            0 1 1 0 0 0 0 0                            96
x = x>>2;                                             0 0 0 1 1 0 0 0                           24

*Each left shift multiplies by 2. Notice that information has been lost after x<<2 because
a bit was shifted off the end.
**Each right shift divides by 2. Notice that subsequent divisions do not bring back any
lost bits.
Table 2-7. Multiplication and Division with Shift Operators

The one's complement operator, ~, reverses the state of each bit in its operand. That
is, all 1's are set to 0, and all 0's are set to 1.
The bitwise operators are often used in cipher routines. If you want to make a disk
file appear unreadable, perform some bitwise manipulations on it. One of the simplest
methods is to complement each byte by using the one's complement to reverse each bit
in the byte, as is shown here:
Notice that a sequence of two complements in a row always produces the original
number. Thus, the first complement represents the coded version of that byte. The
second complement decodes the byte to its original value.
You could use the encode() function shown here to encode a character.
/* A simple cipher function. */
char encode(char ch)
{return(~ch); /* complement it */
}
Of course, a file encoded using encode() would be very easy to crack!

Arithmetic Operators

Table 2-4 lists C/C++'s arithmetic operators. The operators +, −, *, and / work as they
do in most other computer languages. You can apply them to almost any built-in data
type. When you apply / to an integer or character, any remainder will be truncated.
For example, 5/2 will equal 2 in integer division.
The modulus operator % also works in C/C++ as it does in other languages,
yielding the remainder of an integer division. However, you cannot use it on
floating-point types. The following code fragment illustrates %:
int x, y;
x = 5;
y = 2;
printf("%d ", x/y); /* will display 2 */
printf("%d ", x%y); /* will display 1, the remainder of
the integer division */
x = 1;
y = 2;
printf("%d %d", x/y, x%y); /* will display 0 1 */
The last line prints a 0 and a 1 because 1/2 in integer division is 0 with a remainder of 1.
The unary minus multiplies its operand by –1. That is, any number preceded by a
minus sign switches its sign.

Increment and Decrement
C/C++ includes two useful operators not generally found in other computer
languages. These are the increment and decrement operators, ++ and −−. The operator
++ adds 1 to its operand, and −− subtracts one. In other words:
x = x+1;
is the same as
++x;
and

x = x-1;
is the same as
x--;
Both the increment and decrement operators may either precede (prefix) or follow
(postfix) the operand. For example,
x = x+1;
can be written
++x;
or
x++;
There is, however, a difference between the prefix and postfix forms when you use
these operators in an expression. When an increment or decrement operator precedes
its operand, the increment or decrement operation is performed before obtaining the
value of the operand for use in the expression. If the operator follows its operand,
 

Operator                             Action
−                             Subtraction, also unary minus
+                                          Addition
*                                       Multiplication
/                                           Division
%                                        Modulus
– –                                     Decrement
++                                      Increment

Table 2-4. Arithmetic Operators

the value of the operand is obtained before incrementing or decrementing it. For
instance,
x = 10;
y = ++x;
sets y to 11. However, if you write the code as
x = 10;
y = x++;
y is set to 10. Either way, x is set to 11; the difference is in when it happens.
Most C/C++ compilers produce very fast, efficient object code for increment and
decrement operations—code that is better than that generated by using the equivalent
assignment statement. For this reason, you should use the increment and decrement
operators when you can.
Here is the precedence of the arithmetic operators:
highest ++ – –
– (unary minus)
* / %
lowest + –
Operators on the same level of precedence are evaluated by the compiler from left to
right. Of course, you can use parentheses to alter the order of evaluation. C/C++ treats
parentheses in the same way as virtually all other computer languages. Parentheses
force an operation, or set of operations, to have a higher level of precedence.

