Re: [OctDev] Vectorizing code array modulo indexing

[Torbjörn R. <tor...@st...>]

Juan Pablo Carbajal skrev 2012-10-03 14:02:
> On Wed, Oct 3, 2012 at 12:17 PM, Torbjörn Rathsman
> <tor...@st...> wrote:
>> 2012-10-02 22:12, Juan Pablo Carbajal skrev:
>>> On Tue, Oct 2, 2012 at 9:00 PM, Torbjörn Rathsman
>>> <tor...@st...> wrote:
>>>> Is this kind of thing possible to do without do it in a loop?
>>>>
>>>>      notes=[1 1 0 1 1 1 0 1 1 0 1 0]';
>>>>      x=12.*log(f./440)./log(2);
>>>>      spectrum=spectrum.*cos(pi.*x).^16.* notes( mod(round(x),12) + 1 );
>>>>
>>>> f is a vector of 10^7 elements and I want to take each computed index and
>>>> generate corresponding value from notes.
>>>> _______________________________________________
>>>> Help-octave mailing list
>>>> Hel...@oc...
>>>> https://mailman.cae.wisc.edu/listinfo/help-octave
>>> Can you give more details? I do not see a loop.
>>> I see that you are multiplying spectrum by something and then saving
>>> it back into spectrum. What is the initial value of spectrum.
>>>
>>> Sorry for not understanding.
>>>
>>>
>> Implemented using a loop:
>>
>> notes=[1 1 0 1 1 1 0 1 1 0 1 0]';
>>
>> for k=1:N %N huge
>>
>> x=12.*log(f(k)./440)./log(2);
>>      spectrum(k)=spectrum(k).*cos(pi.*x).^16.* notes( mod(round(x),12) + 1 );
>>
>> end
>>
>> I want to implement the same thing without a loop.
>>
> Hi,
>
> Always keep the mailing list in CC.
>
> Here is my attempt tp vectorize your code. I expanded it so that you
> understand all the stages. You do not need to create all those
> intermediate variables
> http://agora.octave.org/snippet/hchr/
>
> Note that I am assuming that the variable spectrum exists, it is a
> column and has length N.
>
> Hope this helps.
>
>
It does not work. You forgot that f is a vector, so is x and x_mod. Now, 
trying to use x_mod as index, will not expand notes into a squarewavish 
function but just fail with

  subscript indices must be either positive integers or logicals