LESSON 77 OF 415 · CONDITIONAL STATEMENTS

Reducing If-Else to a Boolean Expression in Swift

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We will learn a simple trick to improve our code quality: Reducing If-Else to Boolean Expressions.

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Reducing If Else To Boolean Expression in Swift, explained

Problem overview

The method legalAge takes an age and returns a Bool: true when the age is 18 or more, and false otherwise. The exercise asks you to write it in a single line, with no if or else.

Approach: the comparison already is the answer

The first version most people write looks like this:

func legalAge(_ age: Int) -> Bool {
    if age >= 18 {
        return true
    } else {
        return false
    }
}

It works, but it says the same thing twice. The expression age >= 18 already has the value true when the age is legal and false when it is not. The if reads that value, then returns a hand-written copy of it. Returning the expression itself removes the middle step.

The rule is general. Whenever an if returns true in one branch and false in the other, return the condition. If the branches are swapped (false for the condition, true otherwise), return the opposite: !condition, or the opposite comparison.

Step by step

Take the condition the if would have tested: age >= 18.
Put return in front of it. The comparison produces a Bool, which matches the declared return type.
Delete the rest. There are no branches, no braces and no literals true and false left to write.

The solution in Swift

class Solution {
    func legalAge(_ age: Int) -> Bool {
        return age >= 18
    }
}

Swift details worth knowing:

A comparison is a value. age >= 18 is an expression of type Bool, just like 3 is an expression of type Int. It can be returned, stored in a constant, as in let adult = age >= 18, or passed to another function.
Return type. The signature says -> Bool, so anything returned must be a Bool. The comparison qualifies without conversion.
Calling it. After let s = Solution(), s.legalAge(17) returns false, and both s.legalAge(18) and s.legalAge(40) return true.
Single-expression functions. Swift also lets a function whose body is one expression leave out the word return. The lesson keeps it, which is valid and explicit.

Reading the shortened code

A beginner sometimes worries that the short version hides the logic. It doesn't, if you read it aloud: "return whether the age is at least 18". The method answers a yes-or-no question, and the return value is that answer. The long version spells out the same sentence in two extra branches.

The same idea applies to combined conditions. A method that decides whether a number is a single digit could return n >= 0 && n <= 9 directly. Any expression built from comparisons, &&, || and ! is a Bool, so any of them can be returned or stored without an if.

Edge cases and common mistakes

The boundary. The task says 18 or more, so legalAge(18) is true. Writing age > 18 would reject exactly 18.
Redundant comparison with a Boolean. Code such as return (age >= 18) == true works but adds noise. A Boolean value doesn't need to be compared with true.
Keeping the if "to be safe". The long version and the short version behave identically for every input, so there is nothing to gain by keeping it.
Negative ages. legalAge(-5) returns false, since -5 is less than 18. The method does no validation.
When not to reduce. If a branch does more than return a Boolean, such as printing or computing a value, the if is needed.

Testing

legalAge(17) returns false.
legalAge(18) returns true.
legalAge(40) returns true.
legalAge(0) returns false.

Key takeaways

A comparison is an expression with a Bool value, so you can return it directly.
An if that returns true in one branch and false in the other can always become a single return.
Shorter code leaves fewer places for a mistake.

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