# Grouping Array Data

Given an integer matrix a and a nonnegative integer i, output a mapping b that maps the distinct values in the ith column of a to rows of a who have that value in the ith column.

You may assume that i is in the half-open range [0, num_cols(a)) (or [1, num_cols(a)] if you choose to use 1-based indices), and that all integers are within the representable range for your language. Input and output may be done in any reasonable manner, so long as it satisfies the basic requirements of the challenge (2D array -> mapping from ints to 2D arrays of ints). So long as the mapping is clear and consistent, the keys do not need to be included in the output.

## Examples

[], 0 -> {1: []}
[[3, 4, 5], [1, 4, 2], [5, 5, 5], [7, 7, 7], [1, 5, 9]], 1 -> {4: [[3, 4, 5], [1, 4, 2]], 5: [[5, 5, 5], [1, 5, 9]], 7: [[7, 7, 7]]}
[[1, 2, 3, 4, 5], [5, 4, 3, 2, 1], [2, 3, 4, 5, 6], [8, 9, 100, 0, 2]], 4 -> {5: [[1, 2, 3, 4, 5]], 1: [[5, 4, 3, 2, 1]], 6: [[2, 3, 4, 5, 6]], 2: [[8, 9, 100, 0, 2]]}


This is , so the shortest answer in bytes wins.

• Sandbox – Mego May 20 '18 at 3:29
• Just to check, can the mapping be a function? I'm not aware if this is a default, but it seems like something you intend to allow. – FryAmTheEggman May 20 '18 at 3:37
• @FryAmTheEggman Yes, a function that meets our usual requirements is allowed. The I/O is extremely flexible. – Mego May 20 '18 at 3:42
• I like this I/O format very much because the output does not actually need to contain the input in itself. It's completely fine to return a function that accesses the input by reference as long as the function is a mapping. – JungHwan Min May 20 '18 at 4:40
• @JungHwanMin I'm glad. I wanted to experiment with a very loose I/O format, and it's going well so far – Mego May 20 '18 at 4:44

# Octave, 24 bytes

@(a,i)@(n)a(a(:,i)==n,:)


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This creates an anonymous function that returns a matrix whose rows match the criteria. Octave indexes arrays at 1, not zero, and rows of a matrix are separated by a ;.

Matrices are what Octave does best—so well, in fact, that this challenge can be solved using pure syntax, no built-in functions.

## Explanation

@(a,i)                   % creates an anonymous function that...
@(n)               % returns another function that takes input n and
% maps it to the rows of a.
a(         ,:) % Return all the columns of a, with the rows filtered by...
a(:,i)       % whether the ith column of each row of a...
==n    % equals n


# Ruby, 26 bytes

->a,i{a.group_by{|x|x[i]}}


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# Wolfram Language (Mathematica), 21 bytes

#~GroupBy~Extract@#2&


1-indexed. Returns an Association mapping.

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This is a rare case in which a longer function (Extract) reduces the byte count (the shorter one being Part or [[ ... ]]) because Extract can curry. The result is this extremely concise two-function solution.

### Explanation

Extract@#2


Function that extracts the <second input>th element.

#~GroupBy~ ...


Group the <first input> into lists associated with distinct keys <above function>[element].

import Data.List
i!l=[(k,[a|a<-l,a!!i==k])|k<-nub$map(!!i)l]  Try it online! # Clean, 40 bytes import StdEnv  \n l i=filter(\a=a!!n==i)l  Try it online! A lambda (:: Int [[Int]] Int -> [[Int]]) where a partial application of just the first two arguments gives a mapping on the third argument. # J, 16 bytes -3 bytes thanks to FrownyFrog! {"1(~.@[;"0</.)]  Try it online! ## Explanation: A dyadic verb, taking i as its left argument and a as its right one. ] is the right argument, a {"1 finds the numbers at ith column on each row </. boxes groups from the right argument, selected by the keys, provided by the left one ~.@[ finds the unique keys ;"0 links the keys to the selected groups • ;"0 instead of ,: saves 3 – FrownyFrog May 20 '18 at 9:22 • @FrownyFrog Of course! I think I tried it, but apparently not the right way. – Galen Ivanov May 20 '18 at 11:18 # jq, 100 bytes uses an object for output, takes a command line argument $f plus an array on standard input

([.[]|.[$f]]|unique) as$c|[$c[] as$d|{($d|tostring):([.[]|[select(.[$f]==$d)]]|add)}]|add  deobfuscated: .fieldnum as$field |
.input as $input | ([$input[] | .[$field]] | unique) as$categories |
[
$categories[] as$category |
{
($category | tostring) : ([$input[] | [select(.[$field]==$category)]] | add)
}

• Is this the language you're using? – Οurous May 21 '18 at 23:20

# R, 79 55 bytes

function(a,i)for(z in unique(a[,i]))print(a[a[,i]==z,])


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24 bytes shaved off by @JayCe

# Python 3, 45 bytes

lambda a,i:lambda n:[r for r in a if r[i]==n]


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Returns the mapping represented as an anonymous lambda.

# Proton, 29 bytes

a=>i=>n=>filter(r=>r[i]==n,a)


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-3 bytes thanks to Mr. Xcoder using currying and filter (TBH I'm kind of surprised that filter actually worked)

# JavaScript (Node.js), 29 bytes

a=>i=>n=>a.filter(e=>e[i]==n)


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Updated now that I realize the loose output requirements. This uses currying as a golfing technique, and it also returns a function that takes an input n and maps that to the proper arrays.

# Jelly, 5 bytes

ịⱮ⁹¹ƙ


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Omits the keys, but should be clear.

Argument 1: i + 1
Argument 2: a

• I don't think this would qualify as a mapping without the keys. – Dennis May 22 '18 at 3:32
• @Dennis Hm, I had asked in the comments about it and OP said we can omit the keys (exactly what I edited into the question), and I had also linked this solution there (maybe shouldn't have flagged so early...). I did include the keys in a previous revision of this answer (waiting for an answer), so I'll just post another comment and let's see what OP says. – Erik the Outgolfer May 22 '18 at 9:01

# Java 10, 135 64 bytes

m->i->n->new java.util.Stack(){{for(var a:m)if(a[i]==n)add(a);}}


Returns a Function<Integer, List<int[]>> accepting an integer-input n, which returns a List of arrays (matrix-rows) where the i'th values equal the given n.

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Explanation:

m->i->               // Method with int-matrix and int parameters and Function return-type
n->                //  Return a Function with integer as parameter
new java.util.Stack(){{
//  and List of integer-arrays as return-type
for(var a:m)   //   Loop over the arrays of the input-matrix
if(a[i]==n)  //    If the i'the value of the current array equals n: