# How to find a matrix inside a larger matrix?

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Jose Grimaldo on 28 Apr 2020
Answered: SaiDileep Kola on 8 May 2020
Im trying to find a smaller matrix inside a larger matrix using explicit loops. I know there is functions and probably easier to locate if there is matching matrix inside a larger matrix. I want to locate matrix M, inside matrix A. How can i set up the for loops since both matrix are different size?
A,6x6 matrix
A=[8 2 9 7 6 7;
9 5 4 1 7 0;
1 9 8 6 7 2;
5 4 1 0 3 0;
9 8 6 8 6 0;
4 1 0 5 4 1];
B, 3x3 matrix
M=[5 4 1; 9 8 6; 4 1 0];
[r,c]=size(A)
[row,col]=size(M)

jessupj on 28 Apr 2020
if you're beholden to using loops:
you can iterate only a portion of the rows, columnns that fit inside the larger matrix, and test whether the submatrix starting at the appropriate row,col (ie. upper left index) matches the desired matrix. if it matches, store the row,col
match_ul=[]
for rr = 1:(r-row+1)
for cc = 1:(c-col+1);
if isequal( A( rr:(rr+row-1), cc:(cc+col-1) ), M );
match_ul = [match_ul; [rr cc] ];
end
end
end
then you can iterate over the rows of this list to test:
k=1
A( match_ul(k,1)+(0:row-1), match_ul(k,2)+(0:col-1))
ans =
5 4 1
9 8 6
4 1 0
k=2
A( match_ul(k,1)+(0:row-1), match_ul(k,2)+(0:col-1))
ans =
5 4 1
9 8 6
4 1 0
Rik on 28 Apr 2020
Why do you insist on loops? Or is this a homework assignment?

SaiDileep Kola on 8 May 2020