swift: functional programming & arrays

Question

var data: [[String]] = ...
let pos = data.compactMap { Int($0[posColumn]) }
let neg = data.compactMap { Int($0[negColumn]) }

let sum = ?? { pos + neg }

I have a csv data, a column for positive values and a column for negative. Using data.compactMap I get two arrays for both. How can I get [Int] for sum using functional programming?

vacawama · Accepted Answer · 2018-07-05 19:35:22Z

3

You want to sum across your two columns to create a third column.

Use zip(_:_:) to combine the arrays and map(_:) to sum them:

let sum = zip(pos, neg).map(+)

what if I need to get sum AND divide by some number? So, there will be 3 arrays: pos, neg, tot. (pos + neg) / tot

Since zip only works with 2 sequences, you could instead map over the indices of one of your arrays:

let result = pos.indices.map { (pos[$0] + neg[$0]) / tot[$0] }

edited Jul 5, 2018 at 19:35

answered Jul 5, 2018 at 18:54

vacawama

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3 Comments

Anton Over a year ago

Yes! To make it more complex: what if I need to get sum AND divide by some number? So, there will be 3 arrays: pos, neg, tot. (pos + neg) / tot

Anton Over a year ago

As tot is an array with corresponding values, it should be like: let result = zip(pos, neg, tot).map { ($0 + $1)/$2 } - but zip takes only 2 args.

vacawama Over a year ago

Oh, in that case let result = pos.indices.map { (pos[$0] + neg[$0]) / tot[$0] }

hardikdevios · Accepted Answer · 2018-07-05 18:53:23Z

0

let sum = (pos + neg).reduce(0, +)

answered Jul 5, 2018 at 18:53

hardikdevios

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3 Comments

jkdev Over a year ago

This code-only answer would be better if it explained why the code works.

hardikdevios Over a year ago

@jkdev you mean I should mention how this code works?

jkdev Over a year ago

Yes, exactly. That would make the answer easier to understand, and more valuable for other people who find it.

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