MATLAB Answers

Replace elements of matrix

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Ionut  Anghel
Ionut Anghel on 3 Mar 2013
Commented: Mike Lynch on 24 Nov 2020 at 15:45
I have the folowing:
vector=[1 3 8 9];
matrix=[ 100 1 5 9 6; 100 10 13 3 8; 100 9 10 1 4; ];
% I want to search and replace the vector element with "0"in the matrix (i.e new matrix should be : Newmatrix=[ 100 0 5 0 6; 100 10 13 0 0; 100 0 10 0 4; ]; )
The script is:
Newmatrix=zeros(size(matrix));
for i=1:numel(matrix)
for j=1:length(vector)
valvect=vector(j);
if matrix(i)==valvect
Newmatrix(i)=0;
else
Newmatrix(i)=matrix(i);
end
end
end
The results is not the desired one but:
Newmatrix=100 1 5 0 6
100 10 13 3 8
100 0 10 1 4
So what I'm doing wrong?
Thank you

  1 Comment

Mike Lynch
Mike Lynch on 24 Nov 2020 at 15:45
The accepted answer or changem are cleaner and more compact, but for the code you wrote the addition of a "break" should fix the problem.
if matrix(i)==valvect
Newmatrix(i)=0;
break
else ...

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Accepted Answer

Youssef  Khmou
Youssef Khmou on 3 Mar 2013
hi, try ;
F=matrix;
for i=1:length(vector)
F(F==vector(i))=0;
end

  5 Comments

Show 2 older comments
Stephen Cobeldick
Stephen Cobeldick on 12 Mar 2019
"In general, you should be avoiding loops in MATLAB"
Why ?
Joel Bly
Joel Bly on 11 Apr 2019
@BJAnderson how do I change the 1s that come from the ismember function back to the values they are referring to, without changing the zeros?
Walter Roberson
Walter Roberson on 12 Apr 2019
If you use ismember() with two outputs, then the second output is the index at which the element in the first parameter appears in the second parameter. In places that the element does not occur, then the returned value will be 0 there. You can select just the valid indices by indexing the second output by the first output.
[is_it_there, idx] = ismember(A, B);
idx(is_it_there)

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More Answers (4)

Metin Ozturk
Metin Ozturk on 1 Aug 2018
The more vectorized and easier way to do this could be as follows:
new_matrix = changem(matrix,zeros(length(vector),1),vector);

  2 Comments

BJ Anderson
BJ Anderson on 12 Mar 2019
YES YES YES. It looks like hardly anyone knows about changem...I see similar questions asked and typically the "solution" involves a loop. A loop in MATLAB is the first sign you're doing something wrong.
changem covers so many situations, and it is elegant and concise, not to mention avoids the risk of re-reassigning an element.
changem is almost always the right anwer. Let the world know!
Thanks Metin for helping to spread the word.
BJ Anderson
BJ Anderson on 12 Mar 2019
A quick update on changem:
Sadly, if one inspects the actual code within changem, it functions as a loop. While it is a handy one-liner, it does not have the time-savings of moving from a looped function to an matrix-operation function.

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Bruno Luong
Bruno Luong on 12 Mar 2019
F = matrix .* ~ismember(matrix,vector)

  1 Comment

Bruno Luong
Bruno Luong on 13 Mar 2019
A variant is:
matrix(ismember(matrix,vector)) = 0

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Walter Roberson
Walter Roberson on 3 Mar 2013
Suppose you set Newmatrix to 0 because matrix matched vector(1). Now what happens when you go on to the next j to test if matrix matched vector(2) ?

  0 Comments

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Image Analyst
Image Analyst on 3 Mar 2013
Try it this way:
newMatrix = matrix % Initialize
for k = 1 : length(vector)
newMatrix(matrix==vector(k)) = 0
end

  1 Comment

Ionut  Anghel
Ionut Anghel on 3 Mar 2013
Thanks It is working as well

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