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5.8 — Break and continue

Break

Although you have already seen the break statement in the context of switch statements, it deserves a fuller treatment since it can be used with other types of loops as well. The break statement causes a do, for, switch, or while statement to terminate.

Breaking a switch

In the context of a switch statement, a break is typically used at the end of each case to signify the case is finished (which prevents fall-through):

Breaking a loop

In the context of a loop, a break statement can be used to cause the loop to terminate early:

This program allows the user to type up to 10 numbers, and displays the sum of all the numbers entered at the end. If the user enters 0, the break causes the loop to terminate early (before 10 numbers have been entered).

Note that break can be used to get out of an infinite loop:

Break vs return

New programmers often have trouble understanding the difference between break and return. A break statement terminates the switch or loop, and execution continues at the first statement beyond the switch or loop. A return statement terminates the entire function that the loop is within, and execution continues at point where the function was called.

Continue

The continue statement provides a convenient way to jump to the end of the loop body for the current iteration. This is useful when we want to terminate the current iteration early.

Here’s an example of using continue:

This program prints all of the numbers from 0 to 19 that aren’t divisible by 4.

In the case of a for loop, the end-statement of the for loop still executes after a continue (since this happens after the end of the loop body).

Be careful when using a continue statement with while or do-while loops. Because these loops typically increment the loop variables in the body of the loop, using continue can cause the loop to become infinite! Consider the following program:

This program is intended to print every number between 0 and 9 except 5. But it actually prints:

0 1 2 3 4

and then goes into an infinite loop. When count is 5, the if statement evaluates to true, and the loop jumps to the bottom. The count variable is never incremented. Consequently, on the next pass, count is still 5, the if statement is still true, and the program continues to loop forever.

Here’s an example with a do-while loop using continue correctly:

This prints:

0 1 2 3 4 6 7 8 9

Using break and continue

Many textbooks caution readers not to use break and continue, both because it causes the execution flow to jump around and because it can make the flow of logic harder to follow. For example, a break in the middle of a complicated piece of logic could either be missed, or it may not be obvious under what conditions it should be triggered.

However, used judiciously, break and continue can help make loops more readable by keeping the number of nested blocks down and reducing the need for complicated looping logic.

For example, consider the following program:

This program uses a boolean variable to control whether the loop continues or not, as well as a nested block that only runs if the user doesn’t exit.

Here’s a version that’s easier to understand, using a break statement:

In this version, by using a single break statement, we’ve avoided the use of a boolean variable (and having to understand both what its intended use is, and where it is set), an else statement, and a nested block.

Minimizing the number of variables used and keeping the number of nested blocks down both improve code understandability more than a break or continue harms it. For that reason, we believe judicious use of break or continue is acceptable.

5.9 -- Random number generation
Index
5.7 -- For statements

78 comments to 5.8 — Break and continue

  • In the "Break vs return" segment, there is a typo:

    It should be

    The output looks weird but the function name printed would be inline with the function name in the code.

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