---
title: "lexicographicallyPrecedes(_:by:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/prefixsequence/lexicographicallyprecedes(_:by:)"
---

# lexicographicallyPrecedes(_:by:)

Returns a Boolean value indicating whether the sequence precedes another sequence in a lexicographical (dictionary) ordering, using the given predicate to compare elements.

## Declaration

```swift
func lexicographicallyPrecedes<OtherSequence>(_ other: OtherSequence, by areInIncreasingOrder: (Self.Element, Self.Element) throws -> Bool) rethrows -> Bool where OtherSequence : Sequence, Self.Element == OtherSequence.Element
```

## Parameters

- `other`: A sequence to compare to this sequence.
- `areInIncreasingOrder`: A predicate that returns true if its first argument should be ordered before its second argument; otherwise, false.

## Return Value

Return Value true if this sequence precedes other in a dictionary ordering as ordered by areInIncreasingOrder; otherwise, false.

## Discussion

Discussion The predicate must be a strict weak ordering over the elements. That is, for any elements a, b, and c, the following conditions must hold: areInIncreasingOrder(a, a) is always false. (Irreflexivity) If areInIncreasingOrder(a, b) and areInIncreasingOrder(b, c) are both true, then areInIncreasingOrder(a, c) is also true. (Transitive comparability) Two elements are incomparable if neither is ordered before the other according to the predicate. If a and b are incomparable, and b and c are incomparable, then a and c are also incomparable. (Transitive incomparability) note: This method implements the mathematical notion of lexicographical ordering, which has no connection to Unicode.  If you are sorting strings to present to the end user, use String APIs that perform localized comparison instead. note: O(m), where m is the lesser of the length of the sequence and the length of other.