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I m kind of new in functional programming If I have 2 arrays of strings, is there a way using map, filter and zip to return the matching values ? I know a dirty way to do it, by iterating in each array and comapring the values to find matches but there must be a clean way to do it.

so var example i have the following arrays :

array1 = ["max","julie","helmut","igor"] array2 = ["sophie","igor"]

and I would like to retrieve "igor" as a value

another thing, if there s more than one match, witch in my case is not possible, I would like to know how to return a string of matches (just for my own understanding)

Thanks for your help :)

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    What here is functional programming? Commented Mar 4, 2016 at 3:55

5 Answers 5

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Something I whipped up quick. Should work for anything adhering to Equatable (such as a String):

func getMatches<T: Equatable>(firstArray: [T], secondArray: [T]) -> [T] {
    return firstArray.filter({secondArray.contains($0)})
}

Alternately, this is a generic version of Kevin's answer

func getMatches<T: Equatable>(firstArray: [T], secondArray: [T]) -> Set<T> {
    return Set(firstArray).intersect(secondArray)
}
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Set has a intersect method which returns the values that exist in the set and any SequenceType. I'm not sure if this is "functional" but it's certainly not "dirty".

let array1 = ["max","julie","helmut","igor"]
let array2 = ["sophie","igor","helmut"]

let common = Set(array1).intersect(array2)
print(common) //["igor", "helmut"]
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4 Comments

That s why I love internet, I love you also
This will have runtime of O(len(array1)) for constructing the set + O(len(array2)) for intersect(). This will be much faster than O(len(array1)) for filter + O(len(array2) * len(array2)) for contains. This is a great trade if you can afford disregarding duplicates and can spare some extra memory.
nice, I was just wondering what was faster and testing both approaches, my arrays are HUGE, thanks for clearing this out for me
Added a generic version of this to my answer
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This will work for your purposes:

let array1 = ["max","julie","helmut","igor"]
let array2 = ["sophie","igor"]

let arrayOfMatches = array1.filter { array2.contains($0) }
print(arrayOfMatches)

Let me know if you need anything more explained.
This is the shortest way to write this kind of statement.
Essentially filter needs to return a boolean that determines whether or not to keep the value over which it is iterating.

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tks :) it makes sense
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Try this.

let both = array1.filter{ array2.contains($0) }

enter image description here

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Alternative PEEJWEEJ's answer.

If you return value [T] instead of Set<T>, try this. (T must conform to hashable)

func getMatches<T: Hashable>(firstArray: [T], secondArray: [T]) -> [T] {
    return Array(Set(firstArray).intersect(secondArray))
}
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2 Comments

On a more serious note, will the returned array have an expected ordering? jc.
Sorry...! I fixed typo.

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