swift array modification in loop

array_modification.swift

// FAILURE
var array = [["name": "glen"]]
for item in array {
  item["rank"] = "advanced" // Generates an @lvalue error
}

// Even though array is immutable, it's of type <Array<Dictionary<String,String>>,
// item is assigned by let automatically.


// FAILURE
var array = [["name": "glen"]]
for (var item) in array {
  item["rank"] = "advanced" // Compiles, but with unexpected results
}

// item is now mutable, but the assignment is only on item, not array, 
// because array is of type <Array<Dictionary<String,String>>, which is copy by value,
// so array remains unchanged.
// If array was full of objects (like NSDictionary), it would work, because then it would be copy by 
// reference


// SUCCESS
var array = [["name": "glen"]]
for index in 0..<array.count {
  array[index]["rank"] = "advanced" // Expected behavior
}

// This works because we are modifying array directly, not a copy of it's value.


// SUCCESS
var array = [["name": "glen"]]
for index in 0..<array.count {
  array[index]["rank"] = "advanced" // Expected behavior
}

// This works because we are modifying array directly, not a copy of it's value.


// SUCCESS
// You could also use the enumerate function which returns a tuple, for the same outcome
var array = [["name": "glen"]]
for (index, (var item)) = enumerate(array) {
  array[index]["event_date"] = "advanced"
}

// Note the use of (var item). Using item would have the same problem as above, a copy of the value in array.
// It's interesting to note that (var item) is a mutable copy of the value returned in the tuple.
// If item was used by itself, it would be immutable, or equivalent to (let item)