How to remove columns from matrix?

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Sahar abdalah
Sahar abdalah on 15 Apr 2015
Edited: ChangYuan on 16 Apr 2015
I have a matrix A and I want to remove from this matrix each column that contain the value -1.
A= [-1.192 -1.020 -1 -1.050 -1 -1 -1.070;
-1.213 -1.096 -1 -1.045 -1 -1 -1.102;
-1.036 -1.061 -1 -1.085 -1 -1 -1.137;
-1.176 -1.120 -1 -1.004 -1 -1 -1.115;
-1.124 -1.034 -1 -1.073 -1 -1 -1.134;
-1.202 -1.145 -1 -1.078 -1 -1 -1.057;
-1.127 -1.023 -1 -1.056 -1 -1 -1.066]
the size of A is 7*7 and I want to have a matrix 7*4 after removing colums? How can I do please?

Accepted Answer

Star Strider
Star Strider on 15 Apr 2015
A more general solution:
A = reshape(A(~arrayfun(@isequal, A, -ones(size(A)))),size(A,1),[]);
It first finds a logical array of vectors that are equal to -1 using arrayfun, takes the values of the matrix that are not those values and creates a new vector from them. It then takes that vector and reshapes it to the row size of the original matrix to create the new ‘A’ matrix.
  5 Comments
pfb
pfb on 15 Apr 2015
I did test it. Here is what I get
>> A = [-1 2 -1 3 -1; 2 -1 -1 0 1];
>> A==-ones(size(A))
ans =
1 0 1 0 1
0 1 1 0 0
>> arrayfun(@isequal, A, -ones(size(A)))
ans =
1 0 1 0 1
0 1 1 0 0
It seems to me that they are equivalent, at least for this minimal example. I jotted down the matrix randomly, making sure that at least one column was composed of -1 alone. Maybe I was lucky. I have to admit I did not embark in systematic testing. Anyway, that is why I asked.

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More Answers (3)

ChangYuan
ChangYuan on 15 Apr 2015
Edited: ChangYuan on 16 Apr 2015
% A(:, any(A == -1)) = [] %% Please try this.
% revised 'any' to 'all'. 'any' means the columns in which there is any '-1'; 'all'means the columns in which they are all '-1'.
A(:, all(A == -1)) = []
  2 Comments
ChangYuan
ChangYuan on 16 Apr 2015
Yeah. Just a while after I shut down my PC last night, I realized it should be 'all'.
A(:, all(A == -1)) = []
Thanks!

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pfb
pfb on 15 Apr 2015
Edited: pfb on 15 Apr 2015
for that particular matrix you want
B = A(:, [1 2 4 7]);
If this is a more general problem, that's a bit more tricky. Should the column consist entirely of -1 or just one single -1 is sufficient for eliminating it?

Sahar abdalah
Sahar abdalah on 15 Apr 2015
My goal is to remove the columns that entirely contain the value -1. the problem is more general, I have a matrix A: size m * n with m number of test samples and n is the label of the class . The size of A in my case is 1000 * 1181. I want to remove the column and also their index.
  3 Comments
Star Strider
Star Strider on 15 Apr 2015
What do you mean by ‘remove the index’? The resulting matrix will be smaller, so you will have to address its columns differently than you did in the original matrix. You will have to rewrite your code to do that.

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