Won’t it be slightly faster with the view, as doing
x[k] *= -1.
will require a bound check on x[k]?
I also don’t understand the trade-offs between @inbounds vs unsafe_view()…just
in experiments unsafe_view seems to be faster.
On 6 Jan 2014, at 4:53 pm, Stefan Karpinski <[email protected]> wrote:
> Why do you need the view? Why not just write a loop that flips the sign of
> every other element?
>
> On Jan 5, 2014, at 10:29 PM, Sheehan Olver <[email protected]> wrote:
>
>>
>>
>> I'm looking for a fast (BLAS?) way to multiply every other entry of a vector
>> by -1. Attached are some different ways I've tried, all of which seem
>> dreadfully slow. (I include a pre-planned DCT for timing comparison: it
>> should be much faster but is similar in cost)
>>
>>
>>
>> n=10000
>> x=2rand(n)-1
>>
>> @time x.*(-1.).^[0:length(x)-1];
>>
>> # elapsed time: 0.00030401 seconds (241456 bytes allocated)
>>
>>
>> using NumericExtensions
>>
>> y = unsafe_view(x)
>> @time for k =2:2:length(x)
>> y[k] = -y[k]
>> end
>>
>> # elapsed time: 0.000876339 seconds (699616 bytes allocated)
>>
>>
>> ##pre-planned DCT for comparison
>> p=FFTW.plan_r2r(x, FFTW.REDFT00)
>>
>> @time p(x);
>> # elapsed time: 0.000814533 seconds (80928 bytes allocated)
