[SciPy-User] Help with fast fourier transform
Thu Apr 29 20:46:08 CDT 2010
Oscar Gerardo Lazo Arjona wrote:
> Hello! I'm new to this mailing list and to numpy in general.
> I need to calculate fft for my optics class. But I'm having trouble.
> This is the code I'm using (commented):
> import pylab
> import numpy as np
> def g(x):
> if x>450 and x<550:
> return 1
> return 0
> f=[g(x) for x in range(0,1000)]
> #this funtion can be ploted as
> #which is a step function "centered" at 500
> #when calculate the fft of f i get an array of complex numbers
> #whose absolute value can be ploted as
> #But that is not the desired output.
> #Instead of that i expect something that can be ploted like this
> #what i think must be happening because my function f
> #has an offset of 500 (it's supposed to be centereed at 500)
> #So i think it all reduces to two options:
> #somehow telling fft to consider the origin at 500
> #(indicate thetell the offset)
> #Or make fft accept a list of points like
> #so that it can know the position of the step relative to the origin
> Please help!
Take a look at the functions numpy.fft.fftshift and numpy.fft.fftfreq.
The attached script shows an example.
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