Enumerating dictionary in Swift

Question

I guess I noticed a bug in the Swift Dictionary enumeration implementation.

The output of this code snippet:

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
for (key, value) in someDict.enumerated() {
   print("Dictionary key \(key) - Dictionary value \(value)")
}

should be:

Dictionary key 2 - Dictionary value Two
Dictionary key 3 - Dictionary value Three
Dictionary key 1 - Dictionary value One

instead of:

Dictionary key 0 - Dictionary value (key: 2, value: "Two")
Dictionary key 1 - Dictionary value (key: 3, value: "Three")
Dictionary key 2 - Dictionary value (key: 1, value: "One")

Can anyone please explain this behavior?

https://stackoverflow.com/a/31447400/2303865 – Leo Dabus Sep 09 '18 at 22:25 — Leo Dabus, Sep 09 '18 at 22:25

score 28 · Accepted Answer · edited Sep 09 '18 at 16:50

It's not a bug, you are causing the confusion because you are using the wrong API.

You get your expected result with this (dictionary related) syntax

for (key, value) in someDict { ...

where

key is the dictionary key
value is the dictionary value.

Using the (array related) syntax

for (key, value) in someDict.enumerated() { ...

which is actually

for (index, element) in someDict.enumerated() { ...

the dictionary is treated as an array of tuples and

key is the index
value is a tuple ("key": <dictionary key>, "value": <dictionary value>)

Enumerating dictionary in Swift

1 Answers1