Swap

Sometimes it is necessary to exchange the values within 2 variables—this may be useful within certain loops. In Swift 5.9, we can use the built-in swap() function.

Swap receives references (inout) to 2 variables—if we do not provide the reference operator we will receive a compile-time error.

Example

This Swift 5.9 program uses the built-in swap() 2 times, on different variable types. Please note that the variables were declared with the "var" keyword.

Step 1 We declare and initialize 2 strings, using the "var" keyword to allow the strings to be reassigned.

Step 2 We pass the strings as references to the swap method. This changes the values the strings are assigned to.

Step 3 The swap() function can be used on any 2 variables of the same type—here we use Ints.

Step 4 Again, the two variables now have the opposite value. No complicated if-statements were needed.

// Step 1: declare 2 Strings.
var left = "left"
var right = "right"

// Step 2: swap the Strings and print them.
swap(&left, &right)
print(left, right)

// Step 3: declare two Ints.
var zero = 0
var one = 1

// Step 4: swap the Ints and print the swapped values.
swap(&zero, &one)
print(zero, one)right left
1 0

Inout error

If we do use an ampersand when passing a variable, we will get a "passing value" error. This can be easily corrected with a single character.

programs\program.swift:9:6:
error: passing value of type 'String' to an inout parameter requires explicit '&'

With the useful swap() function, built into the Swift standard library, we can swap variables. This function sometimes allows to eliminate a cumbersome if-statement.