Sams Teach Yourself C in 21 Days

Basic Program Control 139

6

The structure of the do...whileloop is as follows:

do

statement

while (condition);

conditionis any C expression, and statementis a single or compound C statement.

When program execution reaches a do...whilestatement, the following events occur:

1. The statements in statementare executed.
   2.conditionis evaluated. If it’s true, execution returns to step 1. If it’s false, the loop
   terminates.
   The operation of a do...whileloop is shown in Figure 6.4.

FIGURE6.4

The operation of a

do...whileloop.

Execute

statements

Start

Evaluate

condition

Done

do statements;

while (condition);

TRUE

FALSE

The statements associated with a do...whileloop are always executed at least once.

This is because the test condition is evaluated at the end, instead of the beginning, of the

loop. In contrast,forloops and whileloops evaluate the test condition at the start of the

loop, so the associated statements are not executed at all if the test condition is initially

false.

Thedo...whileloop is used less frequently than whileandforloops. It is most appro-

priate when the statement(s) associated with the loop must be executed at least once. You

could, of course, accomplish the same thing with a whileloop by making sure that the

test condition is true when execution first reaches the loop. A do...whileloop probably

would be more straightforward, however.

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