Relational and Logical Operators
In the term relational operator, relational refers to the relationships that values can
have with one another. In the term logical operator, logical refers to the ways these
relationships can be connected. Because the relational and logical operators often
work together, they are discussed together here.
The idea of true and false underlies the concepts of relational and logical operators.
In C, true is any value other than zero. False is zero. Expressions that use relational or
logical operators return 0 for false and 1 for true.
C++ fully supports the zero/non-zero concept of true and false. However, it also
defines the bool data type and the Boolean constants true and false. In C++, a 0 value
is automatically converted into false, and a non-zero value is automatically converted
into true. The reverse also applies: true converts to 1 and false converts to 0. In C++,

the outcome of a relational or logical operation is true or false. But since this
automatically converts into 1 or 0, the distinction between C and C++ on this issue is
mostly academic.
Table 2-5 shows the relational and logical operators. The truth table for the logical
operators is shown here using 1's and 0's.
p             q               p && q                 p || q               !p
0             0                    0                          0                 1
0             1                    0                          1                1
1             1                    1                          1                0
1             0                    0                          1                0
Both the relational and logical operators are lower in precedence than the
arithmetic operators. That is, an expression like 10 > 1+12 is evaluated as if it were
written 10 > (1+12). Of course, the result is false.
You can combine several operations together into one expression, as shown here:
10>5 && !(10<9) || 3<=4

Relational Operators
Operator                     Action
>                               Greater than
>=                        Greater than or equal
<                               Less than
<=                       Less than or equal
= =                               Equal
!=                                Not equal

Logical Operators
Operator                       Action
&&                                AND
||                                      OR
!                                     NOT

Table 2-5. Relational and Logical Operators

In this case, the result is true.
Although neither C nor C++ contain an exclusive OR (XOR) logical operator, you
can easily create a function that performs this task using the other logical operators.
The outcome of an XOR operation is true if and only if one operand (but not both) is
true. The following program contains the function xor() , which returns the outcome of
an exclusive OR operation performed on its two arguments:
#include <stdio.h>
int xor(int a, int b);
int main(void)
{
printf("%d", xor(1, 0));
printf("%d", xor(1, 1));
printf("%d", xor(0, 1));
printf("%d", xor(0, 0));
return 0;
}
/* Perform a logical XOR operation using the
two arguments. */
int xor(int a, int b)
{
return (a || b) && !(a && b);
}
The following table shows the relative precedence of the relational and logical
operators:
Highest

!
> >= < <=
== !=
&&
Lowest

  ||

As with arithmetic expressions, you can use parentheses to alter the natural order of
evaluation in a relational and/or logical expression. For example, 

!0&& 0 || 0
is false. However, when you add parentheses to the same expression, as shown here,
the result is true:
!(0 && 0) || 0
Remember, all relational and logical expressions produce either a true or false
result. Therefore, the following program fragment is not only correct, but will print
the number 1.
int x;
x = 100;
printf("%d", x>10);

String and Backslash Character Constants

String Constants

C/C++ supports one other type of constant: the string. A string is a set of characters
enclosed in double quotes. For example, "this is a test" is a string. You have seen
examples of strings in some of the printf() statements in the sample programs.
Although C allows you to define string constants, it does not formally have a string
data type. (C++ does define a string class, however.)
You must not confuse strings with characters. A single character constant is
enclosed in single quotes, as in 'a'. However, "a" is a string containing only one letter.

Backslash Character Constants

Enclosing character constants in single quotes works for most printing characters. A
few, however, such as the carriage return, are impossible to enter into a string from the
keyboard. For this reason, C/C++ include the special backslash character constants
shown in Table 2-2 so that you may easily enter these special characters as constants.
These are also referred to as escape sequences. You should use the backslash codes
instead of their ASCII equivalents to help ensure portability.
For example, the following program outputs a new line and a tab and then prints
the string This is a test.
#include <stdio.h>
int main(void)
{
printf("\n\tThis is a test.");
return 0;

}

Code               Meaning
\b                    Backspace
\f                     Form feed
\n                     New line
\r                  Carriage return
\t                  Horizontal tab
\"                  Double quote
\'                  Single quote
\0                      Null
\\                   Backslash
\v                  Vertical tab
\a                      Alert
\?                Question mark
\N             Octal constant (where N is an octal constant)
\xN           Hexadecimal constant (where N is a hexadecimalconstant)

