Re: vectorization quiz

[Francesco Potortì]

>> I thought a little, but I do not see the way of vectorising this.  It's
>> a loop that is run only few times, but is called very frequently.
>> Essentially, there are few 2-D maps stacked one over the other which
>> must be shifted each by a variable amount and then summed to obtain a
>> new 2-D map.  Here is the code:
>>
>>    for mi = 1:m
>>      llka += llkm(by+offm(mi,1), bx+offm(mi,2), mi);
>>    endfor
>>
>> where:
>> m          ==>     5
>> size(llka) ==>     156 222
>> size(llkm) ==>     186 252 5
>> by         ==>     16:172
>> bx         ==>     16:238
>> size(offm) ==>     5 2
>> offm(m,i)  ==>     a scalar in the range [-15;15]
>>
>> Thanks in advance to those wishing to try :)
>>
>I don't think there's a good way. You can, of course, extract the
>submatrices via cellfun and then use plus, but it's not going to be
>significantly better. 

I think the slow thing is the index computation, so maybe I could try to
compute them all at once and then index, reshape and sum.  Maybe I'll try.

With the new diag implementation now we have a fast way of multiplying
each row of a matrix by a different number.  But I think we have no
euivalently fast way of summing a different number to each row.  Am I
wrong?

>                      Given that m = 5, I think it's not even worth
>trying. If it doesn't seem to be possible to vectorize (part of) the
>outer loop, your best approach is to write a compiled function.
>Perhaps a sufficiently general one would be useful in Octave or
>OctaveForge.

For the moment, the problem does not grant such effort on my part.  But
I'll keep that in mind for the future.  Thanks for the suggestions.

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