index(_:offsetBy:limitedBy:)

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.

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.

Complexity: O(1)