Basic Operators
An operator is a special symbol or phrase that you use to check, change, or combine values. For example, the addition operator (+
) adds two numbers, as in let i = 1 + 2
, and the logical AND operator (&&
) combines two Boolean values, as in if enteredDoorCode && passedRetinaScan
.
Swift supports most standard C operators and improves several capabilities to eliminate common coding errors. The assignment operator (=
) doesn’t return a value, to prevent it from being mistakenly used when the equal to operator (==
) is intended. Arithmetic operators (+, , *, /, %
and so forth) detect and disallow value overflow, to avoid unexpected results when working with numbers that become larger or smaller than the allowed value range of the type that stores them. You can opt in to value overflow behavior by using Swift’s overflow operators, as described in Overflow Operators.
Swift also provides range operators that aren’t found in C, such as a..<b
and a...b
, as a shortcut for expressing a range of values.
This chapter describes the common operators in Swift. Advanced Operators covers Swift’s advanced operators, and describes how to define your own custom operators and implement the standard operators for your own custom types.
Terminology
Operators are unary, binary, or ternary:

Unary operators operate on a single target (such as
a
). Unary prefix operators appear immediately before their target (such as!b
), and unary postfix operators appear immediately after their target (such asc!
). 
Binary operators operate on two targets (such as
2 + 3
) and are infix because they appear in between their two targets. 
Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (
a ? b : c
).
The values that operators affect are operands. In the expression 1 + 2
, the +
symbol is a binary operator and its two operands are the values 1
and 2
.
Assignment Operator
The assignment operator (a = b
) initializes or updates the value of a
with the value of b
:
let b = 10
var a = 5
a = b
// a is now equal to 10
If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple constants or variables at once:
let (x, y) = (1, 2)
// x is equal to 1, and y is equal to 2
Unlike the assignment operator in C and ObjectiveC, the assignment operator in Swift does not itself return a value. The following statement is not valid:
if x = y {
// This is not valid, because x = y does not return a value.
}
This feature prevents the assignment operator (=
) from being used by accident when the equal to operator (==
) is actually intended. By making if x = y
invalid, Swift helps you to avoid these kinds of errors in your code.
Arithmetic Operators
Swift supports the four standard arithmetic operators for all number types:

Addition (
+
) 
Subtraction (

) 
Multiplication (
*
) 
Division (
/
)
1 + 2 // equals 3
5  3 // equals 2
2 * 3 // equals 6
10.0 / 2.5 // equals 4.0
Unlike the arithmetic operators in C and ObjectiveC, the Swift arithmetic operators don’t allow values to overflow by default. You can opt in to value overflow behavior by using Swift’s overflow operators (such as a &+ b
). See Overflow Operators.
The addition operator is also supported for String
concatenation:
"hello, " + "world" // equals "hello, world"
Remainder Operator
The remainder operator (a % b
) works out how many multiples of b
will fit inside a
and returns the value that is left over (known as the remainder).
The remainder operator (
%
) is also known as a modulo operator in other languages. However, its behavior in Swift for negative numbers means that, strictly speaking, it’s a remainder rather than a modulo operation.
Here’s how the remainder operator works. To calculate 9 % 4
, you first work out how many 4s
will fit inside 9
:
You can fit two 4s
inside 9
, and the remainder is 1
(shown in orange).
In Swift, this would be written as:
9 % 4 // equals 1
To determine the answer for a % b
, the %
operator calculates the following equation and returns remainder as its output:
a = (b x some multiplier) + remainder
where some multiplier
is the largest number of multiples of b
that will fit inside a
.
Inserting 9
and 4
into this equation yields:
9 = (4 x 2) + 1
The same method is applied when calculating the remainder for a negative value of a
:
9 % 4 // equals 1
Inserting 9
and 4
into the equation yields:
9 = (4 x 2) + 1
giving a remainder value of 1
.
The sign of b
is ignored for negative values of b
. This means that a % b
and a % b
always give the same answer.
Unary Minus Operator
The sign of a numeric value can be toggled using a prefixed 
, known as the unary minus operator:
let three = 3
let minusThree = three // minusThree equals 3
let plusThree = minusThree // plusThree equals 3, or "minus minus three"
The unary minus operator (
) is prepended directly before the value it operates on, without any white space.
