Swift .updateValue()

The .updateValue() method replaces the value stored in a dictionary for a given key, and returns the replaced value. If the key does not exist, a new key-value pair is created and nil is returned.

Syntax

dictionary.updateValue(value, forKey: key)

• value is the value being updated or added to the dictionary.
• key is the key getting updated or created.

Examples

In the example below, the key Loki is found and updated to the new value, 8. In addition, the key Lenny did not previously exist; a new key-value pair is added to the dictionary:

var petAges = ["Aurora": 10, "Loki": 7]

// Update existing key's value
petAges.updateValue(8, forKey: "Loki")
// Add new key-value pair
petAges.updateValue(12, forKey: "Lenny")

print(petAges)

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This will output:

["Loki": 8, "Lenny": 12, "Aurora": 11]

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Return Values

.updateValue() returns the original value if an existing key is replaced.

The key Dogs exists in the dictionary below, and its value is updated:

var pets = ["Dogs": 1, "Cats": 1]

if let originalValue = pets.updateValue(2, forKey: "Dogs") {
    print("The original value of \(originalValue) was updated.")
} else {
    print("Key was not found in dictionary.")
}

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This will output:

The original value of 1 was replaced.

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If a key does not exist, .updateValue() returns nil.

Parrots does not exist as a key and nil is returned, therefore the else block is run:

if let originalValue = pets.updateValue(2, forKey: "Parrots") {
    print("The original value of \(originalValue) was updated.")
} else {
    print("Key was not found in dictionary.")
}

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This will output:

Key was not found in dictionary.

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