Swift Operators Precedence and Associativity

Jun 13, 2021

Learn how operator precedence and associativity decide the order Swift evaluates an expression, so you know why 1 + 2 * 3 returns 7 and not 9.

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This tutorial belongs to the Swift series

Think about this expression:

let amount = 1 + 2 * 3

The value of amount could drastically change depending if 1 + 2 is calculated before 2 * 3.

The order of calculation is determined by the operator precedence. From higher precedence to lower precedence, as for the most popular operators we have:

Multiplication (*), division (/), remainder (%)
Add (+), subtract (-)
Comparisons (==, !=, <, >, <=, >=)
Logical AND (&&) and OR (||)
Ternary conditional (?:)
Assignment and compound assignment operators (=, += and so on)

This means that the above expression is resolved first calculating the multiplication, and then the sum:

let amount = 1 + 2 * 3 // = 7

The full table of precedence, more complicated, is available at https://developer.apple.com/documentation/swift/swift_standard_library/operator_declarations.

When inside an expression you have multiple operators with the same precedence, we make use of the operator associativity. Associativity is a property we use to determine which operation has priority when the precedence is the same.

For example, consider this:

let amount = 4 / 2 * 5

Depending if we first execute 4 / 2 or 2 * 5, the result could be 10 or 0,4.

Associativity solves this. Multiplication is left associative, so we must first execute the expression on the left. Parentheses help us figure this out:

let amount = (4 / 2) * 5

Multiplication (*), division (/), remainder (%), add (+), subtract (-), logical AND (&&), logical OR (||) are left associative

Assignment and compound assignment operators (=, += and so on) and the ternary conditional (?:) are right associative

Comparisons (==, !=, <, >, <=, >=) don’t have associativity.

Tagged: Swift · All topics

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