Re: Zero entries in one column of a matrix

[Henry F. Mollet]

Thanks for clear explanation.

Can I now also look at it as follows?
In M (: , 2) the ":" operator? says use all entries/rows in the second
column of M;
whereas in M (M(:,2) ~= 0 , 2), the implied ":" operator? looks at the
statement row by row and when it sees a boolean false (=0) says this is
*not* an index and therefore the entry (which was an actual 0) has to be
skipped?
Henry


on 3/31/04 11:54 PM, Miquel Cabanas at address@hidden wrote:

> hello,
> 
> On Wed, Mar 31, 2004 at 10:40:05AM -0800, Henry F. Mollet wrote:
>> 
>> But how does the elimination of the zero entries (in the second
>> column of M) work in M (statement, 1) producing col vector of
>> length 100-25=75, and M (same statement, 2) producing col vector
>> of length 75 also?
> 
> because *statement* (i.e. "M(:,2) ~= 0" in your example) produces
> a boolean matrix, which is regarded as a matrix of indexes and
> handled by octave differently than standard matrices or vectors.
> 
> octave:1> M = [ 1 1; 2 0; 3 1; 4 1; 5 0; 6 1; 7 1; 8 1; 9 0]
> octave:2> M_boolean = M(:,2) ~= 0;
> octave:3> whos
> 
> *** local user variables:
> 
> prot  type                       rows   cols  name
> ====  ====                       ====   ====  ====
> rwd  matrix                        9      2  M
> rwd  bool matrix                   9      1  M_boolean
> 
> note though that as soon as you convert your bool-matrix into a
> plain-matrix this feature is gone,
> 
> octave:4> M_plain = M_boolean .+ 0;
> octave:5> whos
> 
> *** local user variables:
> 
> prot  type                       rows   cols  name
> ====  ====                       ====   ====  ====
> rwd  matrix                        9      2  M
> rwd  bool matrix                   9      1  M_boolean
> rwd  matrix                        9      1  M_plain
> 
> octave:6> M( M_plain , 1);
> error: invalid row index = 0
> 
> 
> Miquel
> 
> 



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