Find given row in a matrix

Question

I have an m by n matrix in MATLAB, say M. I have an n-element row vector, i.e. a one by n column matrix, say X.

I know X is a row somewhere in M. How can I find the index in M?

score 66 · Accepted Answer · edited Jun 20 '20 at 09:12

66

EDIT:

gnovice's suggestion is even simpler than mine:

[~,indx]=ismember(X,M,'rows')

indx =

     3

FIRST SOLUTION:

You can easily do it using find and ismember. Here's an example:

M=magic(4);        %#your matrix

M =

    16     2     3    13
     5    11    10     8
     9     7     6    12
     4    14    15     1

X=[9 7 6 12];      %#your row vector

find(ismember(M,X),1)

ans =

     3

edited Jun 20 '20 at 09:12

Community

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answered Jun 02 '11 at 03:07

abcd

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You could modify your solution slightly using the `'rows'` argument to ISMEMBER to remove the need for FIND: `[~,index] = ismember(X,M,'rows')` – gnovice Jun 02 '11 at 03:19
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I think 'rows'keyword is still needed in find(ismember(M,X.'rows'),1) – Manish Dec 14 '12 at 17:23
ismember is slow. is there a faster alternative? – JustCurious Apr 27 '16 at 14:10

score 8 · Answer 2 · answered Mar 08 '15 at 11:32

8

Before I learned of ismember, I used to do:

index = find(all(bsxfun(@eq, M, X), 2));

But using ismember(X, M, 'rows') is definitely preferable.

answered Mar 08 '15 at 11:32

buzjwa

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This is a nice solution since it returns ALL rows of the matrix in which X occurs (unlike the marked answer). – Erik M Apr 01 '15 at 01:11
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This solution is far faster than the accepted one (about 20 times on a big scale - tested), and it returns all rows, not just the largest index. – Max Aug 08 '16 at 11:18
Interesting. I think I checked for a performance difference before adding this answer and didn't see any, but maybe I didn't use large enough arrays. – buzjwa Aug 08 '16 at 13:18

score 3 · Answer 3 · answered Jan 10 '14 at 21:10

Another solution that returns a row index for each occurrence of X is

find(sum(abs(M-ones(rows(M),1)*X),2)==0)

Also, this solution can be easily adapted to find rows that are within threshold of X as follows (if numerical noise is an issue)

tolerance = 1e-16; %setting the desired tolerance
find(sum(abs(M-ones(rows(M),1)*X),2)<tolerance)

score 1 · Answer 4 · answered Jun 02 '11 at 02:55

This is a non-loop version. It is only suitable, if M (your matrix) is not very large, ie. n and m are small. X is your row:

function ind = findRow(M,X)
    tmp = M - repmat(X,size(M,1),1); 
    ind = find(tmp,1); 
end

If M is too large, it might be faster, to iterate the rows of M and compare every row with your vector.

@Edit: renamed variables to match the names used in the question.

Find given row in a matrix

4 Answers4

EDIT:

FIRST SOLUTION:

Linked

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