Table 2-2. Backslash Codes

Hexadecimal and Octal Constants

It is sometimes easier to use a number system based on 8 or 16 rather than 10 (our
standard decimal system). The number system based on 8 is called octal and uses the digits 0 through 7. In octal, the number 10 is the same as 8 in decimal. The base 16
number system is called hexadecimal and uses the digits 0 through 9 plus the letters
A through F, which stand for 10, 11, 12, 13, 14, and 15, respectively. For example, the
hexadecimal number 10 is 16 in decimal. Because these two number systems are
used frequently, C/C++ allows you to specify integer constants in hexadecimal or octal
instead of decimal. A hexadecimal constant must consist of a 0x followed by the
constant in hexadecimal form. An octal constant begins with a 0. Here are some
examples:
int hex = 0x80; /* 128 in decimal */
int oct = 012; /* 10 in decimal */

Constants in C and C++

Constants refer to fixed values that the program may not alter. Constants can be of any
of the basic data types. The way each constant is represented depends upon its type.
Constants are also called literals.
Character constants are enclosed between single quotes. For example 'a' and '%' are
both character constants. Both C and C++ define wide characters (used mostly in character constant, precede the character with an L. For example,
wchar_t wc;
wc = L'A';
Here, wc is assigned the wide-character constant equivalent of A. The type of wide
characters is wchar_t. In C, this type is defined in a header file and is not a built-in
type. In C++, wchar_t is built in.
Integer constants are specified as numbers without fractional components. For
example, 10 and –100 are integer constants. Floating-point constants require the
decimal point followed by the number's fractional component. For example, 11.123
is a floating-point constant. C/C++ also allows you to use scientific notation for
floating-point numbers.
There are two floating-point types: float and double. There are also several
variations of the basic types that you can generate using the type modifiers. By default,
the compiler fits a numeric constant into the smallest compatible data type that will
hold it. Therefore, assuming 16-bit integers, 10 is int by default, but 103,000 is a long.
Even though the value 10 could fit into a character type, the compiler will not cross
type boundaries. The only exception to the smallest type rule are floating-point
constants, which are assumed to be doubles.
For most programs you will write, the compiler defaults are adequate. However,
you can specify precisely the type of numeric constant you want by using a suffix. For
floating-point types, if you follow the number with an F, the number is treated as a
float. If you follow it with an L, the number becomes a long double. For integer types,
the U suffix stands for unsigned and the L for long. Here are some examples:

Data type                          Constant examples
int                                     1 123 21000 −234
long int                              35000L −34L
unsigned int                     10000U 987U 40000U
float                                   123.23F 4.34e−3F
double                             123.23 1.0 −0.9876324
long double                          1001.2L

Variable Initializations in C and C++

You can give variables a value as you declare them by placing an equal sign and a
value after the variable name. The general form of initialization is
type variable_name = value;
Some examples are
char ch = 'a';
int first = 0;
float balance = 123.23;
Global and static local variables are initialized only at the start of the program. Local
variables (excluding static local variables) are initialized each time the block in which
they are declared is entered. Local variables that are not initialized have unknown
values before the first assignment is made to them. Uninitialized global and static local
variables are automatically set to zero.

Storage Class Specifiers in C

There are four storage class specifiers supported by C:
extern
static
register
auto
These specifiers tell the compiler how to store the subsequent variable. The general
form of a declaration that uses one is shown here.
storage_specifier type var_name;
Notice that the storage specifier precedes the rest of the variable declaration.

extern

Because C/C++ allows separate modules of a large program to be separately compiled
and linked together, there must be some way of telling all the files about the global
variables required by the program. Although C technically allows you to define a
global variable more than once, it is not good practice (and may cause problems when
linking). More importantly, in C++, you may define a global variable only once. How,
then, do you inform all the files in your program about the global variables used by
the program?
The solution to the problem is found in the distinction between the declaration
and the definition of a variable. A declaration declares the name and type of a variable.

