How to Filter an Array in Swift

Swift Array has an instance method, filter(_:), that removes unwanted elements from the resulting array.

The resulting array that pass the filter(_:) contain only the elements that you want.

How to use Array filter

Array filter has the following signature.

func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]

• filter(_:) accepts an isIncluded closure which decided which element we want to include in the final array.
• Each element in the array will pass through this isIncluded closure.
  • If you want to keep that element, you return true from that closure.
  • If you want to remove that element, you return false from that closure.

It is easier to understand this with an example.

Example

Let's say we have an array of people's names, and we want only the names shorter than four characters.

let names = ["Alice", "Bob", "John"]

let shortNames = names.filter { name in
    if name.count < 4 {
        // 1
        return true
    } else {
        // 2
        return false
    }
}
print(shortNames)
// ["Bob"]

1 We return true for an element we want to keep. In this case, we want to keep a name shorter than four characters, name.count < 4.
2 For other names, we return false to discard them.

The resulting array contains only names shorter than four characters which is only **"Bob"**in this case.

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Summary

In summary, to filter an array, you pass a closure telling which element you want to keep in the final array.

• Return true, if you want to keep it.
• Return false to discard.