Programming Tutorials

for loop in C++

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

In our while loop, we used the variable val to control how many times we iterated through the loop. On each pass through the while, the value of val was tested and then in the body the value of val was incremented.

The use of a variable like val to control a loop happens so often that the language defines a second control structure, called a for statement, that abbreviates the code that manages the loop variable. We could rewrite the program to sum the numbers from 1 through 10 using a for loop as follows:

#include <iostream>
int main()
{
int sum = 0;
// sum values from 1 up to 10 inclusive
for (int val = 1; val <= 10; ++val)
sum += val; // equivalent to sum = sum + val
std::cout << "Sum of 1 to 10 inclusive is "
<< sum << std::endl;
return 0;
}

Prior to the for loop, we define sum, which we set to zero. The variable val is used only inside the iteration and is defined as part of the for statement itself. The for statement

for (int val = 1; val <= 10; ++val)
sum += val; // equivalent to sum = sum + val

has two parts: the for header and the for body. The header controls how often the body is executed. The header itself consists of three parts: an init-statement, a condition, and an expression. In this case, the init-statement

int val = 1;

defines an int object named val and gives it an initial value of one. The initstatement is performed only once, on entry to the for. The condition

val <= 10

which compares the current value in val to 10, is tested each time through the loop. As long as val is less than or equal to 10, we execute the for body. Only after executing the body is the expression executed. In this for, the expression uses the prefix increment operator, which as we know adds one to the value of val. After executing the expression, the for retests the condition. If the new value of val is still less than or equal to 10, then the for loop body is executed and val is incremented again. Execution continues until the condition fails.

In this loop, the for body performs the summation

sum += val; // equivalent to sum = sum + val

The body uses the compound assignment operator to add the current value of val to sum, storing the result back into sum.

To recap, the overall execution flow of this for is:

Create val and initialize it to 1.

Test whether val is less than or equal to 10.

If val is less than or equal to 10, execute the for body, which adds val to sum. If val is not less than or equal to 10, then break out of the loop and continue execution with the first statement following the for body.

Increment val.

Repeat the test in step 2, continuing with the remaining steps as long as the condition is true.