# Reorder a matrix, twice

You are given a square $$\n \times n\$$ matrix $$\A\$$, and a list (or vector) $$\u\$$ of length $$\n\$$ containing the numbers $$\1\$$ through $$\n\$$ (or $$\0\$$ through $$\n-1\$$). Your task is to reorder the columns and rows of the matrix $$\A\$$ according to the order specified in $$\u\$$.

That is, you will construct a matrix $$\B\$$ where the $$\(i,j)\$$-th element is the $$\(u(i),u(j))\$$-th element of $$\A\$$. You should also output the inverse of this action; that is, the (i,j)-th element of $$\A\$$ will end up at position $$\(u(i),u(j))\$$ in a new matrix $$\C\$$.

For example, given $$A = \begin{bmatrix} 11 &12& 13 \\ 21& 22& 23 \\ 31& 32& 33 \end{bmatrix},\quad u=\begin{bmatrix}3 & 1 & 2\end{bmatrix}$$

the output should be $$B = \begin{bmatrix}33 & 31 & 32 \\ 13 & 11 & 12 \\ 23 & 21 & 22 \end{bmatrix},\quad C= \begin{bmatrix}22 & 23 & 21 \\32 & 33 & 31 \\ 12 & 13 & 11 \end{bmatrix}$$

You may take input and output through any of the default I/O methods. You do not have to specify which matrix is $$\B\$$ or $$\C\$$, as long as you output both. You may assume $$\A\$$ only contains positive integers, and you may use 1- or 0-based indexing for $$\u\$$. You must support matrices up to at least size $$\64 \times 64\$$.

## Example

===== Input =====
A =
35     1     6    26    19    24
3    32     7    21    23    25
31     9     2    22    27    20
8    28    33    17    10    15
30     5    34    12    14    16
4    36    29    13    18    11
u=
3 5 6 1 4 2

==== Output =====
B =
2    27    20    31    22     9
34    14    16    30    12     5
29    18    11     4    13    36
6    19    24    35    26     1
33    10    15     8    17    28
7    23    25     3    21    32
C =
17    15     8    10    28    33
13    11     4    18    36    29
26    24    35    19     1     6
12    16    30    14     5    34
21    25     3    23    32     7
22    20    31    27     9     2

• Sandbox Sep 20 '19 at 20:01
• Can we output without the empty line here, that is, like this? (there is no ambiguity) Or, failing that, use 0 as separator? Sep 20 '19 at 20:10
• @LuisMendo Sure no problem. Sep 20 '19 at 20:32
• Is 1-indexing required for this? Can we use 0-indexing and input u = [2, 0, 1]? Sep 20 '19 at 23:50
• @ValueInk See the first sentence, [...] containing the numbers 1 through n (or 0 through n−1) Sep 21 '19 at 7:13

# R, 42 bytes

function(A,o)list(A[o,o],A[I<-order(o),I])


Try it online!

Takes A as a matrix and 1-based indexes o.

# MATL, 15 13 bytes

t3$)&Gw&St3$)


Inputs u, then A.

Outputs B, then C without a separator, as there is no ambiguity.

Try it online!

### Explanation

t     % Take input u implicitly. Duplicate u
3$) % Take input A implicitly. Index A with u as row and column indices &G % Push the two inputs again: u, A w % Swap &S % Push indices that would make u sorted. Call that v t % Duplicate v 3$)   % Index A with v as row as column indices. Display implcitly


# Octave, 33 bytes

@(A,u){A(u,u) A([~,v]=sort(u),v)}


Try it online!

Thanks to Luis for correcting an error and saving several bytes!

Basic indexing works here for both tasks, by defining a vector $$\ v \$$ equal to the permutation that undoes $$\ u \$$. That is, if $$\ u = ( 3, 1, 2 ) \$$ then the first element of $$\ v \$$ is 2, since 1 is in the second position of $$\ u \$$. This is accomplished with Octave's sort function.

# Python 3 with numpy, 51 45 bytes

lambda m,p:[m[x][:,x]for x in(p,p.argsort())]


Try it online!

-6 bytes thanks to @xnor

The function takes two arguments: a numpy matrix and a permutation vector having values from $$\0\$$ to $$\n-1\$$.

• 45 bytes as lambda
– xnor
Sep 21 '19 at 4:50
• @xnor Thanks ! I felt that it could be shortened in some way but the idea of using a for-loop did not come to my mind.
– Joel
Sep 21 '19 at 4:55

# Wolfram Language (Mathematica), 30 bytes

ii[[#,#]]&/@{#,Ordering@#}&


Try it online!

Input as f[A][u].

