Suppose you want the user to enter three integers for sides of a right triangle. If they do not make a right triangle, say so and make the user try again.
One way to look at the while statement rubric is:
set data for conditions
while (condition) {
accomplish something
set data for condition
}
As we have pointed out before this involves setting data in two places. With the triangle problem, three pieces fo data need to be entered, while the condition to test is fairly simple (and in any case the condition could be calculated in a function).
A do-while loop will help here. It tests the condition at the end of the loop, so there is no need to gather data before the loop:
int a, b, c;
do {
Console.WriteLine("Think of integer sides for a right triangle.");
a = IntInput("Enter integer leg: ");
b = IntInput("Enter another integer leg: ");
c = IntInput("Enter integer hypotenuse: ");
if (a*a + b*b != c*c) {
Console.WriteLine("Not a right triangle: Try again!");
} while (a*a + b*b != c*c);
The general form of a do-while statement is
do {statement(s)} while ( continuationCondition );
Here the block of statement(s) is always executed, but it continues to be executed in a loop only so long as the condition tested after the loop body is true.
Note
A do-while loop is the one place where you do want a semicolon right after a condition, unlike the places mentioned in Dangerous Semicolon. At least if you omit it here you are likely to get a compiler error rather than a difficult logical bug.
A do-while loop, like the example above, that accomplishes exactly the same thing as the while loop rubric above, can have the form:
do {
set data for condition
if (condition) {
accomplish something
}
} while (condition);
It only sets the data to be tested once. (The tradeoff is that the condition is tested twice.)