Using Functional programming to untangle deeply nested loops (Kotlin)

Question

Problem statement for better understanding: Let's say we assign a unique number to each alphabet,

a=1, b=2, c=3...z=26

then find the frequency of the sum of each word for all possible three letter words (with or without meaning). E.g.

abc has sum of 6, yzx has a sum of 75, zzz has the sum of 78, cat has a sum of 24

and so on.

Following is my code in Kotlin.

    var h = mutableMapOf<Int, Int>()
    for(i in 1..26){
        for(j in 1..26){
            for(k in 1..26){
                val t = i+j+k
                if(h.containsKey(t)){
                    val count = h.get(t)
                    h[t] = count!!.plus(1)
                } else {
                    h.put(t, 1)
                }
            }
        }
    }
    println(h)

The output is:

{3=1, 4=3, 5=6, 6=10, 7=15, 8=21, 9=28, 10=36, 11=45, 12=55, 13=66, 14=78, 15=91, 16=105, 17=120, 18=136, 19=153, 20=171, 21=190, 22=210, 23=231, 24=253, 25=276, 26=300, 27=325, 28=351, 29=375, 30=397, 31=417, 32=435, 33=451, 34=465, 35=477, 36=487, 37=495, 38=501, 39=505, 40=507, 41=507, 42=505, 43=501, 44=495, 45=487, 46=477, 47=465, 48=451, 49=435, 50=417, 51=397, 52=375, 53=351, 54=325, 55=300, 56=276, 57=253, 58=231, 59=210, 60=190, 61=171, 62=153, 63=136, 64=120, 65=105, 66=91, 67=78, 68=66, 69=55, 70=45, 71=36, 72=28, 73=21, 74=15, 75=10, 76=6, 77=3, 78=1}

Regardless of the efficiency of the algorithm, how can I use functional programming in Kotlin to unwrangle the ugly loops to make it more readable and pleasant to the eyes?

EDIT: The solution need not be strictly in Kotlin

Simon Forsberg · Accepted Answer · 2021-06-05 21:30:21Z

3

There are three things you are doing here:

Three loops
Sum
Grouping and counting

The way I would recommend to do these things in Kotlin are:

One loop: val loop = 1..26

Three loops and sum: loop.flatMap {x -> loop.flatMap { y -> loop.map { z -> x + y + z } } }

Grouping by and counting: .groupingBy { it }.eachCount()

Resulting code:

val loop = 1..26
val result = loop.flatMap { x -> 
    loop.flatMap { y -> 
        loop.map { z -> x + y + z }
    }
}.groupingBy { it }.eachCount()

Still though, I have to tell you that this is not an effective way and you might want to learn more about combinatorics in order to make a more efficient solution.

answered Jun 5, 2021 at 21:30

Simon Forsberg

59.8k9 gold badges160 silver badges312 bronze badges

\$\begingroup\$ Thanks for your response. I will try this out and revert. \$\endgroup\$

Sid
– Sid

2021-06-06 10:39:41 +00:00
Commented Jun 6, 2021 at 10:39

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