Unary Plus Operator
The unary plus operator (+
) simply returns the value it operates on, without any change:
let minusSix = 6
let alsoMinusSix = +minusSix // alsoMinusSix equals 6
Although the unary plus operator doesn’t actually do anything, you can use it to provide symmetry in your code for positive numbers when also using the unary minus operator for negative numbers.
Compound Assignment Operators
Like C, Swift provides compound assignment operators that combine assignment (=
) with another operation. One example is the addition assignment operator (+=
):
var a = 1
a += 2
// a is now equal to 3
The expression a += 2
is shorthand for a = a + 2
. Effectively, the addition and the assignment are combined into one operator that performs both tasks at the same time.
The compound assignment operators don’t return a value. For example, you can’t write let b = a += 2.
For information about the operators provided by the Swift standard library, see Operator Declarations.
Comparison Operators
Swift supports all standard C comparison operators:

Equal to (
a == b
) 
Not equal to (
a != b
) 
Greater than (
a > b
) 
Less than (
a < b
) 
Greater than or equal to (
a >= b
) 
Less than or equal to (
a <= b
)
Swift also provides two identity operators (
===
and!==
), which you use to test whether two object references both refer to the same object instance. For more information, see Structures and Classes.
Each of the comparison operators returns a Bool
value to indicate whether or not the statement is true:
1 == 1 // true because 1 is equal to 1
2 != 1 // true because 2 is not equal to 1
2 > 1 // true because 2 is greater than 1
1 < 2 // true because 1 is less than 2
1 >= 1 // true because 1 is greater than or equal to 1
2 <= 1 // false because 2 is not less than or equal to 1
Comparison operators are often used in conditional statements, such as the if
statement:
let name = "world"
if name == "world" {
print("hello, world")
} else {
print("I'm sorry \(name), but I don't recognize you")
}
// Prints "hello, world", because name is indeed equal to "world".
For more about the if statement, see Control Flow.
You can compare two tuples if they have the same type and the same number of values. Tuples are compared from left to right, one value at a time, until the comparison finds two values that aren’t equal. Those two values are compared, and the result of that comparison determines the overall result of the tuple comparison. If all the elements are equal, then the tuples themselves are equal. For example:
(1, "zebra") < (2, "apple") // true because 1 is less than 2; "zebra" and "apple" are not compared
(3, "apple") < (3, "bird") // true because 3 is equal to 3, and "apple" is less than "bird"
(4, "dog") == (4, "dog") // true because 4 is equal to 4, and "dog" is equal to "dog"
In the example above, you can see the lefttoright comparison behavior on the first line. Because 1
is less than 2
, (1, "zebra")
is considered less than (2, "apple")
, regardless of any other values in the tuples. It doesn’t matter that "zebra
" isn’t less than "apple
", because the comparison is already determined by the tuples’ first elements. However, when the tuples’ first elements are the same, their second elements are compared — this is what happens on the second and third line.
Tuples can be compared with a given operator only if the operator can be applied to each value in the respective tuples. For example, as demonstrated in the code below, you can compare two tuples of type (String, Int)
because both String
and Int
values can be compared using the <
operator. In contrast, two tuples of type (String, Bool)
can’t be compared with the <
operator because the <
operator can’t be applied to Bool
values.
("blue", 1) < ("purple", 1) // OK, evaluates to true
("blue", false) < ("purple", true) // Error because < can't compare Boolean values
The Swift standard library includes tuple comparison operators for tuples with fewer than seven elements. To compare tuples with seven or more elements, you must implement the comparison operators yourself.
Ternary Conditional Operator
The ternary conditional operator is a special operator with three parts, which takes the form question ? answer1 : answer2
. It’s a shortcut for evaluating one of two expressions based on whether question is true
or false
. If question is true, it evaluates answer1
and returns its value; otherwise, it evaluates answer2
and returns its value.
