Swift forEach with assignment inside the block

user_340354 0
Viewed 241

Let's say I have this

var array = [1,2,3]

array.forEach {
    array.append(17)
    print($0)
}

It will not loop forever, instead it will print 1, 2 and 3 and afterwards the array has 6 elements: [1,2,3,17,17,17]

How can this be understood?

the array inside the block is a copy of the array the moment i call .append(17), right? And then after the loop it's assigned to the array var? Or is the copy made before hand?

which is actually what is going on?

This:

var array = [1,2,3]
var arrayCopy: [Int]

array.forEach {
    arrayCopy.append(17)
    print($0)
}

array = arrayCopy

or this:

var array = [1,2,3]
var arrayCopy = array

arrayCopy.forEach {
    array.append(17)
    print($0)
}

Or something else?

copy-on-write swift struct foreach
2 Answers

Intersting observation. Second answer looks more accurate when u create a forEach a copy of current array is created and looped through it's elements.

var array = [1,2,3]
var arrayCopy = array

arrayCopy.forEach {
    array.append(17)
    print($0)
}

If u run following code in playground u will get output showing element added to main array for each cycle of the loop

var array = [1,2,3]

array.forEach {_ in
    array.append(17)
    print(array)
}


//You will get following output 
//[1, 2, 3, 17]
//[1, 2, 3, 17, 17]
//[1, 2, 3, 17, 17, 17]
user_30993770

The first one is accurate and works fine but you have to initialize your arrayCopy as an empty array like below:

var array = [1,2,3]
var arrayCopy: [Int] = []

array.forEach {
    arrayCopy.append(17)
    print($0)
}

array = arrayCopy

Which will give you the same output as you already added the output into your array but this Valid only if you wanna make a copy and put it in your original array.

user_144374110
Related