Programming Tutorials

if in C++

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

A logical extension of summing the values between 1 and 10 is to sum the values between two numbers our user supplies. We might use the numbers directly in our for loop, using the first input as the lower bound for the range and the second as the upper bound. However, if the user gives us the higher number first, that strategy would fail: Our program would exit the for loop immediately. Instead, we should adjust the range so that the larger number is the upper bound and the smaller is the lower. To do so, we need a way to see which number is larger.

Like most languages, C++ provides an if statement that supports conditional execution. We can use an if to write our revised sum program:

#include <iostream>
int main()
{
    std::cout << "Enter two numbers:" << std::endl;
    int v1, v2;
    std::cin >> v1 >> v2; // read input
    // use smaller number as lower bound for summation
    // and larger number as upper bound
    int lower, upper;
    if (v1 <= v2)
    {
        lower = v1;
        upper = v2;
    }
    else
    {
        lower = v2;
        upper = v1;
    }
    int sum = 0;
    // sum values from lower up to and including upper
    for (int val = lower; val <= upper; ++val)
        sum += val; // sum = sum + val
    std::cout << "Sum of " << lower
              << " to " << upper
              << " inclusive is "
              << sum << std::endl;
    return 0;
}

If we compile and execute this program and give it as input the numbers 7 and 3, then the output of our program will be

Sum of 3 to 7 inclusive is 25

Most of the code in this program should already be familiar from our earlier examples. The program starts by writing a prompt to the user and defines four int variables. It then reads from the standard input into v1 and v2. The only new code is the if statement

// use smaller number as lower bound for summation
// and larger number as upper bound
int lower, upper;
if (v1 <= v2) {
	lower = v1;
	upper = v2;
} else {
	lower = v2;
	upper = v1;
}

The effect of this code is to set upper and lower appropriately. The if condition tests whether v1 is less than or equal to v2. If so, we perform the block that immediately follows the condition. This block contains two statements, each of which does an assignment. The first statement assigns v1 to lower and the second assigns v2 to upper.

If the condition is false that is, if v1 is larger than v2then we execute the statement following the else. Again, this statement is a block consisting of two assignments. We assign v2 to lower and v1 to upper.