Using nested reduce in Swift

Question

I have an array which contains an arrays of Double, like in the screenshot:

My goal is to get the sum of the multiplication of the Double elements of each array. It means, I want to multiply all elements of each array then, in my case, I will have 3 values so I get the sum of them.

I want to use reduce, flatMap ? or any elegant solution.

What I have tried ?

totalCombinations.reduce(0.0) { $0 + ($1[0]*$1[1]*$1[2])  }

but this work only when I know the size of the arrays that contains the doubles.

Luca Angeletti · Accepted Answer · 2016-07-28 12:05:20Z

8

Given these values

let lists: [[Double]] = [[1.1, 2.2, 3.3], [4.4, 5.5, 6.6]]

let's take a look at several possible approaches

Solution #1

let sum =  lists.reduce(0) { $0 + $1.reduce(1, combine: *) }

Solution #2

If you define this extension

extension SequenceType where Generator.Element == Double {
    var product : Double { return reduce(1.0, combine: *) }
}

then you can write

let sum = lists.reduce(0) { $0 + $1.product }

Solution #3

With the extension defined above you can also write

let sum = lists.map { $0.product }.reduce(0, combine:+)

Solution #4

If we define these 2 postfix operators

postfix operator +>{}
postfix func +>(values:[Double]) -> Double {
    return values.reduce(0, combine: +)
}

postfix operator *>{}
postfix func *>(values:[Double]) -> Double {
    return values.reduce(1, combine: *)
}

we can write

lists.map(*>)+>

edited Jul 28, 2016 at 12:05

answered Jul 28, 2016 at 11:09

Luca Angeletti

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Comments

OOPer · Accepted Answer · 2016-07-28 11:05:00Z

2

You can write something like this:

let totalCombinations: [[Double]] = [
    [2.4,1.45,3.35],
    [2.4,1.45,1.42],
    [2.4,3.35,1.42],
    [1.45,3.35,1.42],
]

let result = totalCombinations.reduce(0.0) {$0 + $1.reduce(1.0) {$0 * $1} }

print(result) //->34.91405

But I'm not sure it's "elegant" or not.

answered Jul 28, 2016 at 11:05

OOPer

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2 Comments

Hamish Over a year ago

Not sure if considered more elegant in this case, but you can pass the * operator to reduce directly ($1.reduce(1, combine: *))

OOPer Over a year ago

@Hamish, good point. But it's already included in appzYourLife's answer. So, I think I'm late.

ielyamani · Accepted Answer · 2016-07-28 11:07:47Z

2

Maybe this is what you're looking for

let a = [1.0, 2.0, 3.0]
let b = [4.0, 5.0, 6.0]
let c = [7.0, 8.0, 9.0, 10.0]

let d = [a, b, c]

let sum = d.reduce(0.0) { $0 + $1.reduce(1.0) {$0 * $1}}
print(sum) // prints 5166.0

answered Jul 28, 2016 at 11:07

ielyamani

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