iOS Swift - Reduce function

Question

I have a piece of code in SWIFT that I don't understand:

var peers: [String:NSSet] = [:]
for s in squares {
  var ps = reduce(units[s]!, NSMutableSet()) { set, u in
    set.addObjectsFromArray(u)
    return set
  }
  ps.removeObject(s)
  peers[s] = ps
}

squares is an array of String.

So far I have realized that peers probably is a key/value data structure with keys of String and values of NSSet. NSSet is similar to Array but it cannot accept duplicate items. The main part that I don't understand is actually the reduce function. Any explanation or instructive article/webpage is appreciated.

Ian MacDonald · Accepted Answer · 2015-02-02 17:52:25Z

2

reduce is a method that's used to reduce an array into single value using the operator that you provide to construct the final result. Most demonstrations of this available in tutorials use + or * to reduce an array of numbers into a single sum or multiplication result.

The method you're using takes the input array units[s] and an initial value NSMutableSet() (an empty set), then applies the closure to each element in sequence.

Your code seems to indicate that the elements of units[s] are again arrays; so your data might look something like this:

units[s]: [
            [1, 2, 3, 4],
            [5, 6, 7, 8],
            [1, 3, 5, 7]
          ]

Making ps be:

ps: [ 1, 2, 3, 4, 5, 6, 7, 8 ]

after your reduce call.

answered Feb 2, 2015 at 17:52

Ian MacDonald

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MirekE · Accepted Answer · 2015-02-02 17:47:56Z

1

var ps = reduce(units[s]!, NSMutableSet()) { set, u in
    set.addObjectsFromArray(u)
    return set
  }

Reduce combines elements from array in the first parameter (units[s] should be an array) into second parameter (here NSMutableSet). The code in curly brackets that follows tells how to combine the elements. The "set" and "u" refer to the units[s] and the NSMutable set. So it takes each element in units[s] and adds them to the NSMutableSet.

answered Feb 2, 2015 at 17:47

MirekE

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