Ch3.3: if-else

The if-else Statement

An if chooses whether to run a statement. An if-else chooses between two statements:

if (cond) {
    ;
} else {
    ;
}

As before, ; is just an empty statement.

else if

For more than two cases, you can chain conditions using else if:

if (x < 0) {
    print("negative\n");
} else if (x == 0) {
    print("zero\n");
} else {
    print("positive\n");
}

Only the first matching branch runs.

Init-statement

An if or else if may declare a variable before the condition. C++ allows two different ways to write this. The variable exists only inside the entire if / else if / else chain.

Form 1

::std::uint_least32_t a{3};
::std::uint_least32_t b{9};

if (::std::int_least32_t sum = a + b) {   // condition is: sum != 0
    println("sum != 0, a=", a, ", b=", b, ", sum=", sum);
} else {
    println("sum == 0, a=", a, ", b=", b, ", sum=", sum);
}

In this form, the condition is simply whether the variable is nonzero.

Form 2

if (auto sum = a + b; sum > 10) {
    println("sum > 10, a=", a, ", b=", b, ", sum=", sum);
} else if (sum == 10) {
    println("sum == 10, a=", a, ", b=", b, ", sum=", sum);
} else {
    println("sum < 10, a=", a, ", b=", b, ", sum=", sum);
}

In this form, the variable is declared first, and then a separate condition is checked.

In both forms, the variable (sum) is destroyed after the whole chain ends.

Always Use { }

Beginners should always use braces for if, else if, and else. Without braces, it is extremely easy to attach the else to the wrong if.

❌ Wrong (ambiguous)

if (a)
    if (b)
        ;
    else
        ;   // attaches to the inner if, not the outer one

This is the classic “dangling else” problem.

✔️ Correct

if (a) {
    if (b) {
        ;
    } else {
        ;
    }
}

Braces make the structure clear and prevent mistakes.

Another Confusing Example

Without braces, this code looks like the else belongs to the first if:

❌ Wrong

if (x > 0)
    if (x == 1)
        print("one\n");
    else
        print("not one\n");  // actually belongs to the inner if

But the else always pairs with the nearest unmatched if.

✔️ Correct

if (x > 0) {
    if (x == 1) {
        print("one\n");
    } else {
        print("not one\n");
    }
}

Always use braces. It removes all ambiguity.

Key takeaways

• if-else chooses: if picks one path; if-else picks between two.
• else if chains: use else if for multiple ordered conditions.
• Init-statement: both forms of if (init) ... and if (init; cond) declare a variable valid for the entire chain.
• Single-statement rule: without braces, each branch controls only one statement.
• Empty statement: if (cond) ; or else ; does nothing and is almost always a bug.
• Dangling else: else binds to the nearest if; braces prevent confusion.
• Clarity: consistent braces make code easier to read and safer to modify.