The ternary conditional operator is shorthand for the code below:
if question {
answer1
} else {
answer2
}
Here’s an example, which calculates the height for a table row. The row height should be 50 points taller than the content height if the row has a header, and 20 points taller if the row doesn’t have a header:
let contentHeight = 40
let hasHeader = true
let rowHeight = contentHeight + (hasHeader ? 50 : 20)
// rowHeight is equal to 90
The example above is shorthand for the code below:
let contentHeight = 40
let hasHeader = true
let rowHeight: Int
if hasHeader {
rowHeight = contentHeight + 50
} else {
rowHeight = contentHeight + 20
}
// rowHeight is equal to 90
The first example’s use of the ternary conditional operator means that rowHeight
can be set to the correct value on a single line of code, which is more concise than the code used in the second example.
The ternary conditional operator provides an efficient shorthand for deciding which of two expressions to consider. Use the ternary conditional operator with care, however. Its conciseness can lead to hardtoread code if overused. Avoid combining multiple instances of the ternary conditional operator into one compound statement.
NilCoalescing Operator
The nilcoalescing operator (a ?? b
) unwraps an optional a
if it contains a value, or returns a default value b
if a is nil
. The expression a is always of an optional type. The expression b
must match the type that is stored inside a
.
The nilcoalescing operator is shorthand for the code below:
a != nil ? a! : b
The code above uses the ternary conditional operator and forced unwrapping (a!
) to access the value wrapped inside a when a
is not nil
, and to return b
otherwise. The nil
coalescing operator provides a more elegant way to encapsulate this conditional checking and unwrapping in a concise and readable form.
If the value of a is non
nil
, the value of b is not evaluated. This is known as shortcircuit evaluation.
The example below uses the nil
coalescing operator to choose between a default color name and an optional userdefined color name:
let defaultColorName = "red"
var userDefinedColorName: String? // defaults to nil
var colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is nil, so colorNameToUse is set to the default of "red"
The userDefinedColorName
variable is defined as an optional String
, with a default value of nil
. Because userDefinedColorName
is of an optional type, you can use the nil
coalescing operator to consider its value. In the example above, the operator is used to determine an initial value for a String
variable called colorNameToUse
. Because userDefinedColorName
is nil
, the expression userDefinedColorName ?? defaultColorName
returns the value of defaultColorName
, or "red"
.
If you assign a nonnil value to userDefinedColorName
and perform the nilcoalescing operator check again, the value wrapped inside userDefinedColorName
is used instead of the default:
userDefinedColorName = "green"
colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is not nil, so colorNameToUse is set to "green"
Range Operators
Swift includes several range operators, which are shortcuts for expressing a range of values.
Closed Range Operator
The closed range operator (a...b
) defines a range that runs from a
to b
, and includes the values a
and b
. The value of a
must not be greater than b
.
The closed range operator is useful when iterating over a range in which you want all of the values to be used, such as with a forin
loop:
for index in 1...5 {
print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25
For more about forin
loops, see Control Flow.
HalfOpen Range Operator
The halfopen range operator (a..<b
) defines a range that runs from a
to b
, but doesn’t include b
. It’s said to be halfopen because it contains its first value, but not its final value. As with the closed range operator, the value of a
must not be greater than b
. If the value of a
is equal to b
, then the resulting range will be empty.
Halfopen ranges are particularly useful when you work with zerobased lists such as arrays, where it’s useful to count up to (but not including) the length of the list:
let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
print("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack
Note that the array contains four items, but 0..<count
only counts as far as 3
(the index of the last item in the array), because it’s a halfopen range. For more about arrays, see Arrays.
OneSided Ranges
The closed range operator has an alternative form for ranges that continue as far as possible in one direction — for example, a range that includes all the elements of an array from index 2
to the end of the array. In these cases, you can omit the value from one side of the range operator. This kind of range is called a onesided range because the operator has a value on only one side. For example:
for name in names[2...] {
print(name)
}
// Brian
// Jack
for name in names[...2] {
print(name)
}
// Anna
// Alex
// Brian
The halfopen range operator also has a onesided form that’s written with only its final value. Just like when you include a value on both sides, the final value isn’t part of the range. For example:
for name in names[..<2] {
print(name)
}
// Anna
// Alex
Onesided ranges can be used in other contexts, not just in subscripts. You can’t iterate over a onesided range that omits a first value, because it isn’t clear where iteration should begin. You can iterate over a onesided range that omits its final value; however, because the range continues indefinitely, make sure you add an explicit end condition for the loop. You can also check whether a onesided range contains a particular value, as shown in the code below.
