“无法分配”错误迭代结构数组

Question

I have an array of structs:

struct CalendarDate {
    var date: NSDate?
    var selected = false
}

private var collectionData = [CalendarDate]()

Which I simply populate with a date like this:

    for _ in 1...7 {
        collectionData.append(CalendarDate(date: NSDate(), selected: false))
    }

So when you tap on a collectionView, I simply want to loop through the data and mark them all as False.

    for c in collectionData {
        c.selected = false  ///ERROR: Cannot assign to 'selected' in 'c'
    }

Why do I get this error?

If I do this, it works fine but I want to know what I did wrong above:

    for i in 0..<collectionData.count {
        collectionData[i].selected = false
    }

Answer 1

As I understand it, the iterator

for c in collectionData

returns copies of the items in collectionData - ( structs are value types, not reference types, see http://www.objc.io/issue-16/swift-classes-vs-structs.html ), whereas the iteration

for i in 0..<collectionData.count

accesses the actual values. If I am right in that, it is pointless to assign to the c returned from the iterator... it does not "point" at the original value, whereas the

collectionData[i].selected = false

in the iteration is the original value.

Some of the other commentators suggested

for (var c) in collectionData

but although this allows you to assign to c , it is still a copy, not a pointer to the original, and though you can modify c , collectionData remains untouched.

The answer is either A) use the iteration as you originally noted or B) change the data type to a class, rather than a struct.

Answer 2

because each 'c' is by default let, and this is a new instance of CalendarDate and the value of array at index copied to this for each step of for, and 'c' isn't pointer to the index of the array and it is just a copy of index, so if you set a new value to this, the new value does not apply in array. but 'i' is used as index of array and can directly manipulate the values of array.