Instance MethodSwift

index(_:offsetBy:limitedBy:)

Returns an index that is the specified distance from the given index, unless that distance is beyond a given limiting index.

func index(_ i: Int, offsetBy distance: Int, limitedBy limit: Int) -> Int?

Parameters

i: A valid index of the array.
distance: The distance to offset i.
limit: A valid index of the collection to use as a limit. If distance > 0, limit has no effect if it is less than i. Likewise, if distance < 0, limit has no effect if it is greater than i.

Returns

An index offset by distance from the index i, unless that index would be beyond limit in the direction of movement. In that case, the method returns nil.

Overview

The following example obtains an index advanced four positions from an array’s starting index and then prints the element at that position. The operation doesn’t require going beyond the limiting numbers.endIndex value, so it succeeds.

let numbers = [10, 20, 30, 40, 50]
if let i = numbers.index(numbers.startIndex,
                         offsetBy: 4,
                         limitedBy: numbers.endIndex) {
    print(numbers[i])
}
// Prints "50"

The next example attempts to retrieve an index ten positions from numbers.startIndex, but fails, because that distance is beyond the index passed as limit.

let j = numbers.index(numbers.startIndex,
                      offsetBy: 10,
                      limitedBy: numbers.endIndex)
print(j)
// Prints "nil"

The value passed as distance must not offset i beyond the bounds of the collection, unless the index passed as limit prevents offsetting beyond those bounds.