if Statements

So far, we have not been making decisions based on the values our variables hold. Let’s see how to do this. The if statement allows us to test on values and take actions.

The basic form is:

if (condition1) {
   // do things if condition1 is true

} else if (condition2) {
   // do things if condition1 is false but condition 2 is true

} else {
   // do things only if both condition1 and condition2 are false

}

Here condition is something that will evaluate to true or false, and this is a place where we will use the Relational operators we’ve seen previously.

Again we use { } to group the statements into blocks for each part of the conditional.

Caution

There is a form of an if-statement that does not use brackets if there is only a single statement to execute:

if (condition)
    statement;

This is potentially dangerous—if someone later edits the code and decides that they want to add another statement to that condition, they might do:

if (condition)
    statement;
    another_statement;

But since there are no braces, only statement is conditionally-executed. another_statement is not part of the if-test and will always be executed.

For this reason, it is always best to use brackets.

Simple example

Here’s an example that reads an integer from the user and then outputs whether it is positive, negative, or zero:

Listing 46 if-example.cpp

#include <iostream>

int main() {

    std::cout << "Enter a number: ";
    int n{};
    std::cin >> n;

    if (n < 0) {
        std::cout << "you entered a negative number" << std::endl;
    } else if (n > 0) {
        std::cout << "you entered a positive number" << std::endl;
    } else {
        std::cout << "you entered zero" << std::endl;
    }

}

Note

We read input from the user using std::cin as discussed in Reading Input.

Scope

Consider the following example:

Listing 47 if-scope.cpp

#include <iostream>

int main() {

    int n{5};

    if (n % 2 != 0) {
        int rem = n % 2;
    }

    // note that rem is not available here

}

Since we declare the variable rem inside of an if block (i.e., inside the { }), it is not available to us outside of the block.

If we want to set a variable inside of an if condition, then we need to declare the variable outside. This is an issue of scope. Most variables in C++ have lifetimes limited to the block ({ }) in which they are defined.

Ternary operator

There is also a simple ternary operator in C++ of the form:

condition ? true-result : false result

Where true-result is the value used if condition is true and false-result otherwise.

For instance:

int i{10};

double x = (i > 5) ? 1.0 : 0.0;

This tends not to be very readable, but there are some rare instances where it is needed.

switch statement

C++ also has a switch statement, which we will not consider. We can accomplish any logic needed with the if - else constructs.