# What can be a reverse permutation formula of n*m matrix

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dani elias le 7 Sep 2020
Commenté : dani elias le 8 Sep 2020
The first row remain as it is, but from the second to the end it changed as shown below: Given A as original matrix, B the result matrix. the first row at B remain as it was in A, but at second row, the first element is obtained After add the first element at row above and it's last element in corresponding row, such as B[2,1]=A[1,1]+A[2,4] which is 9=5+4. But the second element at row two is B[2,2]=A[2,1]+A[1,2] which is 13=10+3. So 13=11+2,4=3+1, again for next row B[3,1]=8+10, B[3,2]= 6+11 etc..
A=[4 3 2 1;10 11 3 5; 6 4 7 8; 9 10 40 5]
B=[ 4 3 2 1; 9 13 13 4; 18 16 7 12; 11 23 17 48].
I need the reverse formula to its original matrix.. Assume Matrix A is of size 256*256 or 512*512
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Matt J le 8 Sep 2020
I think you have incorrect entries in B(3:4,2). I think the real B matrix should be,
B =
4 3 2 1
9 13 13 4
18 17 7 12
11 13 17 48

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### Réponse acceptée

Matt J le 8 Sep 2020
Modifié(e) : Matt J le 8 Sep 2020
Assuming A and B are always square,
n=length(A);
E=speye(n);
S=circshift(E,[1,0]);
T=kron(S,E)+kron(E,S);
T(1:n,1:n)=E;
T(1:n,n+1:end)=0;
fun=@(Z) reshape(Z.',[],1);
ifun=@(Z) reshape(Z,n,n).';
A=ifun(T\fun(B))
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Matt J le 8 Sep 2020
B=ifun(T*fun(A));
dani elias le 8 Sep 2020
Thank you. I do appreciate.

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### Plus de réponses (1)

Matt J le 8 Sep 2020
This version is also quite fast - maybe even faster than my other, fully vectorized answer.
n=length(A);
%% Forward
tic;
B=A;
B(2:end,:)=A(1:end-1,:)+circshift(A(2:end,:),[0,1]);
toc
%% Inverse
A=B;
tic
for i=2:n
A(i,:)=circshift(A(i,:)-A(i-1,:),[0,-1]);
end
toc
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dani elias le 8 Sep 2020
Thank you. I do appreciate your contributions.

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