[Numpy-discussion] Finding indices and setting values

Todd Miller jmiller at stsci.edu
Wed May 21 09:01:14 EDT 2003

On Wed, 2003-05-21 at 11:08, Cliff Martin wrote:
> Hi,
> I have a problem where I want to set values in a 2D array based on 
> conditions I’ve established. The variables are the NA of the system, its 
> wavelength and the correct units on the device I’m modeling. Using these 
> variables I define a variable N. I set up a 512 by 512 array [x,y]. Then 
> I set up a radius , r = x^2 +y^2 and ask it to give me all r’s(to the 
> closest integer) <= N(the variable I’ve defined above). This gives all 
> the index values in that radius and then I set all those locations to a 
> value of 1. After this I do some FFT ‘s, etc. So how do I do this in 
> Numerical Python without having to index through i,j steps which would 
> be incredibly tedious. This works fairly well in MatLab but I want to 
> port my program to Python(for lots of reasons). If you’d rather I write 
> the MatLab code snippet I can do that. Thanks.
> Cliff Martin

I think part of your code looks like this:

x,y = Numeric.indices((512,512))
r = x**2 + y**2
c = r < N 

In numarray you can then say:

a[ c ] = 1

In Numeric, I think you say:

a = a.flat
c = c.flat
Numeric.put(a, Numeric.nonzero(c), 1)
a.shape = (512,512)


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