logical operation on a column within a matrix

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Christopher 2014-2-28

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https://ww2.mathworks.cn/matlabcentral/answers/119566-logical-operation-on-a-column-within-a-matrix

编辑： lvn 2014-2-28

I have the m-by-n matrix POS

I want to focus on a single column and perform an element-wise logical operation. For instance, turning all zero's to ones:

POS(POS(:,2)==0)=1;

However, this does nothing. On the other hand, this works on the whole matrix:

POS(POS==0)=1;

But, obviously, I just want to perform this operation on column 2. What am I missing? I do not want another line of code since speed is very important here.

Even more confusing, the following works on the first column, but not on the second:

 POS(POS(:,1)==1)=2;  % works
 POS(POS(:,2)==1)=2;  % not working

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Answer 1

lvn 2014-2-28

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https://ww2.mathworks.cn/matlabcentral/answers/119566-logical-operation-on-a-column-within-a-matrix#answer_126559

在 MATLAB Online 中打开

This should work:

POS(POS(:,2)==0,2)=1;

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Christopher 2014-2-28

Thanks. But why does it work? Where can I read more about this type of referencing?

lvn 2014-2-28

编辑：lvn 2014-2-28

在 MATLAB Online 中打开

filter=(POS(:,2)==0)

gives you a vector (of 0's and 1's).

POS(filter)=1

normally assumes 'filter' to be a matrix. This is why filter=(POS==0) gives you the expected answer. What you want is just to change the second column, hence you only want to target the second column:

POS(filter,2)=0

This tells matlab to change only the second column, using the filter as argument. Note sure this is very clearly explained, but read more here http://www.mathworks.com/help/matlab/math/matrix-indexing.html

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