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Next: Example Program: Sum of
Up: Introduction to C++ Programming
Previous: Exercises
The following example is a version using a do-while statement of the
problem considered at the beginning of the Lesson on the while
statement. The program has to accept positive numbers entered by a
user and to accumulate their sum, terminating when a negative value is
entered.
sum = 0.0;
cin >> x;
do {
sum += x;
cin >> x;
}
while (x > 0.0);
Again the accumulator variable sum is initialised to zero and the
first value is entered from the user before the do-while statement is
entered for the first time. The statement between the do and the
while is then executed before the condition
x > 0.0 is tested. This of course is different from the while
statement in which the condition is tested before the
statement is executed. This means that the compound statement
between the do and the while would be executed at least
once, even if the user entered a negative value initially. This value
would then be added to sum and the computer would await entry of
another value from the user! Thus do-while statements are not
used where there is a possibility that the statement inside the loop
should not be executed.
The general form of the do-while statement is:
do
statement
while (condition); //note the brackets!
In the do-while statement the body of the loop is executed
before the first test of the condition. The loop is terminated
when the condition becomes false. As noted above the
loop statement is always executed at least once, unlike the
while statement where the body of the loop is not executed at
all if the condition is initially false. The statement may
be a single statement or a compound statement. The effect of a
do-while statement can always be simulated by a while
statement so it is not strictly necessary. However in some situations
it can be more convenient than a while statement.