Different ways to map over Dictionary in Swift
Table of Contents
When talking about a map
function over a Dictionary, there are many ways to think about it.
You might want to do either one of these three things.
- Transform a value, but keep the same key.
- Transform a key, but keep the same value.
- Transform both key and value.
Swift only supports the first case out of the box.
Transform Value
If you want to transform the value part of a dictionary and keep the same key, you can use mapValues(_:)
.
The syntax is similar to mapping over an array. We provide a transform closure that accepts a value of the dictionary and returns a transformed value.
func mapValues<T>(_ transform: (Value) throws -> T) rethrows -> Dictionary<Key, T>
In this example, I want to double the integer value of the dictionary.
let dict: [String: Int] = [
"a": 1,
"b": 2,
"c": 3
]
let doubleDict = dict.mapValues { value in
// 1
return value * 2
}
// 2
print(doubleDict)
// ["b": 4, "c": 6, "a": 2]
1 We get a value of each element in the dictionary and return the transformed one from the closure.
2 Beware that Dictionary is not gurantee the order.
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Transform Key
When I said that Swift only support the transform of dictionary value, it isn't entirely true.
I meant we don't have a dedicated method like mapKeys
to do so.
But we can accomplish this by combining two Swift methods.
map(_:)
: Dictionary'smap(_:)
method will let you transform each key and value into a new array (Not dictionary).Dictionary.init(uniqueKeysWithValues:)
: You can create a new dictionary from an array of key-value pair Tuple with this initializer.
We can use these two methods to transform the key of a dictionary.
In this example, we transform each key to uppercase.
let dict: [String: Int] = [
"a": 1,
"b": 2,
"c": 3
]
// 1
let uppercasedKeyTuple = dict.map { (key, value) in
return (key.uppercased(), value)
}
print(uppercasedKeyTuple)
// [("C", 3), ("B", 2), ("A", 1)]
// 2
let uppercasedKeyDict = Dictionary(uniqueKeysWithValues: uppercasedKeyTuple)
print(uppercasedKeyDict)
// ["C": 3, "A": 1, "B": 2]
1 Transform the dictionary into an array of key-value pairs. This is a place where you transform your key.
2 Create a dictionary out of this new key-value pair.
The first part of the Tuple will become the key of a dictionary, and the second part will become its value.
Caveat
Every key in the key-value pair Tuple must be unique.
If you provide a Tuple with the same key to Dictionary.init(uniqueKeysWithValues:)
, you will get a runtime error.
The following code will produce "Fatal error: Duplicate values for key: 'A'" error.
let dict = Dictionary(uniqueKeysWithValues: [("A", 1), ("A", 2)])
// Fatal error: Duplicate values for key: 'A'
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Transform Both Key and Value
We can transform both the key and value of a dictionary using the same method as the transform key.
Instead of transforming only the key path in the map(_:)
method, we do it for both key and value.
let dict: [String: Int] = [
"a": 1,
"b": 2,
"c": 3
]
let mapDict = dict.map { (key, value) in
// 1
return (key.uppercased(), value * 2)
}
print(mapDict)
// [("A", 2), ("C", 6), ("B", 4)]
let transformedDict = Dictionary(uniqueKeysWithValues: mapDict)
print(transformedDict)
// ["B": 4, "A": 2, "C": 6]
1 We transform both the key and value here.
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