How to initialize an array before referencing it Swift

Question

Why is it that when I create arr, an array then try to populate it with integers using a for-in loop it, gives me an error when I don't initialize it. From what I can tell it is initialized when I write "var arr," but obviously not so what does writing var arr actually do if not initialize it.

Error Example

var arr : [Int] //Error Message: Variable 'arr' passed by reference before being initialized
for i in 1...10 {
    arr += [i]
}
arr //Error Message: Variable 'arr' used before being initialized

Working Example

var arr : [Int] = [] //Allocating memory?
for i in 1...10 {
    arr += [i]
}
arr

That is because the array was not initialized. You should initialize all the variables before use it. If you put an ! after [Int] I think you shouldn't receive that message. — CrApHeR
– CrApHeR, Commented Mar 29, 2015 at 23:33
I get the following error instead. '[Int]!' is not identical to 'UInt8' — jeanpier_re
– jeanpier_re, Commented Mar 29, 2015 at 23:37

GoZoner · Accepted Answer · 2015-03-29 23:38:54Z

7

Just saying var arr does not initialize your array. Should the initial value be an empty array? Should it have N copies of a given value? If it is declared to hold type A objects, should it be initialized with subtypes of A.

So you provide an initial value; the simplest is [] - an empty array.

edited Mar 29, 2015 at 23:38

answered Mar 29, 2015 at 23:33

GoZoner

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4 Comments

jeanpier_re Over a year ago

Does that mean that not initializing your variables is a bad practice?

GoZoner Over a year ago

All variables need to be initialized either as part of their declaration or in the class's init() method. The compiler enforces this and thus it is worse than 'bad practice' it is illegal Swift.

nhgrif Over a year ago

@GoZoner All non-optional instance variables, you mean.

GoZoner Over a year ago

Right. Optionals are a special case. Thanks.

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