Property testing in lists

[Remco Gerlich]

Dan Brown wrote in comp.lang.python:
> I'm a python newbie (my work is mostly in scientific computing, so I use
> 
> Matlab for most of my coding needs).
> 
> I'd like to be able to identify the indexes of all members of a list
> that
> have a particular property.  Certainly, there's lots of ways to do it;
> here's two examples with testing for being one more than a multiple of
> 4.
> 
> >>> N = [1, 10, 194, 148, -403]
> >>> map (lambda x: x[1],filter (lambda x: x[0] % 4 == 1, map (None, N,
> range(len(N)))))
> [0, 4]
> >>> index = []
> >>> for i in range (len(N)):
> ... if (N[i] % 4 == 1):
> ...  index.append(i)
> ...
> >>> index
> [0, 4]
> 
> However, these are both much more disgusting than the equivalent Matlab
> code: [Lists in Matlab are indexed starting at 1.]
> 
> >> N = [1 10 194 148 -403];
> >> find (mod (N,4) == 1)
> 
> ans =
> 
>      1     5
> 
> Is there an equivalent form in Python which is pretty like the Matlab
> code,
> or am I just barking up the wrong tree?

How about this one:

N = [1, 10, 194, 148, -403]
filter(lambda x,N=N: N[x] % 4 == 1, range(len(N)))

It's more verbose than Matlab, but well, just make it into a function once,
and you'll never need to type it again...

Python has nice ways to make a list of the *values* that have the property,
like filter() and list comprehensions in 2.0(beta), but not for the indices
I think. So just view it as a property of the index :-)

-- 
Remco Gerlich,  scarblac at pino.selwerd.nl
"This gubblick contains many nonsklarkish English flutzpahs, but the
 overall pluggandisp can be glorked from context"  (David Moser)