Programming Tutorials

while loop in C++

By: Stanley B. in C++ Tutorials on 2011-02-19  

A while statement provides for iterative execution. We could use a while to write a program to sum the numbers from 1 through 10 inclusive as follows:

#include <iostream>
int main()
{
int sum = 0, val = 1;
// keep executing the while until val is greater than 10
while (val <= 10) {
sum += val; // assigns sum + val to sum
++val; // add 1 to val
}
std::cout << "Sum of 1 to 10 inclusive is "
<< sum << std::endl;
return 0;
}

This program when compiled and executed will print:

Sum of 1 to 10 inclusive is 55

As before, we begin by including the iostream header and define a main function. Inside main we define two int variables: sum, which will hold our summation, and val, which will represent each of the values from 1 through 10. We give sum an initial value of zero and start val off with the value one.

The important part is the while statement. A while has the form

while (condition) while_body_statement;

A while executes by (repeatedly) testing the condition and executing the associated while_body_statement until the condition is false.

A condition is an expression that is evaluated so that its result can be tested. If the resulting value is nonzero, then the condition is true; if the value is zero then the condition is false.

If the condition is true (the expression evaluates to a value other than zero) then while_body_statement is executed. After executing while_body_statement, the condition is tested again. If condition remains true, then the while_body_statement is again executed. The while continues, alternatively testing the condition and executing while_body_statement until the condition is false.

In this program, the while statement is:

// keep executing the while until val is greater than 10
while (val <= 10) {
sum += val; // assigns sum + val to sum
++val; // add 1 to val
}

The condition in the while uses the less-than-or-equal operator (the <= operator) to compare the current value of val and 10. As long as val is less than or equal to 10, we execute the body of the while. In this case, the body of the while is a block containing two statements:

{
sum += val; // assigns sum + val to sum
++val; // add 1 to val
}

A block is a sequence of statements enclosed by curly braces. In C++, a block may be used wherever a statement is expected. The first statement in the block uses the compound assignment operator, (the += operator). This operator adds its right-hand operand to its left-hand operand. It has the same effect as writing an addition and an assignment:

sum = sum + val; // assign sum + val to sum

Thus, the first statement adds the value of val to the current value of sum and stores the result back into sum.

The next statement

++val; // add 1 to val

uses the prefix increment operator (the ++ operator). The increment operator adds one to its operand. Writing ++val is the same as writing val = val + 1.

After executing the while body we again execute the condition in the while. If the (now incremented) value of val is still less than or equal to 10, then the body of the while is executed again. The loop continues, testing the condition and executing the body, until val is no longer less than or equal to 10.

Once val is greater than 10, we fall out of the while loop and execute the statement following the while. In this case, that statement prints our output, followed by the return, which completes our main program.