# Using a column vector of indices to replace values in a matrix

6 views (last 30 days)
Teddy Fisher on 13 Dec 2019
Commented: sruizpp on 18 Nov 2020
Hello,
I have a column vector, C, (55x1 double) which are the indices of my values of interest in Matrix, M which is 370x29 double.
I want to replace all the values in M that are not these values of interest with 0.
I am trying to use C to index into M and replace ~C with 0.
However when I try
M(~C)=0
all it does is change all the values in M to just different values and I'm not sure why.
When I try to create a new Matrix
N=M(~C==0) it returns a columnn vector with just the first 55 values in the first column of M.
Does anyone know how I can replace all the elements in M that are not listed in C with 0? I want my output to be another 370x29 double matrix.
Thanks!
##### 3 CommentsShow 1 older commentHide 1 older comment
Teddy Fisher on 13 Dec 2019
I had another Matrix, R, that was 0s and 1s, also 370x29 double
I used
C=find(R) to get the indexes of the 1s in R (which correspond to the values I want in M)
So the values in C are numbers (between 1-10730), not [row,column] format
Is there any way I can use C to index into M ?
Fangjun Jiang on 13 Dec 2019

Fangjun Jiang on 13 Dec 2019
Edited: Fangjun Jiang on 13 Dec 2019
C=[1,2,3]';
M=magic(5);
index=setdiff(1:numel(M),C);
M(index)=0
Teddy Fisher on 13 Dec 2019
Thank you so much!! I can't tell you how much this helped
sruizpp on 18 Nov 2020
c00L

Guillaume on 13 Dec 2019
Edited: Guillaume on 13 Dec 2019
Well, now you have explained how you got C, a much simpler method than the accepted one is not to create C and use:
M(~R) = 0; %that's all that is needed!
As usual in matlab, find is completely unnecessary and what you were trying to do initially would have worked had you kept your original logical vector.
Fangjun Jiang on 13 Dec 2019
indeed!

### Categories

Find more on Matrix Indexing in Help Center and File Exchange

R2017b

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!