Swift - How to mutate a struct object when iterating over it

You are working with struct objects which are copied to local variable when using for in loop. Also array is a struct object, so if you want to mutate all members of the array, you have to create modified copy of original array filled by modified copies of original objects.

arrayOfMyStruct = arrayOfMyStruct.map { obj in
   var obj = obj
   obj.backgroundColor = .red
   return obj
}

It can be simplified by adding this Array extension.

Swift 4

extension Array {
    mutating func mutateEach(by transform: (inout Element) throws -> Void) rethrows {
        self = try map { el in
            var el = el
            try transform(&el)
            return el
        }
     }
}

Usage

arrayOfMyStruct.mutateEach { obj in
    obj.backgroundColor = .red
}

struct are value types, thus in the for loop you are dealing with a copy.

Just as a test you might try this:

Swift 3:

struct Options {
   var backgroundColor = UIColor.black
}

var arrayOfMyStruct = [Options]()

for (index, _) in arrayOfMyStruct.enumerated() {
   arrayOfMyStruct[index].backgroundColor = UIColor.red
}

Swift 2:

struct Options {
    var backgroundColor = UIColor.blackColor()
}

var arrayOfMyStruct = [Options]()

for (index, _) in enumerate(arrayOfMyStruct) {
    arrayOfMyStruct[index].backgroundColor = UIColor.redColor() 
}

Here you just enumerate the index, and access directly the value stored in the array.

Hope this helps.

You can use use Array.indices:

for index in arrayOfMyStruct.indices {
    arrayOfMyStruct[index].backgroundColor = UIColor.red
}