Default Value in Swift Dictionary

When you try to access a value from a dictionary with a key that doesn't exist, you will get nil.

This makes perfect sense since nil represent "the absence of a valid object".

But sometimes, you might want to get a default value if the key isn't found. Swift dictionary has an easy way to do that.

Access dictionary with a default value

To get a default value instead of nil when the key isn't found, you can specify the default value along with the subscription key, dict["key", default: "default value"].

Let's say we have a dictionary that stores the number of times we see each alphabet.

let counter: [String: Int] = [
    "a": 1,
    "b": 2,
    "c": 3
]

print(counter["d"])
// nil

print(counter["d", default: 0])
// 0

Reading the value of the key "d" would return a nil, counter["d"].

0 makes more sense than nil in this case. We can specify the default value using this form counter["d", default: 0].

This might not seem useful since you can define the default value using Nil-coalescing operator.

counter["d", default: 0]
counter["d"] ?? 0

But I think this feature shrine when you try to assign the value.

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Assign dictionary with a default value

Here is an example where we count the number of strings in an array, source.

Without using the default value, we might need to do something similar to this.

let source = ["a", "b", "c", "s", "w", "a", "a", "s"]
var counter: [String: Int] = [:]

for c in source {
    // 1
    if let count = counter[c] {
        counter[c] = count + 1
    } else {
        counter[c] = 1
    }
}

print(counter)
// ["s": 2, "a": 3, "w": 1, "b": 1, "c": 1]

1 We must check whether the value exists and act accordingly.

If we use a dictionary default value, we can remove the if-let part.

let source = ["a", "b", "c", "s", "w", "a", "a", "s"]
var counter: [String: Int] = [:]

for c in source {
    // 1
    counter[c, default: 0] += 1
}

print(counter)
// ["s": 2, "a": 3, "c": 1, "b": 1, "w": 1]