indexing lists/arrays question

[Matthew Wilson]

On Thu 13 May 2010 10:36:58 AM EDT, a wrote:
> this must be easy but its taken me a couple of hours already
>
> i have
>
> a=[2,3,3,4,5,6]
>
> i want to know the indices where a==3 (ie 1 and 2)
>
> then i want to reference these in a
>
> ie what i would do in IDL is
>
> b=where(a eq 3)
> a1=a(b)


There's several solutions.  Here's one:

It is a recipe for madness to use a list of integers and then talk
about the position of those integers, so I renamed your list to use
strings.

    >>> a = ['two', 'three', 'three', 'four','five', 'six']

Now I'll use the enumerate function to iterate through each element and
get its position::

    >>> for position, element in enumerate(a):
    ...     print position, element
    ...
    0 two
    1 three
    2 three
    3 four
    4 five
    5 six

And now filter:

    >>> for position, element in enumerate(a):
    ...    if element == 'three':
    ...        print position, element

    1 three
    2 three

And now do something different besides printing:

    >>> b = []
    >>> for position, element in enumerate(a):
    ...     if element == 'three':
    ...         b.append(position)

And now we can rewrite the whole thing from scratch to use a list
comprehension:

    >>> [position for (position, element) in enumerate(a) if element == 'three']
    [1, 2]

HTH

Matt