Re: [Numpy-discussion] Finding indices and setting values

[Todd M. <jm...@st...>]

On Wed, 2003-05-21 at 11:08, Cliff Martin wrote:
> Hi,
>=20
> I have a problem where I want to set values in a 2D array based on=20
> conditions I=E2=80=99ve established. The variables are the NA of the syst=
em, its=20
> wavelength and the correct units on the device I=E2=80=99m modeling. Usin=
g these=20
> variables I define a variable N. I set up a 512 by 512 array [x,y]. Then=20
> I set up a radius , r =3D x^2 +y^2 and ask it to give me all r=E2=80=99s(=
to the=20
> closest integer) <=3D N(the variable I=E2=80=99ve defined above). This gi=
ves all=20
> the index values in that radius and then I set all those locations to a=20
> value of 1. After this I do some FFT =E2=80=98s, etc. So how do I do this=
 in=20
> Numerical Python without having to index through i,j steps which would=20
> be incredibly tedious. This works fairly well in MatLab but I want to=20
> port my program to Python(for lots of reasons). If you=E2=80=99d rather I=
 write=20
> the MatLab code snippet I can do that. Thanks.
>=20
> Cliff Martin

I think part of your code looks like this:

x,y =3D Numeric.indices((512,512))
r =3D x**2 + y**2
c =3D r < N=20

In numarray you can then say:

a[ c ] =3D 1

In Numeric, I think you say:

a =3D a.flat
c =3D c.flat
Numeric.put(a, Numeric.nonzero(c), 1)
a.shape =3D (512,512)


Todd