Cannot assign to 'X' in 'Y' in Swift

Question

I have a dictionary with Structs in it. I am trying to assign the values of the struct when I loop through the dictionary. Swift is telling me cannot assign to 'isRunning' in 'blockStatus'. I haven't been able to find anything in the docs on this particular immutability of dictionaries or structs.
Straight from the playground:

import Cocoa

struct BlockStatus{
 var isRunning = false
 var timeGapForNextRun = UInt32(0)
 var currentInterval = UInt32(0) 
}

var statuses = ["block1":BlockStatus(),"block2":BlockStatus()]

for (block, blockStatus) in statuses{
 blockStatus.isRunning = true
}

cannot assign to 'isRunning' in 'blockStatus'
blockStatus.isRunning = true

This does work if I change the struct to a class.

I am guessing it has something to do with the fact that structs are copied and classes are always referenced?

EDIT: So even if it is copying it.. Why can't I change it? It would net me the wrong result but you can change members of constants just not the constant themselves. For example you can do this:

class A {
    var b = 5
}

let a = A()
a.b = 6

Answer 1

Your guess is true.

By accessing blockStatus, you are creating a copy of it, in this case, it's a constant copy (iterators are always constant).

This is similar to the following:

var numbers = [1, 2, 3]

for i in numbers {
   i = 10  //cannot assign here
}

References:

Control Flow

In the example above, index is a constant whose value is automatically set at the start of each iteration of the loop.

Classes and Structures

A value type is a type that is copied when it is assigned to a variable or constant, or when it is passed to a function. [...] All structures and enumerations are value types in Swift

Methods

Structures and enumerations are value types. By default, the properties of a value type cannot be modified from within its instance methods.

However, if you need to modify the properties of your structure or enumeration within a particular method, you can opt in to mutating behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit self property, and this new instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the mutating keyword before the func keyword for that method:

Answer 2

You could loop through the array with an index

for index in 0..<statuses.count {
    // Use your array - statuses[index]
}

that should work without getting "cannot assign"