How does the if-else statement work in C, and what's the dangling else problem?

The condition goes in parentheses and each branch takes one statement (use braces for more); the "dangling else" trap is that else binds to the nearest unmatched if.

// Single statement, no braces needed
if (a > b) c = a-b;
if (b > a) c = b-a;
// else is optional
else c = a-b;

// Braces for multiple statements
if (2*a == b) {
    b = b*2;
    a = a+1;
} else {
    c = 3;
    d = 4;
}

The dangling else problem:

if (a > b)
    if (b > a) c = b-a;
// Which 'if' does this 'else' belong to?
else c = a-b;

Rule: else matches the closest unmatched if.

So the above is actually:

if (a > b) {
    if (b > a) c = b-a;
    // Belongs to inner if!
    else c = a-b;
}

Best practice: Always use braces to avoid ambiguity:

if (a > b) {
    if (b > a) {
        c = b-a;
    }
} else {
    c = a-b;
}

Go deeper:

• Dangling else — Wikipedia — the ambiguity and how grammars and conventions resolve it.