Remove rows in matrix A if not member of matrix B

Hi all,

I have two matrices, and I am trying to remove rows in one for numbers which are not part of the other. Below is my attempt:

trials;
for x = 1:256
    if ~ismember(trials,x);
       rows2remove=[x];
       imagesAll(rows2remove,:)=[];
    end 
%     imagesCut = 
end

But it is not working and I get the following error message (I guess because it keeps deleting rows?):

Matrix index is out of range for deletion.

Error in samplecode (line 21)
       imagesAll(rows2remove,:)=[];

Please could anyone help me with my code? Many thanks

Answer 1

[Stephen23]

MATLAB works best when working on entire arrays at once, whereas using a loop and removing rows like that is inefficient and not recommended.

>> tryIdx = [2,5,7,8];
>> allIdx = 1:size(imagesAll,1);

Method one: setdiff:

>> idx = setdiff(allIdx,tryIdx)
idx =
   1   3   4   6   9

Method two: ismember:

>> idx = ~ismember(allIdx,tryIdx)
idx =
1  0  1  1  0  1  0  0  1

And then simply:

imagesAll(idx,:)

[Stephen23]

Lulu Dulac's "Answer" moved here:

Hi! Thank you a lot for your answer. I am not 100% sure that's what I want to do though, sorry for being unclear:

imagesAll is a matrix with 256 rows and 1 column, and trials is a matrix with 253 rows whose values go from 1 to 256, with 3 deletions in between, and 1 column. Some rows in trials had to be deleted, and I am trying to find the values of the deletions, so that I can delete the corresponding rows in imagesAll. So for instance if value 3 is missing in trials, I would like to delete the 3rd row in imagesAll, and then save the shorten imagesAll.

Does this make any sense? thanks a lot for your help

[Stephen23]

@Lulu Duvec: given your new description, it seems that simple indexing will do what you want:

imagesAll(trials,:)

[Lulu Dulac]

Thank you a lot!