4.10 — Introduction to if statements

Consider a case where you’re going to go to the market, and your roommate tells you, “if they have strawberries on sale, buy some”. This is a conditional statement, meaning that you’ll execute some action (“buy some”) only if the condition (“they have strawberries on sale”) is true.

Such conditions are common in programming, as they allow us to implement conditional behavior into our programs. The simplest kind of conditional statement in C++ is called an if statement. An if statement allows us to execute one (or more) lines of code only if some condition is true.

The simplest if statement takes the following form:

if (condition) true_statement;

For readability, this is more often written as following:

if (condition)
    true_statement;

A condition (also called a conditional expression) is an expression that evaluates to a Boolean value.

If the condition of an if statement evaluates to Boolean value true, then true_statement is executed. If the condition instead evaluates to Boolean value false, then true_statement is skipped.

A sample program using an if statement

Given the following program:

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x {};
    std::cin >> x;

    if (x == 0)
        std::cout << "The value is zero\n";

    return 0;
}

Here’s output from one run of this program:

Enter an integer: 0
The value is zero

Let’s examine how this works in more detail.

First, the user enters an integer. Then the condition x == 0 is evaluated. The equality operator (==) is used to test whether two values are equal. Operator== returns true if the operands are equal, and false if they are not. Since x has value 0, and 0 == 0 is true, this expression evaluates to true.

Because the condition has evaluated to true, the subsequent statement executes, printing The value is zero.

Here’s another run of this program:

Enter an integer: 5

In this case, x == 0 evaluates to false. The subsequent statement is skipped, the program ends, and nothing else is printed.

If-else

Given the above example, what if we wanted to tell the user that the number they entered was non-zero?

We could write something like this:

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x {};
    std::cin >> x;

    if (x == 0)
        std::cout << "The value is zero\n";
    if (x != 0)
        std::cout << "The value is non-zero\n";

    return 0;
}

Or this:

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x {};
    std::cin >> x;

    bool zero { (x == 0) };
    if (zero)
        std::cout << "The value is zero\n";
    if (!zero)
        std::cout << "The value is non-zero\n";

    return 0;
}

Both of these programs are more complex than they need to be. Instead, we can use an alternative form of the if statement called if-else. If-else takes the following form:

if (condition)
    true_statement;
else
    false_statement;

If the condition evaluates to Boolean true, true_statement executes. Otherwise false_statement executes.

Let’s amend our previous program to use an if-else.

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x {};
    std::cin >> x;

    if (x == 0)
        std::cout << "The value is zero\n";
    else
        std::cout << "The value is non-zero\n";

    return 0;
}

Now our program will produce the following output:

Enter an integer: 0
The value is zero

Enter an integer: 5
The value is non-zero

Chaining if statements

Sometimes we want to check if several things are true or false in sequence. We can do so by chaining an if-statement (or if-else) to a prior if-else, like so:

#include <iostream>

int main()
{
    std::cout << "Enter an integer: ";
    int x {};
    std::cin >> x;

    if (x > 0)
        std::cout << "The value is positive\n";
    else if (x < 0)
        std::cout << "The value is negative\n";
    else 
        std::cout << "The value is zero\n";

    return 0;
}

The less than operator (<) is used to test whether one value is less than another. Similarly, the greater than operator (>) is used to test whether one value is greater than another. These operators both return Boolean values.

Here’s output from a few runs of this program:

Enter an integer: 4
The value is positive

Enter an integer: -3
The value is negative

Enter an integer: 0
The value is zero

Note that you can chain if statements as many times as you have conditions you want to evaluate. We’ll see an example in the quiz where this is useful.

Boolean return values and if statements

In the previous lesson (