logical operation on a column within a matrix

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Christopher
Christopher 2014-2-28
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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lvn
lvn 2014-2-28
This should work:
POS(POS(:,2)==0,2)=1;
  2 个评论
Christopher
Christopher 2014-2-28
Thanks. But why does it work? Where can I read more about this type of referencing?
lvn
lvn 2014-2-28
编辑：lvn 2014-2-28
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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