File One                   File Two
int x, y;                   extern int x, y;
char ch;                  extern char ch;
int main(void)           void func22(void)
{                               {
/* ... */                     x = y / 10;
}                              }
void func1(void)       void func23(void)
{                                {
x = 123;                        y = 10;
}                               }

Figure 2-1. Using global variables in separately compiled modules

A definition causes storage to be allocated for the variable. In most cases, variable
declarations are also definitions. However, by preceding a variable name with the
extern specifier, you can declare a variable without defining it. Thus, in a multifile
program, you can declare all of your global variables in one file and use extern
declarations in the other, as in Figure 2-1.
In File Two, the global variable list was copied from File One and the extern
specifier was added to the declarations. The extern specifier tells the compiler that the
variable types and names that follow it have been defined elsewhere. In other words,
extern lets the compiler know what the types and names are for these global variables
without actually creating storage for them again. When the linker links the two
modules, all references to the external variables are resolved.
The extern keyword has this general form:
extern var-list;
There is another, optional use of extern that you may occasionally see. When you
use a global variable inside a function, you can declare it as extern, as shown here:
int first, last; /* global definition of first
and last */

main(void)
{
extern int first; /* optional use of the
extern declaration */
.
.
.
}

static Variables
static variables are permanent variables within their own function or file. Unlike global
variables, they are not known outside their function or file, but they maintain their
values between calls. This feature makes them useful when you write generalized
functions and function libraries that other programmers may use. static has different
effects upon local variables and global variables.
static Local Variables
When you apply the static modifier to a local variable, the compiler creates permanent
storage for it, much as it creates storage for a global variable. The key difference
between a static local variable and a global variable is that the static local variable
remains known only to the block in which it is declared. In simple terms, a static
local variable is a local variable that retains its value between function calls.
static local variables are very important to the creation of stand-alone functions
because several types of routines must preserve a value between calls. If static variables
were not allowed, globals would have to be used, opening the door to possible side
effects. An example of a function that benefits from a static local variable is a numberseries
generator that produces a new value based on the previous one. You could use
a global variable to hold this value. However, each time the function is used in a
program, you would have to declare that global variable and make sure that it did not
conflict with any other global variables already in place. The better solution is to declare
the variable that holds the generated number to be static, as in this program fragment:
int series(void)

{
static int series_num;
series_num = series_num+23;
return series_num;
}
In this example, the variable series_num stays in existence between function calls,
instead of coming and going the way a normal local variable would. This means that
each call to series() can produce a new member in the series based on the preceding
number without declaring that variable globally.
You can give a static local variable an initialization value. This value is assigned
only once, at program start-up—not each time the block of code is entered, as with
normal local variables. For example, this version of series() initializes series_num
to 100:
int series(void)
{
static int series_num = 100;
series_num = series_num+23;
return series_num;
}
As the function now stands, the series will always begin with the value 123. While this
is acceptable for some applications, most series generators need to let the user specify
the starting point. One way to give series_num a user-specified value is to make it a
global variable and then let the user set its value. However, not defining series_num
as global was the point of making it static. This leads to the second use of static.
static Global Variables
Applying the specifier static to a global variable instructs the compiler to create a
global variable that is known only to the file in which you declared it. This means
that even though the variable is global, routines in other files may have no knowledge
of it or alter its contents directly, keeping it free from side effects. For the few situations
where a local static cannot do the job, you can create a small file that contains only the
functions that need the global static variable, separately compile that file, and use it
without fear of side effects.
To illustrate a global static, the series generator example from the previous section
is recoded so that a seed value initializes the series through a call to a second function
called series_start() . The entire file containing series() , series_start() , and series_num
is shown here:

/* This must all be in one file - preferably by itself. */
static int series_num;
void series_start(int seed);
int series(void);
int series(void)
{
series_num = series_num+23;
return series_num;
}
/* initialize series_num */
void series_start(int seed)
{
series_num = seed;
}
Calling series_start() with some known integer value initializes the series generator.
After that, calls to series() generate the next element in the series.
To review: The names of local static variables are known only to the block of code
in which they are declared; the names of global static variables are known only to the
file in which they reside. If you place the series() and series_start() functions in a
library, you can use the functions but cannot reference the variable series_num, which
is hidden from the rest of the code in your program. In fact, you can even declare and
use another variable called series_num in your program (in another file, of course). In
essence, the static modifier permits variables that are known only to the functions that
need them, without unwanted side effects.
static variables enable you to hide portions of your program from other portions.
This can be a tremendous advantage when you are trying to manage a very large and
complex program.
In C++, the preceding use of static is still supported, but deprecated. This means
that it is not recommended for new code. Instead, you should use a namespace,
which is described in Part Two.

