The break and continue Statements

break

• Causes immediate exit from a while, for, do/while or switch structure

• Program execution continues with the first statement after the structure

• Common uses of the break statement:

  1. Escape early from a loop.

  2. Skip the remainder of a switch structure.

• Note: If you have nested loops , the break will exit only the inner loop

continue

• Skips the remaining statements in the body of a while, for or do/while structure .

• Proceeds with the next iteration of the loop

• while and do/while

  1. Loop-continuation test is evaluated immediately after the continue statement is executed.

  2. Then increment is done.

• for

  1. Increment expression is executed.

  2. Then the loop-continuation test is evaluated.

To put everything we've seen in this chapter together, as well as demonstrate the use of the break statement, here is a program for printing prime numbers between 1 and 100:

#include <stdio.h> #include <math.h> main() { int i, j; printf("%d\n", 2); for(i = 3; i <= 100; i = i + 1) { for(j = 2; j < i; j = j + 1) { if(i % j == 0) break; if(j > sqrt(i)) { printf("%d\n", i); break; } } } return 0; }

The outer loop steps the variable i through the numbers from 3 to 100; the code tests to see if each number has any divisors other than 1 and itself. The trial divisor j loops from 2 up to i. j is a divisor of i if the remainder of i divided by j is 0, so the code uses C's ``remainder'' or ``modulus'' operator % to make this test. (Remember that i % j gives the remainder when i is divided by j.)

If the program finds a divisor, it uses break to break out of the inner loop, without printing anything. But if it notices that j has risen higher than the square root of i, without its having found any divisors, then i must not have any divisors, so i is prime, and its value is printed. (Once we've determined that i is prime by noticing that j > sqrt(i), there's no need to try the other trial divisors, so we use a second break statement to break out of the loop in that case, too.)

The simple algorithm and implementation we used here (like many simple prime number algorithms) does not work for 2, the only even prime number, so the program ``cheats'' and prints out 2 no matter what, before going on to test the numbers from 3 to 100.

Many improvements to this simple program are of course possible; you might experiment with it. (Did you notice that the ``test'' expression of the inner loop for(j = 2; j < i; j = j + 1) is in a sense unnecessary, because the loop always terminates early due to one of the two break statements?)