[Numpy-discussion] Indexing in Numpy vs. IDL?

[Robert Kern]

On Sun, Nov 16, 2008 at 16:15, Jason Woolard <Jason.Woolard at noaa.gov> wrote:
> hi all,
>
> I'm fairly new to Numpy and I've been trying to port over some IDL code
> to become more familiar. I've been moderately successful with
> numpy.where and numpy.compress to do some of things that were pretty
> easy to do in IDL. I'm a bit confused about how the indexing of arrays
> works though.

You may want to look at this section of the reference manual:

http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html

> This is pretty straightforward:
>
> in IDL
> =============================
> data = [50.00, 100.00, 150.00, 200.00, 250.00, 300.00, 350.00]
> index = WHERE((data GT 100.00) AND (data LT 300.00))
> new_data = data[index]
> print, new_data
>
> 150.000      200.000      250.000
>
> in Python
> ==============================
>  >>> import numpy
>  >>> from numpy import *
>  >>> data = [50.00, 100.00, 150.00, 200.00, 250.00, 300.00, 350.00]
>  >>> data = array(data, dtype=float32) #Convert list to array
>  >>> index_mask = numpy.where((data > 100.00) & (data < 300.00), 1,0)

index_mask = (data > 100.) & (data < 300.0)

> #Test for the condition.
>  >>> index_one = numpy.compress(index_mask, data)

index_one = data[index_mask]

>  >>> print index_one
> [ 150.  200.  250.]

Note that this is not an index; this is equivalent to new_data in your
IDL example. Are you sure you wanted to call this index_one?

> But I'm having a bit of trouble with the Python equivalent of this:
>
> in IDL:
> =============================
> index_two = WHERE ((data[index_one]  GT bottom) AND (data[index_one] LE
> top)

Well, since index_one are not indices, then data[index_one] doesn't
mean anything. Can you show us the equivalent IDL that generates
index_one?

> and also this:
>
> result = MAX(data[index_one[index_two]])

Ditto.

>  From what I've read it looks like numpy.take() might work to do the
> indexing. I've tried to test this but I'm not getting the answers I'd
> expect. Am I overlooking something obvious here?

So-called "advanced indexing" where the indices are boolean or integer
arrays will probably solve your problem, but we need more information
on what you mean by index_one.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
  -- Umberto Eco