# How to find indices of a rectangular region inside big matrix? | Efficiently

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JAI PRAKASH on 2 Nov 2018
Edited: JAI PRAKASH on 24 Nov 2018
How can indices of elements belong to a rectangular region can be found?
A = [4 4 4 4 4 4 4
4 1 1 1 1 3 0
4 1 3 3 1 3 0
4 1 3 3 1 3 0
4 1 1 1 1 3 0
4 4 4 4 4 4 4];
Input: Matrix's size[height, width] , [Row_start Row_end], [Col_start Col_end]
Output: [21 22 23 27 28 29 33 34 35]
Why efficiently : to do same for multiple combinations of rows & columns
Thank you
JAI PRAKASH on 2 Nov 2018
Sorry if I am not clear in my question.
I want linear indices of all the elements present in rectangular region.
e.g

Bruno Luong on 2 Nov 2018
Edited: Bruno Luong on 2 Nov 2018
recidx = (Row_start:Row_End)' + height*(Col_start-1:Col_end-1)
Bruno Luong on 3 Nov 2018
James's solution is very well. I doubt I can do better.

Geoffrey Schivre on 2 Nov 2018
Hello,
I'm not sure if it's efficient enough but try :
p = nchoosek([Row_start:Row_end,Col_start:Col_end],2);
idx = unique(sub2ind(size(A),p(:,1),p(:,2)));
Geoffrey
JAI PRAKASH on 2 Nov 2018
'nchoosek' is new to me
Thanks

Caglar on 2 Nov 2018
Edited: Caglar on 2 Nov 2018
function result = stack (A,row_start,row_end,col_start,col_end)
% A = [4 4 4 4 4 4 4
% 4 1 1 1 1 3 0
% 4 1 3 3 1 3 0
% 4 1 3 3 1 3 0
% 4 1 1 1 1 3 0
% 4 4 4 4 4 4 4];
% row_start=3; col_start=4;
% row_end=5; col_end=6;
height=(size(A,1));
result=(row_start:row_end)+(height)*((col_start:col_end)'-1);
result=transpose(result); result=result(:);
end

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