Re: bin2dec behavior different from Matlab?

[Daniel J Sebald]

On 03/16/2012 10:15 PM, Daniel J Sebald wrote:

> octave:19> tvec = char (randi ([48 49], 1e6, 10));
> octave:20> tvec(randi(1e7, 1e6,1)) = " ";
> octave:21> ctvec = cellstr(tvec);
> octave:22>
> octave:22> cpuzero = cputime();
> octave:23> junk = bin2dec(tvec);
> octave:24> cputime() - cpuzero
> ans = 423.04
> octave:25>
> octave:25> cpuzero = cputime();
> octave:26> junk = test_bin2dec(ctvec);
> octave:27> cputime() - cpuzero
> ans = 68.398
>
> So two questions come to mind from this:
>
> 1) The cellfun() based approach is five times faster than the version
> 3.2.4 approach (granted, I left out several things), and the conversion
> ctvec = cellstr(tvec) is relatively fast compared to these benchmark
> times so maybe converting to a cell array approach is better. (John's
> point I believe.) There may be a better approach than the power()
> routine, but I was just trying to illustrate cellfun().
>
> 2) What machine are you using that is so fast?!

I had the vector backward in my little routine, anyway...

OK, now I'm getting in tune here.  The 3.2.4 bin2dec uses looping so it 
is no wonder the it takes 423 seconds.  I downloaded just the bin2dec() 
and base2dec() scripts and now get an amazing speed up to 1.6 seconds.  Wow.

Well, that leads to the question, Why is cellfun() so noticeably slow? 
Is it just in 3.2.4 that cellfun() is slow?  John, any reason that 
something like

function retval = test_bin2dec (d)
  retval = cellfun('binchar2dec', d);
endfunction

function retval = binchar2dec(s)
  s = s(!isspace(s));
  retval = (s-'0')*(2.^[length(s)-1:-1:0])';
endfunction

runs about 40 times slower than the matrix multiply method Rik is using? 
 I'm thinking, sure it might be slower, but by a factor of two or 
three, not 40.

cellstr() would fall in this category too.  That takes 1.0 second.  I 
understand that the code is creating cells and there is memory 
allocation, but the thing about cellstr() is that we know the input is a 
string of a fixed length for *all* the cells so internally cells could 
be created with one big hunk of memory.  All the cells look the same 
except for a different string.  cell2str should be fast too.

Sorry if I've missed something with being stuck on 3.2.4 still.

Dan