Generic Instance Method

relative(to:)

Returns the range of indices described by this range expression within the given collection.

Declaration

func relative<C>(to collection: C) -> Range<Bound> where Bound == C.Index, C : Collection

Parameters

collection

The collection to evaluate this range expression in relation to.

Return Value

A range suitable for slicing collection. The returned range is not guaranteed to be inside the bounds of collection. Callers should apply the same preconditions to the return value as they would to a range provided directly by the user.

Discussion

You can use the relative(to:) method to convert a range expression, which could be missing one or both of its endpoints, into a concrete range that is bounded on both sides. The following example uses this method to convert a partial range up to 4 into a half-open range, using an array instance to add the range’s lower bound.

let numbers = [10, 20, 30, 40, 50, 60, 70]
let upToFour = ..<4

let r1 = upToFour.relative(to: numbers)
// r1 == 0..<4

The r1 range is bounded on the lower end by 0 because that is the starting index of the numbers array. When the collection passed to relative(to:) starts with a different index, that index is used as the lower bound instead. The next example creates a slice of numbers starting at index 2, and then uses the slice with relative(to:) to convert upToFour to a concrete range.

let numbersSuffix = numbers[2...]
// numbersSuffix == [30, 40, 50, 60, 70]

let r2 = upToFour.relative(to: numbersSuffix)
// r2 == 2..<4

Use this method only if you need the concrete range it produces. To access a slice of a collection using a range expression, use the collection’s generic subscript that uses a range expression as its parameter.

let numbersPrefix = numbers[upToFour]
// numbersPrefix == [10, 20, 30, 40]

Relationships

From Protocol

See Also

Converting Ranges

init(Range<Bound>)

Creates an instance equivalent to the given Range.