The do-while loop is similar to a while loop, only the conditional statement is only evaluated after the body has run at once. There are very few situations where a do-while loop is more natural than a while loop, however, one is where you prompt the user for input and quit when a certain value is reached.

Program 1 asks the user to enter a number and the program prints n! allowing the user to also enter 0 to quit.

Program 1. Printing the smallest factor.

#include <iostream>

using namespace std;

int main() {
	do {
		cout << "Enter a positive integer (0 to quit): " << endl;
		cin >> n;

		result = 1;

		for ( int i = 1; i <= n; ++i ) {
			result *= i;
		}

		cout << n << "! = " << result << endl;
	} while ( n != 0 );

	// end of loop

	cout << "You entered a negative number.  Bye." << endl;

	return 0;
}

Very seldom does a situation require a do-while loop; often the loop can be written as naturally using a while loop. If the do-while structure is necessary, you must document this.

Example

The function in Program 2 uses Newton's method to find the square root of the argument. It starts by using the approximation x to the root and calculates the next approximation, halting if the two are equal.

Program 2. Finding the square root of a number.

double sqrt( double x ) {
	double current = 0.0;
	double next = x;

	// Perform at least one iteration of Newton's method before
	// comparing if the current value equals the next value.
	//
	// Note this is a simplification of Newton's method which
	// normally would look like
	//   next = current - (current*current - x)/(2.0*current);

	do {
		current = next;
		next = 0.5*(current + x/current);
	} while ( next != current );

	return current;
}

To write this as a while loop, you would have to either seed current with a value like ∞ or do something equally unnatural.

Questions

1. Write a do-while loop which continues to prompt the user for an integer between 1 and 10 and repeats the question if the user enters an integer less than 1 or greater than 10.

2. Write a do-while loop which prompts the user for a positive integer and repeats the question if the user enters a negative integer.

3. Write a do-while loop which continues to prompt the user to enter a character 'y' or 'n'. If the user enters any other character, prompt the user again.

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