Dictionary won't let me append a string to its values - ios

Question

I'm trying to create a function which takes an array of names, and sorts it in a dictionary with letters and names as keys and values. But now the dictionary wont let me append the string to the values. My code looks like this:
func listCounter(usernames: [String])->Dictionary <String,[String]> {
var dict=Dictionary<String,[String]>()
var letterList = [String]()
for user in usernames{
var index = user.substringToIndex(advance(user.startIndex,1))
index = index.lowercaseString as String
if find(letterList, index) != 0{
dict[index] += [user]
}else{
dict[index] = [user]
letterList += [index]
}
}
return dict
}
The error comes in the line where I'm trying to add the new string to the dictionary, it says: "Cannot invoke '+=' with an argument list of type '$T4,$T6'" which is telling me there's something wrong with the types, but I don't know how to fix it.
Any suggestions on how to solve this would be appreciated.

Answer 1

That happens because a dictionary lookup always returns an optional - since the preceding if should ensure that the element exists, you can safely apply the forced unwrapping operator on that:
dict[index]! += [user]
However running a test on a playground resulted in a runtime exception - I think this condition:
if find(letterList, index) != 0 {
is not reliable.
I replaced with an explicit check for the key existence, and it worked:
if dict[index] != nil {
dict[index]! += [user]
Note: I didn't use optional binding like this:
if var element = dict[index] {
element += [user]
because arrays are value types, copied by value. Assigning the array to a variable actually creates a copy of it, so the addition is done on the copy, leaving the original array unchanged.

Answer 2

if find(letterList, index) != 0 { ... }
should actually be
if find(letterList, index) != nil { ... }
or just
if contains(letterList, index) { ... }
But #Antonio has already explained the error message and given a solution. As an alternative, you can also take advantage of
optional chaining:
for user in usernames {
var index = user.substringToIndex(advance(user.startIndex,1))
index = index.lowercaseString as String
if (dict[index]?.append(user)) == nil {
dict[index] = [user]
letterList.append(index)
}
}
How does it work? If dict[index] is nil, then
dict[index]?.append(user)
does nothing and returns nil, so that the if-block is executed.
Otherwise
dict[index]?.append(user)
appends the user to the array in dict[index] and the if-block
is not executed.
You could also write it as a one-liner, using the "nil-coalescing operator" ??:
for user in usernames {
var index = user.substringToIndex(advance(user.startIndex,1))
index = index.lowercaseString as String
dict[index] = (dict[index] ?? []) + [user]
}
Here, dict[index] ?? [] evaluates to the dictionary value if that
already exists, and to an empty array otherwise. And the array
of all indices can also be computed after the loop with
letterList = Array(dict.keys)