# [Numpy-discussion] Correlation function about a factor of 100 slower than matlab/mathcad ... but with fft even worse ?

qubax@g... qubax@g...
Wed Nov 25 12:23:20 CST 2009

```The correlation of a large data (about 250k points) v can be checked
via

correlate(v,v,mode='full')

and ought to give the same result as the matlab function

xcorr(v)

FFT might speed up the evaluation ...

In my specific case:

xcorr takes about 0.2 seconds.
correlate takes about 70 seconds.
fftconvolve takes about 400 seconds.

on the irc channel a second person happened to run into the same
problem using mathcad and data consisting of about 300k points:

correl takes about 1 second
correlate takes 127 seconds
fftconvolve was aborded because it took to long

These tests were checked and confirmed by two other persons on the
irc channel. The computers involved were 32bit as well as 64bit
machines. All 4 persons are sure that lapack/atlas libraries are
properly installed.

Could someone please investigate why correlate and especially
fftconvolve are orders of magnitude slower?

Should more details / sample data be required, please let me know.

Thanks,
q

----------------------

executed code:

tic=time.time()
cor_c1=correlate(c1data[:,1],c1data[:,1],mode='full')
toc=time.time()

tic=time.time()
cor_c1=fftconvolve(c1data[:,1],c1data[:,1],mode='full')
toc=time.time()

xcorr(data)

--
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A beggar's mistake harms no one but the beggar. A king's mistake,
however, harms everyone but the king. Too often, the measure of
power lies not in the number who obey your will, but in the number
who suffer your stupidity.
```