register Variables
The register storage specifier originally applied only to variables of type int, char, or
pointer types. However, in Standard C, register's definition has been broadened so that
it applies to any type of variable.
Originally, the register specifier requested that the compiler keep the value of a
variable in a register of the CPU rather than in memory, where normal variables are stored. This meant that operations on a register variable could occur much faster than
on a normal variable because the register variable was actually held in the CPU and
did not require a memory access to determine or modify its value.
Today, the definition of register has been greatly expanded and it now may be
applied to any type of variable. Standard C simply states "that access to the object be
as fast as possible." (Standard C++ states that register is a "hint to the implementation
that the object so declared will be heavily used.") In practice, characters and integers
are still stored in registers in the CPU. Larger objects like arrays obviously cannot be
stored in a register, but they may still receive preferential treatment by the compiler.
Depending upon the implementation of the C/C++ compiler and its operating
environment, register variables may be handled in any way deemed fit by the
compiler's implementor. In fact, it is technically permissible for a compiler to ignore
the register specifier altogether and treat variables modified by it as if they weren't,
but this is seldom done in practice.
You can only apply the register specifier to local variables and to the formal
parameters in a function. Global register variables are not allowed. Here is an example
that uses register variables. This function computes the result of Me for integers:
int int_pwr(register int m, register int e)
{
register int temp;
temp = 1;
for(; e; e--) temp = temp * m;
return temp;
}
In this example, e, m, and temp are declared as register variables because they
are all used within the loop. The fact that register variables are optimized for speed
makes them ideal for control of or use in loops. Generally, register variables are used
where they will do the most good, which are often places where many references will
be made to the same variable. This is important because you can declare any number
of variables as being of type register, but not all will receive the same access speed
optimization.
The number of register variables optimized for speed allowed within any one code
block is determined by both the environment and the specific implementation of
C/C++. You don't have to worry about declaring too many register variables because
the compiler automatically transforms register variables into nonregister variables
when the limit is reached. (This ensures portability of code across a broad line of
processors.)

Usually at least two register variables of type char or int can actually be held in the
registers of the CPU. Because environments vary widely, consult your compiler's user
manual to determine if you can apply any other types of optimization options.
In C, you cannot find the address of a register variable using the & operator
(discussed later in this chapter). This makes sense because a register variable might be
stored in a register of the CPU, which is not usually addressable. But this restriction
does not apply to C++. However, taking the address of a register variable in C++ may
prevent it from being fully optimized.
Although the description of register has been broadened beyond its traditional
meaning, in practice it still generally has a significant effect only with integer and
character types. Thus, you should probably not count on substantial speed
improvements for other variable types.

volatile datatype in C and C++

The modifier volatile tells the compiler that a variable's value may be changed in ways
not explicitly specified by the program. For example, a global variable's address may
be passed to the operating system's clock routine and used to hold the real time of thesystem. In this situation, the contents of the variable are altered without any explicit
assignment statements in the program. This is important because most C/C++
compilers automatically optimize certain expressions by assuming that a variable's
content is unchanging if it does not occur on the left side of an assignment statement;
thus, it might not be reexamined each time it is referenced. Also, some compilers
change the order of evaluation of an expression during the compilation process. The
volatile modifier prevents these changes.
You can use const and volatile together. For example, if 0x30 is assumed to be the
value of a port that is changed by external conditions only, the following declaration
would prevent any possibility of accidental side effects:
const volatile char *port = (const volatile char *) 0x30;

