if, else-if and else Statements

Making decisions

Programs constantly need to choose what to do based on conditions — is the user an adult, did they pass, is the number positive? C++ handles this with the if family of statements. A condition is any expression that is true or false.

The if statement

The simplest form runs a block only when the condition is true:

int age = 20;
if (age >= 18) {
    cout << "You are an adult.\n";
}

if-else

Add an else to handle the false case, so exactly one block always runs:

int n = 7;
if (n % 2 == 0) {
    cout << "Even\n";
} else {
    cout << "Odd\n";
}

The else-if ladder

To pick among several ranges, chain conditions with else if. C++ checks them top to bottom and stops at the first true one:

int marks = 82;
if (marks >= 90) {
    cout << "Grade A\n";
} else if (marks >= 75) {
    cout << "Grade B\n";
} else if (marks >= 50) {
    cout << "Grade C\n";
} else {
    cout << "Fail\n";
}

Nested if

You can put an if inside another to check a second condition only when the first holds:

if (loggedIn) {
    if (isAdmin) {
        cout << "Welcome, admin\n";
    } else {
        cout << "Welcome, user\n";
    }
}

Often a nested check can be written more cleanly with &&: if (loggedIn && isAdmin).

A quick note on the ternary

For a simple either/or that produces a value, the ternary operator is shorter:

string result = (score >= 50) ? "Pass" : "Fail";

See the operators lesson for more on the ternary.

Common mistakes

• Using = (assign) instead of == (compare) in the condition.
• Forgetting braces, so only the first line belongs to the if.
• Putting a stray semicolon after if (x) — if (x); does nothing.
• Ordering an else-if ladder wrongly so a broad condition hides narrower ones.

Summary

• if runs a block when a condition is true.
• if-else chooses between two blocks.
• An else-if ladder picks among several ranges, top to bottom.
• Nested if checks a second condition inside the first.
• Always use braces, and use == (not =) to compare.