while loopwhile is most commonly used for indefinite looping. Preferred for variable conditions.
Syntax:
while (condition) {
// statements
}
The while loop follows the basic syntax of if statement. When the conditional expression is true, the loop executes an iteration.
Eg:
#include <stdio.h>
int main() {
int i = 1;
while (i <= 10){
printf("%d ", i++);
// The incrementation will cause the loop termination change
}
return 0;
}
The
whileloop should definitely contain an expression that will cause the termination of loop sooner or later.
do...while loopThe do...while looping is yet another looping statement that is used in indefinite cases. It works similar to the while loop where truthfulness of a conditional expression decides the flow.
Syntax:
do {
// statements
} while(condition);
Eg:
#include <stdio.h>
int main() {
int i = 1;
do {
printf("%d ", i++);
// The incrementation will cause the loop termination change
} while (i <= 10);
return 0;
}
whileConsider the problem:
Given a number 'n', find the number of steps taken to reach 1 using the following function:
$$ \displaystyle n = \begin{cases} n/2, ~~~~~~~~~~~~\rm{if}~ n~ \text{is even}\\ 3 \times n + 1, ~~ \rm{if}~n~ \text{is odd} \end{cases} $$
Here, the number of steps it takes to reach 1 is not clear (indefinite).
It is implemented using while loop as below:
#include <stdio.h>
int main() {
int n, steps = 0; // steps variable to count number of steps taken
printf("Enter n ");
scanf("%d", &n); // getting n
while (n != 1) {
++steps;
if (n % 2 == 0) {
// if n is even
n /= 2;
} else {
n = (3 * n) + 1;
}
}
printf("Steps taken = %d", steps);
return 0;
}
10Steps taken = 611Steps taken = 14while vs do...while loopsBoth while and do…while loops are indefinite loops, working similar. But still they have differences.
In while loop, the iteration starts with a condition check. If the result is false, the loop is not entered and execution is passed to after the loop.
In do...while loop, the condition check is done after iteration. This means an iteration may execute even if condition is not satisfied.
Eg:
while loop:
#include<stdio.h>
int main(){
int n = 11;
while(n < 10){
printf("%d ", n);
}
printf("Outside Loop")
return 0;
}
output
Outside Loop
do…while loop:
#include<stdio.h>
int main(){
int n = 11;
do{
printf("%d ", n);
}while(n < 10);
printf("Outside Loop");
return 0;
}
output
11 Outside Loop
You can use one or more loops inside any other while, for, or do...while loop.
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.
While loops can also be used for infinite loops. In that case:
while(1){
}
// or
while(true){
}
Terminate an infinite loop (code execution) by pressing
Ctrl + C.
Loop control Statements are statements that alter the flow of loop. C supports the following statements
breakcontinuegoto1. break Statement:
The break statement has the following two usages −
break statement is encountered inside a loop, the loop is immediately terminated and the program control resumes at the next statement following the loop.In nested loops, the
breakstatement will stop the execution of the innermost loop and start executing the next line of code after the block.
Syntax:
break;
Eg:
#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;
}
Output
value of a: 10
value of a: 11
value of a: 12
value of a: 13
value of a: 14
value of a: 15
It can be seen that, on encountering the break statement, the execution of loop is interrupted.
2. continue statement:
The continue statement is used to interrupt the current iteration. 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.
Syntax:
continue;
Eg:
#include <stdio.h>
int main () {
/* local variable definition */
int a = 10;
/* do loop execution */
do {
if( a == 15) {
/* skip the iteration */
++a;
continue;
}
printf("value of a: %d\n", a);
a++;
} while( a < 20 );
return 0;
}
Output:
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
It can observed that, when the continue statement is encountered, the current iteration is skipped (when a = 15).
3. goto statement:
A goto statement in C programming provides an unconditional jump from the 'goto' to a labeled statement in the same function.
Syntax:
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.
Use of
gotostatement is highly discouraged in any programming language
It is 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.
Eg:
#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;
}
Output:
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