Operator precedence is a collection of rules used in order to evaluate a given mathematical expression. When there are several operators used in a single expression, each part is evaluated in a certain order called as operator precedence. Certain operators have higher priority than others which affects how an expression is evaluated.
The table below lists the precedence of operators in Swift. Higher it appears in the table, the higher its precedence.
Operator Groups | Examples |
---|---|
Bitwise shift precedence | >> &<< &>> >> |
Multiplication precedence | &* % & * / |
Addition precedence | | &+ &- + - ^ |
Range Formation Precedence | ..< ... |
Casting Precedence | is as |
Nil-Coalescing Precedence | ?? |
Comparison Precedence | != > < >= <= === == |
Logical Conjunction Precedence | && |
Logical Disjunction Precedence | || |
Default Precedence | ~> |
Ternary Precedence | ?: |
Function Arrow precedence | ( ) |
Assignment Precedence | |= %= /= &<<= &>>= &= *= >>= <<= ^= += -= |
let x = 4 + 5 * 5
print(x)
When you run the above program, the output will be:
29
In the above example, if you read the expression from left to right, you might expect the output to be 45. But, multiplication operator has a higher precedence than addition operator so the expression 4 + 5 * 5
is evaluated as 4 + (5 * 5)
. Therefore print(x)
outputs 29 in the screen.
Steps | Operator | Expression | Value returned from expression |
---|---|---|---|
1 | * | 5 * 5 | 25 |
2 | + | 4 + 25 | 29 |
3 | = | print(x) | 29 |
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 var y = 15
assigns value 15 in the variable y.
In the next expression y += 10 - 2 * 3
the multiplication operator has a higher precedence than subtraction & compound assignment operator(+=
).
Therefore, the expression y += 10 - 2 * 3
is evaluated as y = y + (10 - (2 * 3))
.
Steps | Operator | Expression | Value returned from expression |
1 | = | var y = 15 | 15 |
2 | * | 2 * 3 | 6 |
3 | - | 10 - 6 | 4 |
4 | += | 15 + 4 | 19 |
Although there are predefined rules defined by operator precedence to evaluate an expression, you might be wondering what happens if there are multiple operators of the same precedence level. The operator associativity defines how operators of the same precedence are grouped together.
In Swift, operators can be either left-associative, right-associative or have no associativity. Operators that are left-associative, group their operands from the left of the statement, right-associative operators from the right and operators with no associativity have no defined behaviour when used in sequence in an expression.
The table below shows the associativity of Swift operators along with the precedence.
Operator Groups | Examples | Associativity |
---|---|---|
Bitwise shift precedence | >> &<< &>> >> | none |
Multiplication precedence | &* % & * / | left |
Addition precedence | | &+ &- + - ^ | left |
Range Formation Precedence | ..< ... | none |
Casting Precedence | is as | none |
Nil-Coalescing Precedence | ?? | right |
Comparison Precedence | != > < >= <= === == | none |
Logical Conjunction Precedence | && | left |
Logical Disjunction Precedence | || | left |
Default Precedence | ~> | none |
Ternary Precedence | ?: | right |
Function Arrow precedence | ( ) | right |
Assignment Precedence | |= %= /= &<<= &>>= &= *= >>= <<= ^= | right |
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 evaluated from left to right because the operators fall into multiplication precedence groups and has left associativity. Therefore, the division operation takes place earlier and you get the result 200. What if you want to execute the multiplication first? You need to wrap the 2 * 10
expression into () braces as:
let x = 40 / (2 * 10)
print(x)
When you run the above program, the output will be:
2
You don't need to memorize the precedence and associative table. Most of the time, the precedence and associativity of operators makes sense in itself. You can always use the table for reference when in doubt. Also, it's better to use parenthesis to make your code easier to understand.
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