let range = ...5
range.contains(7) // false
range.contains(4) // true
range.contains(1) // true
Logical Operators
Logical operators modify or combine the Boolean logic values true
and false
. Swift supports the three standard logical operators found in Cbased languages:

Logical NOT (
!a)

Logical AND (
a && b
) 
Logical OR (
a  b
)
Logical NOT Operator
The logical NOT operator (!a
) inverts a Boolean value so that true
becomes false
, and false
becomes true
.
The logical NOT operator is a prefix operator, and appears immediately before the value it operates on, without any white space. It can be read as “not a”, as seen in the following example:
let allowedEntry = false
if !allowedEntry {
print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"
The phrase if !allowedEntry
can be read as “if not allowed entry.” The subsequent line is only executed if “not allowed entry” is true
; that is, if allowedEntry is false
.
As in this example, careful choice of Boolean constant and variable names can help to keep code readable and concise, while avoiding double negatives or confusing logic statements.
Logical AND Operator
The logical AND operator (a && b
) creates logical expressions where both values must be true for the overall expression to also be true.
If either value is false
, the overall expression will also be false
. In fact, if the first value is false
, the second value won’t even be evaluated, because it can’t possibly make the overall expression equate to true
. This is known as shortcircuit evaluation.
This example considers two Bool
values and only allows access if both values are true:
let enteredDoorCode = true
let passedRetinaScan = false
if enteredDoorCode && passedRetinaScan {
print("Welcome!")
} else {
print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"
Logical OR Operator
The logical OR operator (a  b
) is an infix operator made from two adjacent pipe characters. You use it to create logical expressions in which only one of the two values has to be true
for the overall expression to be true
.
Like the Logical AND operator above, the Logical OR operator uses shortcircuit evaluation to consider its expressions. If the left side of a Logical OR expression is true
, the right side is not evaluated, because it can’t change the outcome of the overall expression.
In the example below, the first Bool
value (hasDoorKey
) is false
, but the second value (knowsOverridePassword
) is true
. Because one value is true
, the overall expression also evaluates to true
, and access is allowed:
let hasDoorKey = false
let knowsOverridePassword = true
if hasDoorKey  knowsOverridePassword {
print("Welcome!")
} else {
print("ACCESS DENIED")
}
// Prints "Welcome!"
Combining Logical Operators
You can combine multiple logical operators to create longer compound expressions:
if enteredDoorCode && passedRetinaScan  hasDoorKey  knowsOverridePassword {
print("Welcome!")
} else {
print("ACCESS DENIED")
}
// Prints "Welcome!"
This example uses multiple &&
and 
operators to create a longer compound expression. However, the &&
and 
operators still operate on only two values, so this is actually three smaller expressions chained together. The example can be read as:
If we’ve entered the correct door code and passed the retina scan, or if we have a valid door key, or if we know the emergency override password, then allow access.
Based on the values of enteredDoorCode, passedRetinaScan, and hasDoorKey, the first two subexpressions are false. However, the emergency override password is known, so the overall compound expression still evaluates to true.
The Swift logical operators
&&
and
are leftassociative, meaning that compound expressions with multiple logical operators evaluate the leftmost subexpression first.
Explicit Parentheses
It’s sometimes useful to include parentheses when they’re not strictly needed, to make the intention of a complex expression easier to read. In the door access example above, it’s useful to add parentheses around the first part of the compound expression to make its intent explicit:
if (enteredDoorCode && passedRetinaScan)  hasDoorKey  knowsOverridePassword {
print("Welcome!")
} else {
print("ACCESS DENIED")
}
// Prints "Welcome!"
The parentheses make it clear that the first two values are considered as part of a separate possible state in the overall logic. The output of the compound expression doesn’t change, but the overall intention is clearer to the reader. Readability is always preferred over brevity; use parentheses where they help to make your intentions clear.