Re: bin2dec behavior different from Matlab?

[Daniel J Sebald]

On 03/16/2012 12:52 PM, Rik wrote:
> On 03/16/2012 04:53 AM, Jordi Gutiérrez Hermoso wrote:
> > On 16 March 2012 01:30, Daniel J Sebald<address@hidden>  wrote:
> > > This change happened with changeset 11172:7e8ce65f73cf shown here
> > >
> > > http://hg.savannah.gnu.org/hgweb/octave/rev/7e8ce65f73cf
> > >
> > > which overhauled the string number conversion routines.  (Maybe the intent
> > > was to move white space removal inside base2dec.)
> > >
> > > In my opinion tossing away white space makes better sense than treating
> > > white space as zeros.
> > >
> > > I also suggest adding a test to the file.  There currently are several, but
> > > none that test the behavior of white space.  E.g.,
> > >
> > > %!assert (bin2dec ("1 0 1"), 5)
> > This seems like a clear unintentional regression. Can you prepare a changeset?
> 3/16/12
>
> Daniel,
>
> The change is unintentional, however the fix is not absolutely
> straightforward.  Matlab's version of the function is quite limited in that
> it works on just a single string.  Octave's version also accepts character
> matrices (one string per row) and cell array of strings.  In base2dec the
> space character is treated as 0 so that the following will work
>
> bin2dec ([" 101";
>                  "1101"])
>
> For the first string in the char matrix , the leftmost value is 2^3
> (determined by position) * 0 (value of space) so that any spaces used for
> padding out a character matrix do not influence the resulting conversion.

I was wondering if this sort of construct tied in somehow.  The thing 
is, that use of strings is sort of falling out of favor because it was 
always a little clumsy to work with character arrays.  The cell array 
construct is preferred.


> The code for 3.2.4 handled "1 0 1" by using a for loop over every row of
> the character matrix to remove spaces BEFORE calling base2dec (str,2).
> Looping in Octave is abysmally slow and not the way to do this.  Even if we
> have to go back to some sort of looping structure it would be better to use
> regexprep, arrayfun, or cellfun.

Well, my philosophy for optimization is to optimize the method which is 
the most likely to be used.  Since the character matrix approach isn't 
so common anymore, optimizing that is lower priority in deference to a 
more often used technique.  Plus, the for loop is only slow if it is in 
fact used.  Also, if someone is using the string matrix approach, I 
would guess there is somewhere else in the person's code that is just as 
inefficient as the bin2dec and base2dec routines might be on character 
matrices.


> One option might be to write a custom string function that both right
> justifies and removes excess spaces.  This would replace this part of
> base2dec.m starting at line 84.
>
> ## Right justify the values before anything else.
> s = strjust (s, "right");

Rather than redundantly call strjust and s = s(s != ' '), the latter 
would be better.


> This hypothetical function, strjustsqueeze [justify and squeeze] can be
> based on strjust.m which is efficient.
>
> Perhaps you can investigate and propose some possible solutions and we can
> take it from there.

I would say to simplify as much as possible with string cell functions 
in mind as the most likely scenario.  I would guess that putting the 
white space removal inside base2dec makes the most sense if there are 
other string/number conversion routines utilizing base2dec.  Otherwise, 
leave it in bin2dec with the idea that base2dec remains lean and fast.

Dan