You may encounter situations when a block of code needs to be executed several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.
Programming languages provide various control structures that allow for more complicated execution paths.
A loop statement allows us to execute a statement or group of statements multiple times. Given below is the general form of a loop statement in most of the programming languages:
C programming language provides the following types of loops to handle looping requirements.
| Loop type | Description |
|---|---|
| while Type | Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body. |
| for loop | Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable. |
| do...while loop | It is more like a while statement, except that it tests the condition at the end of the loop body. |
| nested loops | You can use one or more loops inside any other while, for, or do..while loop. |
A while loop in C programming repeatedly executes a target statement as long as a given condition is true.
The syntax of a while loop in C programming language is:
while(condition)
{
statement(s);
}
Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any nonzero value. The loop iterates while the condition is true.
When the condition becomes false, the program control passes to the line immediately following the loop.
Here, the key point to note is that a while loop might not execute at all. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* while loop execution */
while( a < 20 )
{
printf("value of a: %d\n", a);
a++;
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
value of a: 16
value of a: 17
value of a: 18
value of a: 19
A for loop is a repetition control structure that allows you to efficiently write a loop that needs to execute a specific number of times.
The syntax of a for loop in C programming language is:
for ( init; condition; increment )
{
statement(s);
}
Here is the flow of control in a ‘for’ loop:
#include <stdio.h>
int main ()
{
/* for loop execution */
for( int a = 10; a < 20; a = a + 1 )
{
printf("value of a: %d\n", a);
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 15 value of a: 16 value of a: 17 value of a: 18 value of a: 19
Unlike for and while loops, which test the loop condition at the top of the loop, the do...while loop in C programming checks its condition at the bottom of the loop.
A do...while loop is similar to a while loop, except the fact that it is guaranteed to execute at least one time.
The syntax of a do...while loop in C programming language is:
do
{
statement(s);
}while( condition );
Notice that the conditional expression appears at the end of the loop, so the statement(s) in the loop executes once before the condition is tested.
If the condition is true, the flow of control jumps back up to do, and the statement(s) in the loop executes again. This process repeats until the given condition becomes false.
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
do
{
printf("value of a: %d\n", a);
a = a + 1;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces the following result:
C programming allows to use one loop inside another loop. The following section shows a few examples to illustrate the concept.
The syntax for a nested for loop statement in C is as follows:
for ( init; condition; increment )
{
for ( init; condition; increment )
{
statement(s);
}
statement(s);
}
The syntax for a nested while loop statement in C programming language is as follows:
while(condition)
{
while(condition)
{
statement(s);
}
statement(s);
}
The syntax for a nested do...while loop statement in C programming language is as follows:
do
{
statement(s);
do
{
statement(s);
}while( condition );
}while( condition );
A final note on loop nesting is that you can put any type of loop inside any other type of loop. For example, a ‘for’ loop can be inside a ‘while’ loop or vice versa.
The following program uses a nested for loop to find the prime numbers from 2 to 100:
#include <stdio.h>
int main ()
{
/* local variable definition */
int i, j;
for(i=2; i<100; i++) {
for(j=2; j <= (i/j); j++)
if(!(i%j)) break; // if factor found, not prime
if(j > (i/j)) printf("%d is prime\n", i);
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 53 is prime 59 is prime 61 is prime 67 is prime 71 is prime 73 is prime 79 is prime 83 is prime 89 is prime 97 is prime
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.
C supports the following control statements.
| Control Statement | Description |
|---|---|
| break statement | Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch. |
| continue statement | Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. |
| goto statement | Transfers control to the labeled statement. |
The break statement in C programming has the following two usages:
If you are using nested loops, the break statement will stop the execution of the innermost loop and start executing the next line of code after the block.
The syntax for a break statement in C is as follows:
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* while loop execution */
while( a < 20 )
{
printf("value of a: %d\n", a);
a++;
if( a > 15)
{
/* terminate the loop using break statement */
break;
}
}
return 0;
}
When the above code is compiled and executed, it produces the following result:
The continue statement in C programming works somewhat like the break statement. Instead of forcing termination, it forces the next iteration of the loop to take place, skipping any code in between.
For the for loop, continue statement causes the conditional test and increment portions of the loop to execute. For the while and do...while loops, continue statement causes the program control to pass to the conditional tests.
The syntax for a continue statement in C is as follows:
continue;
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
do
{
if( a == 15)
{
/* skip the iteration */
a = a + 1;
continue;
}
printf("value of a: %d\n", a);
a++;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces the following result:
A goto statement in C programming provides an unconditional jump from the ‘goto’ to a labeled statement in the same function.
NOTE: Use of goto statement is highly discouraged in any programming language because it makes difficult to trace the control flow of a program, making the program hard to understand and hard to modify. Any program that uses a goto can be rewritten to avoid them.
The syntax for a goto statement in C is as follows:
goto label;
..
.
label: statement;
Here label can be any plain text except C keyword and it can be set anywhere in the C program above or below to goto statement.
#include <stdio.h>
int main ()
{
/* local variable definition */
int a = 10;
/* do loop execution */
LOOP:do
{
if( a == 15)
{
/* skip the iteration */
a = a + 1;
goto LOOP;
}
printf("value of a: %d\n", a);
a++;
}while( a < 20 );
return 0;
}
When the above code is compiled and executed, it produces the following result:
value of a: 10 value of a: 11 value of a: 12 value of a: 13 value of a: 14 value of a: 16 value of a: 17 value of a: 18 value of a: 19
A loop becomes an infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty.
#include <stdio.h>
int main ()
{
for( ; ; )
{
printf("This loop will run forever.\n");
}
return 0;
}
When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C programmers more commonly use the for(;;) construct to signify an infinite loop.
NOTE: You can terminate an infinite loop by pressing Ctrl + C keys.