The break and continue statement in C

break statement #

Consider the following problem.

Suppose that we creating a program to search for a particular number among 1000 numbers. In 10th iteration, we have found the desired number. Now there is no need to traverse all remaining 990 numbers. Such conditions demand break statement. The break statement when encountered, immediately breaks out of a while, do or for loop to execute statements that follow the loop. The break statement is commonly used in conjunction with a condition.

Let's take an example:

#include<stdio.h>

int main()
{
    int i;

    for(i = 1; i < 10 ; i++)
    {
        if(i==5)
        {
            break; // breaks out of the for loop
        }
        printf("Value of i = %d\n", i);
    }

    // signal to operating system everything works fine
    return 0;
}

Expected Output:

Value of i = 1
Value of i = 2
Value of i = 3
Value of i = 4

How it works ?

In the 5th iteration value of i becomes 5. The condition (i==5) is checked since it is true. The break statement is executed and the control comes out the for loop. Had there been no break statement, this loop would have been executed 9 times.

Let's create something more useful. The following program determines whether the number entered by the user is prime or not.

#include<stdio.h>

int main()
{
    int i, num, flag=1;

    printf("Enter a number: ");
    scanf("%d", &num);

    for(i = 2; i < num-1 ; i++)
    {
        /*
            when this condition becomes true,
            it means that entered number is
            divisible by a number between 2 and num-1.
            So it can't be prime.
        */
        if(num%i==0)
        {
            flag = 0; // number is not prime
        }
    }

    if(flag==1)
    {
        printf("%d is prime", num);
    }

    else
    {
        printf("%d is not prime prime", num);
    }

    // signal to operating system everything works fine
    return 0;
}

Expected Output:

1st run:

Enter a number: 5
5 is prime

2nd run:

Enter a number: 6
6 is not prime

*How it works ?

A number is prime if it is only divisible by 1 and itself. Here is one way to test whether a number is prime or not. Try dividing the number from 2 to one less than the number, if it is found to be divisible by any number within that range, then is not prime, otherwise it is a prime number.

After looping through all the numbers from 2 to n-1 . If the value of the variable flag remains set to 1. Then the number n is prime, otherwise it is not.

Let's say in the first run the user entered 5.

In for loop i is initialized to 2. The condition (i < num-1) or (2 < 4) is checked, since it is true, control moves ahead to execute the body of the loop. In the body of for loop, the if condition num%i == 0 i.e 5%2 == 0 is tested, since it is false. The statement in the body of the if is omitted. Then the value of i is incremented using i++, condition (i < num-1) i.e 2 < 4 is checked again, since it is true the body of the for is executed once more. The process repeats until i < num-1. Even after third and fourth iteration value of flag remains the same i.e 1. When control comes out of for loop, the if condition i.e flag==1 is checked, in this case, it is true and thus the statement inside the body of if is executed.

Again Let's say in the second run the user entered 6.

Everything work exactly as described above. Except in the 2nd iteration when i reaches 3, the if condition (num%i==0) i.e 6%3 == 0 is checked, since it is true. Control moves ahead to execute statement inside the body of the if block. Inside the if block the variable flag is initialized to 1 , this means that the number is not prime. The break statement breaks out of the for loop, effectively terminating the loop. If condition outside the loop is tested again i.e flag==1, as it is false, the statement in the else block is executed.

break inside nested loop #

When the break statement is used inside a nested loop then it causes exit only from the innermost loop.

#include<stdio.h>
int main()
{
    int i, j;

    for(i = 1; i < 5 ; i++)
    {
        printf("Outer for loop, i = %d\n", i);

        for(j = 1; j < 5 ; j++)
        {
            if(j==3)
            {
                 printf("Breaking out from the inner loop \n");  
                 break; // break out from inner loop only
            }
            printf("Inner for loop, j = %d\n", j);
        }
        printf("\n");
    }

    // signal to operating system program ran fine
    return 0;
}

Expected Output:

Outer for loop, i = 1
Inner for loop, j = 1
Inner for loop, j = 2
Breaking out from the inner loop

Outer for loop, i = 2
Inner for loop, j = 1
Inner for loop, j = 2
Breaking out from the inner loop

Outer for loop, i = 3
Inner for loop, j = 1
Inner for loop, j = 2
Breaking out from the inner loop

Outer for loop, i = 4
Inner for loop, j = 1
Inner for loop, j = 2
Breaking out from the inner loop

continue statement #

The continue statement is used to prematurely end the current iteration and move on the to the next iteration. When continue statement is encountered in a loop, all the statements after the continue statement are omitted and the loop continues with the next iteration. The continue statement is used in conjunction with a condition.

Sometimes people get confused with between break and continue statement. Always remember break statement when encountered breaks out of the loop, effectively terminating the loop, but when continue statement is encountered, the loop is not terminated instead the control is passed to the beginning of the loop.

When continue statement is encountered in the while and do while loop, the control is transferred to the test condition and then the loop continues. whereas in for loop when continue statement is encountered the control is transferred to update expression, then the condition is tested.

The following program prints all the numbers between 0 to 10 which are not divisible by 4.

#include<stdio.h>

int main()
{
    int i;

    for(i = 0; i < 10; i++)
    {
        if( i % 4 == 0 )
        {
            /*
                when i is divisible by 4
                continue to the next iteration
            */
            continue;
        }
        printf("%d\n", i);
    }

    // signal to operating system everything works fine
    return 0;
}

Expected Output:

1
2
3
5
6
7
9

How it works ?

In the for loop variable i is initialized to 0, the condition (i < 10) or (0 < 10) is checked, since it is true the control moves ahead to execute the body the for loop. if condition i.e (i%4==0) or (0%4==0) is checked, since it is false, statement inside the body of if block is skipped. In line 20, printf() statement is executed. This completes the first iteration.

In the 5th iteration value of i reaches 4, the if condition (i % 4 == 0) or (4%4==0) is tested, since it is true, the continue statement is executed, and control is transferred to the update expression, the statement i++ is executed, again the condition i < 10 is tested. This process continues until i < 10.