logical operation on a column within a matrix

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

Answer 1

[lvn]

This should work:

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

[Christopher]

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

[lvn]

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