Swift: Find index of first element of sequence matching a predicate

findIndex.swift

/// Find the index of the first element of a sequence that satisfies a predicate
///
/// :param: sequence A sequence to be searched
/// :param: predicate A function applied to each element in turn until it returns true
///
/// :returns: Zero-based index of first element that satisfies the predicate, or nil if no such element was found
public func findIndex<S: SequenceType>(sequence: S, predicate: (S.Generator.Element) -> Bool) -> Int? {
    for (index, element) in enumerate(sequence) {
        if predicate(element) {
            return index
        }
    }
    return nil
}



// Example

let array = ["One", "Two", "Three", "Four"]

// find index of first item that starts with "Th"
let firstTh = findIndex(array) { $0.hasPrefix("Th") }
println("firstTh = \(firstTh)")                                            // "firstTh = Optional(2)"

Yes, it would be better to extend SequenceType. This was written before protocol extensions were available in Swift.

Note that CollectionType has an indexOf method which is similar. (I don't remember whether it was present when I wrote this.)