making column of zero matrix equal to one based on another matrix

Hi ,

I have a zero matrix (A) of size mxn where m in number of observation and n number of feature. I have another non zero matrix (B) of size (mx1) where numbers respresent column in matrix A which should be replaced by one.

How to do this ?

Answer 1

[Jos (10584)]

Perhaps you want to set in a row K of A, the entry in column B(k) to one?

%data
  A = zeros(3,3)
  B = [3; 1; 2]       % column index
% engine
  R = 1:numel(B)      % row index
  idx = sub2ind(size(A), R, B) % convert to linear indices into A
  A(idx) = 1

Note that the statement A(R,B) = 1 does not do what you might think it should do.

[Sukuchha]

thnks everyone, may be i should have phrased it more clearly. Jos understood what i wanted and it worked. Thanks every one. I knew this function sub2ind but it didnot cross my mind.

[Image Analyst]

Strange, I just copied and pasted that code and it errored out:

Error using sub2ind (line 47)
The subscript vectors must all be of the same size.
Error in test2 (line 6)
  idx = sub2ind(size(A), R, B) % convert to linear indices into A

[Sukuchha]

yes, the error you got is right because R in above is [1,3] and B is [3,1].

The exact code that i adapted is: %data

A = zeros(3,3)
  B = [3; 1; 2]       % column index
% engine
  R = 1:numel(B)      % row index
  idx = sub2ind(size(A), R, B') % convert to linear indices into A
  A(idx) = 1

thanx once again.

[Jos (10584)]

@Image Analyst My apologies, I should have tested the code properly, rather than write it down theoretically.

@Sukuchha, I am glad you solved this issue yourself.

[Image Analyst]

No problem - I sometimes post untested code off the top of my head also, and mistakes do happen. Actually I'm kind of impressed that you (1) actually figured out what he meant, and (2) thought up that somewhat cryptic solution off the top of your head.

Answer 2

[Image Analyst]

Did you try

A(:, B) = 1;

[Sukuchha]

hi image analyst, your suggestion didnot work.

A = zeros(3,3)
B = [3;1;2]
A(:,B)=1;

As you can see every element in A is one.

[Paul]

Well, that is what you said you want. Since all three the column indices are present in B, all values of A are set to 1.

[Image Analyst]

I agree with Paul. You said any column number of A that shows up in B should have that entire column of A set to 1. For example:

A = randi(9, 5, 8) % Random integers
B = [1,3,7]; % Set columns 1, 3, and 7 to all 1's.
A(:, B) = 1