Mutability of the Iterator Element in a For-In loop with an Array in Swift

问题

I have some code in Swift 3.0 like so for trying to update the property in a array of elements...

for point in listOfPoints {
    var pointInFrame : Float = Float(point.position.x * sensorIncomingViewPortSize.width) + Float(point.position.y)
    point.status = getUpdateStatus( pointInFrame )
}

However I get a compile error of: 'Cannot assign to property: 'point' is a 'let' constant' [for line 3]

Is there anyway to make the iterator (point) mutable in Swift, like how you can use 'inout' for a function parameter?

Or are you supposed to do this task another way?

Thanks in Advance.

Stan

回答1:

Just change it to var instead of let where you declare point. let is a constant.

回答2:

Another way to accomplish this:

for i in 0 ... myStructArray.count - 1 {
    var st = myStructArray[i]
    st.someStringVariable = "xxx"   // do whatever you need with the struct
    st.someIntVariable = 123        // do more stuff

    // last step (important!):
    myStructArray[i] = st           // necessary because structs are VALUE types, not reference types.
}

If you only need to make one change, you can omit the steps of defining the local variable (st in the example) as the array element and then afterwards setting the array element as equal to the local variable. But if you're going to make lots of changes to the element, if may be cleaner to make the local variable, make all the changes to it, and then assign that variable back to the array element.

If the array was of a Class rather than a Struct, that last step of assigning back wouldn't be necessary (reference type -- Class, vs value type -- Struct).

来源：https://stackoverflow.com/questions/40160113/mutability-of-the-iterator-element-in-a-for-in-loop-with-an-array-in-swift

标签

arrays

swift

iterator

immutability

for-in-loop