Access Modifiers and constants in C and C++

Access Modifiers
There are two modifiers that control how variables may be accessed or modified. These
qualifiers are const and volatile. They must precede the type modifiers and the type
names that they qualify. These modifiers are also referred to as cv-qualifiers.

const
Variables of type const may not be changed by your program. (A const variable can be
given an initial value, however.) The compiler is free to place variables of this type into
read-only memory (ROM). For example,
const int a=10;
creates an integer variable called a with an initial value of 10 that your program may
not modify. However, you can use the variable a in other types of expressions. A const
variable will receive its value either from an explicit initialization or by some
hardware-dependent means.
The const qualifier can be used to protect the objects pointed to by the arguments
to a function from being modified by that function. That is, when a pointer is passed
to a function, that function can modify the actual variable pointed to by the pointer.
However, if the pointer is specified as const in the parameter declaration, the function
code won't be able to modify what it points to. For example, the sp_to_dash() function
in the following program prints a dash for each space in its string argument. That is,
the string "this is a test" will be printed as "this-is-a-test". The use of const in the
parameter declaration ensures that the code inside the function cannot modify the
object pointed to by the parameter.
#include <stdio.h>
void sp_to_dash(const char *str);
int main(void)
{
sp_to_dash("this is a test");
return 0;
}

void sp_to_dash(const char *str)
{
while(*str) {
if(*str== ' ') printf("%c", '-');
else printf("%c", *str);
str++;
}
}
If you had written sp_to_dash() in such a way that the string would be modified, it
would not compile. For example, if you had coded sp_to_dash() as follows, you would
receive a compile-time error:
/* This is wrong. */
void sp_to_dash(const char *str)
{
while(*str) {
if(*str==' ' ) *str = '-'; /* can't do this; str is const */
printf("%c", *str);
str++;
}
}
Many functions in the standard library use const in their parameter declarations.
For example, the strlen() function has this prototype:
size_t strlen(const char *str);
Specifying str as const ensures that strlen() will not modify the string pointed to by str.
In general, when a standard library function has no need to modify an object pointed to
by a calling argument, it is declared as const.
You can also use const to verify that your program does not modify a variable.
Remember, a variable of type const can be modified by something outside your
program. For example, a hardware device may set its value. However, by declaring
a variable as const, you can prove that any changes to that variable occur because of
external events.

Global Variables

Unlike local variables, global variables are known throughout the program and may be
used by any piece of code. Also, they will hold their value throughout the program's
execution. You create global variables by declaring them outside of any function. Any
expression may access them, regardless of what block of code that expression is in.
In the following program, the variable count has been declared outside of all
functions. Although its declaration occurs before the main() function, you could have
placed it anywhere before its first use as long as it was not in a function. However, it is
usually best to declare global variables at the top of the program.
#include <stdio.h>
int count; /* count is global */
void func1(void);

void func2(void);
int main(void)
{
count = 100;
func1();
return 0;
}
void func1(void)
{
int temp;
temp = count;
func2();
printf("count is %d", count); /* will print 100 */
}
void func2(void)
{
int count;
for(count=1; count<10; count++)
putchar('.');
}
Look closely at this program. Notice that although neither main() nor func1() has
declared the variable count, both may use it. func2() , however, has declared a local
variable called count. When func2() refers to count, it refers to only its local variable,
not the global one. If a global variable and a local variable have the same name, all
references to that variable name inside the code block in which the local variable is
declared will refer to that local variable and have no effect on the global variable.
This can be convenient, but forgetting it can cause your program to act strangely,
even though it looks correct.
Storage for global variables is in a fixed region of memory set aside for this purpose
by the compiler. Global variables are helpful when many functions in your program
use the same data. You should avoid using unnecessary global variables, however.
They take up memory the entire time your program is executing, not just when they are
needed. In addition, using a global where a local variable would do makes a function
less general because it relies on something that must be defined outside itself. Finally,
using a large number of global variables can lead to program errors because of unknown and unwanted side effects. Amajor problem in developing large programs is the
accidental changing of a variable's value because it was used elsewhere in the program.
This can happen in C/C++ if you use too many global variables in your programs.