# PowerShell, 7873 71 bytes

($A,$u=$args)|%{$A[$u]|%{''+$_[$u]}$u=1..$u.count|%{$u.indexof($_-1)}}  # Jelly, 13 bytes ŒJṁ⁸ịⱮ⁹,Ụ¤œị⁸  Try it online! # J, 19 bytes (]/:~"1/:)"_ 1],:/:  Try it online! • Main verb ]/:~"1/: • The right most /: sorts the left arg (matrix) according to the order that would sort the right arg (specified order). This sorts rows. • Now that result get sorted /:~"1 again according to the order specified ]. But this time we're sorting with rank 1, ie, we're sorting each row, which has the effect of sorting columns. • ],:/: We apply the above using both the order specified ] and the grade up of the order specified /:. This gives us the 2 results we want. • Nice! I was thinking about applying sort+transpose twice, but will end up longer. Sep 22 '19 at 7:32 • u is allowed to be 0-based, so sort (/:) could be indexing ({) with swapped args – ngn Sep 22 '19 at 10:01 # JavaScript (Node.js), 7770 68 bytes a=>g=(u,v=[])=>[u.map((i,x)=>u.map(j=>a[i][j],v[i]=x)),v&&g(v,0)[0]]  Try it online! • It took me a minute to figure out what v was. It's neat how you found a use for non-strict mode silent failure of property assignment to a primitive value, and used that for your recursion base-case. Sep 23 '19 at 15:34 # APL (Dyalog Extended), 12 bytesSBCS Full program. Prompts for $$\u\$$ and then $$\A\$$. Prints $$\C\$$ next to $$\B\$$, separated by two spaces ⌷∘⎕¨⍋¨⍛⍮⍨⍮⍨⎕  Try it online! ⎕ prompt for $$\u\$$; [3,1,2] ⍮⍨ juxtaposition-selfie; [[3,1,2],[3,1,2]] ⍋¨ permutation-inversion of each; [[2,3,1],[2,3,1]] ⍛ then ⍮⍨ juxtapose with itself [[[2,3,1],[2,3,1]],[[3,1,2],[3,1,2]]] ⌷ reorder ∘ the value of ⎕$$\A\$$ as inputted ¨ using each pair as a set of orders, one order per axis # J, 17 16 15 14 bytes -1 thanks to @Jonah ([{"1{)~(,:/:)  Try it online! • Nice! You can get down to 14 with ([{"1{)~(,:/:): Try it online! Sep 22 '19 at 18:08 • Btw, random question: I noticed you golf (very well) in J, APL and K. Curious which you prefer overall? Also I seem to recall you saying you used K professionally, am I remembering that right? Sep 22 '19 at 18:23 • @Jonah if i must choose one, that would definitely be k (pls ping me in the k chat if you wanna know the reasons), but i do enjoy golfing in all array languages. sadly, i'm not one of the lucky few who can have a k-language job – ngn Sep 22 '19 at 18:54 # Charcoal, 24 bytes Ｅ⟦ηＥη⌕ηκ⟧Ｅθ⪫Ｅθ§§θ§ιμ§ιξ  Try it online! Link is to verbose version of code. 0-indexed. Note: Trailing space. Explanation:  η Input u Ｅ Map over elements ⌕ Index of κ Current index in η Input u η Input u Ｅ⟦ ⟧ Map over u and its inverse θ Input A Ｅ Map over elements θ Input A Ｅ Map over elements θ Input A § Indexed by ι Current vector § Indexed by μ Row index § Indexed by ι Current vector § Indexed by ξ Column index ⪫ Join with spaces for readability Implicitly print  # Kotlin, 213 bytes {a:List<List<Int>>,u:List<Int>->val s=u.size for(l in List(s){r->List(s){c->a[u[r]][u[c]]}})println(l.joinToString(" ")) for(l in List(s){r->List(s){c->a[u.indexOf(r)][u.indexOf(c)]}})println(l.joinToString(" "))}  Try it online! # APL+WIN, 21 bytes Prompts for input of u followed by a. Outputs b immediately over the top of c with no separator: (a←⎕)[u;u←⎕]⋄a[⍋u;⍋u]  Try it online! Courtesy of Dyalog Classic # Perl 5, 79 bytes sub{$[=1;($A,$u)=@_;@$v[@$u]=(1..@$u);map{$x=$_;[map[@$_[@$x]],@$A[@$x]]}$u,$v}  Try it online! # Jelly, 12 11 13 bytes +2 :( to fix cases when B = C ṭþœị¥@Ƭị@2,0  A dyadic Link accepting a list of lists, A (n by n), on the left and a list of the first n integers on the right, u, which yields a list of lists of lists, [B, C]. Try it online! ### How? ṭþœị¥@Ƭị@2,0 - Link: A, u Ƭ - collect up while the results are no longer unique, applying: ¥@ - last two links as a dyad with swapped arguments:  - use left (u) as both arguments of: þ - outer product with: ṭ - tack œị - multi-dimensional index into last result (starting with A) ...at the end of the Ƭ-loop we have [A,B,...,C] or [A] if A=B=C or [A,B] if B=C but A!=B 2,0 - literal pair [2,0] @ - with swapped arguments: ị - index into (1-based & modular) -> [B,C] or [A,A]=[B,C] if A=B=C or [B,B]=[B,C] if B=C  # q, 26 bytes {Y:iasc y;(x[y;y];x[Y;Y])}  iasc returns indexes to sort it's argument. # Clean, 91 bytes import StdEnv$a u=map(\l={{a.[i,j]\\j<-l}\\i<-l})[u,[k\\i<-[0..]&_<-u,j<-u&k<-[0..]|j==i]]


Try it online!

Defines \$ :: {{a}} [Int] -> [{{a}}] (used with a = Int) taking an array of arrays and a list of zero-based indices, returning a list of arrays of arrays containing B and C.

# Python 3, 91 bytes

lambda a,u:[[[a[y][x]for x in t]for y in t]for t in[u,[u.index(i)for i in range(len(u))]]]



Try it online!

Takes parameters as a 2D and 1D list and returns a list containing two 2D lists B and C. I'm not sure if there's a cleaner way to do all the for-loops.

# C++ (gcc), 148 142 bytes

#import<queue>
#define q[o[i/z]*z+o[i%z]]
using V=std::vector<int>;int f(V m,V o,V&r,V&R,int z){int i=z*z;for(r=R=V(i);i--;r[i]=m q)R q=m[i];}
`

Try it online!

Thanks to @ceilingcat suggestion to use #import <queue> instead of <vector> which mysteriously brings std::vector

• @ceilingcat now I see that import queue gives me access to vector.. Is it compiler dependant? I'm trying to search information about this but found nothing Dec 10 '19 at 5:57
• Tips for golfing in C++ Dec 10 '19 at 9:59