Swift Operator Precedence and Associativity

In this article, you will learn the rules for calculating expressions with operators and operands.

Swift operator precedence

Operator precedence is a set of rules used to calculate a given mathematical expression. When multiple operators are used in a single expression, each part is calculated in a specific order called operator precedence. Some operators have a higher precedence than others, which affects the way the expression is calculated.

Operator precedence table

The following table lists theOperatorThe higher it is displayed in the table, the higher its precedence is.

Example1:Swift operator precedence

let x =　4　+　5　*　5
print(x)

When you run the above program, the output will be:

29

In the above example, if the expression is read from left to right, it may be expected that the output will be45. However, the precedence of the multiplication operator is higher than that of the addition operator, so the expression4 + 5 * 5Is calculated to be4 + (5 * 5). Therefore, the output printed on the screen is29.

Example2Has complex assignment operator precedence in Swift

var　y　=　15
y　+=　10　-　2　*　3
print(y)

When you run the above program, the output will be:

19

In the above example, the expression of the variable y var y = 15Is assigned a value15.

In the next expression y + = 10-2 * 3In which, the precedence of the multiplication operator is higher than that of the subtraction compound assignment operator(+ =)

Therefore, the expression y + = 10-2 * 3 Is calculated as y = y + (10-(2 * 3))

Swift operator associativity

Although there are predefined rules defined by operator precedence to evaluate expressions, you may wonder what happens if there are multiple operators with the same precedence. Operator associativity defines how operators with the same precedence are combined.

In Swift, operators can be left-associative, right-associative, or non-associative. When used in an expression in order, left-associative operators group operands from the left side of the statement, right-associative operators and non-associated operators do not have a defined behavior.

Operator associativity table

The following table shows the associativity and precedence of Swift operators.

Example3：Swift Operator Associativity

let x =　　40　　/　2　*　10
print(x)

When you run the above program, the output will be:

200

In the above program, the expression is calculated from left to right because the operators belong to the multiplication precedence group and have left associativity. Therefore, the division operation is executed early, resulting in200. If you want to perform the multiplication operation first, what should you do? You need to put2 * 10Expressions wrapped in parentheses, as shown below:

let x =　　40　　/　(2　*　10)
print(x)

When you run the above program, the output will be:

2

You do not need to remember the precedence and associativity table. In most cases, the precedence and associativity of operators are quite meaningful in themselves. If you have any doubts, you can always use this table as a reference. It is also best to use parentheses to make the code easier to understand.