# [portland] Still Having List/Tuple Problems

kirby urner kirby.urner at gmail.com
Wed Apr 16 21:13:39 CEST 2008

```You could use list comprensions:

>>> mydata
[('A', 1), ('A', 2), ('A', 8), ('B', 1), ('B', 2), ('B', 8), ('C', 1),
('C', 2), ('C', 8)]

>>> astuff = [j for (i,j) in mydata if i == 'A']

>>> astuff
[1, 2, 8]

>>> bstuff = [j for (i,j) in mydata if i == 'B']

>>> bstuff
[1, 2, 8]

If you don't know what all the keys are gonna
be you could build a list of unique ones:

>>> thekeys = set([i for (i,j) in mydata])

>>> thekeys
set(['A', 'C', 'B'])

>>> for thekey in thekeys:
print thekey
print [j for (i,j) in mydata if i == thekey]

A
[1, 2, 8]
C
[1, 2, 8]
B
[1, 2, 8]

Note there's no presumption the As, Bs are clumped together.

Kirby

On Wed, Apr 16, 2008 at 11:48 AM, Rich Shepard <rshepard at appl-ecosys.com> wrote:
>    I have a mental block on understanding just how to index a list of tuples
>  so I can extract the data in an orderly fashion.
>
>    The list looks like this: [(A,1),(A,2), ... (A,8),(B,1),(B,2), ...
>  (B,8),(C,1),(C2), ... (C,8)]
>
>    What I want to do is extract all tuples grouped by the first element, then
>  list the associated second elements). That is,
>  A
>         1
>         2
>         ...
>         8
>
>  B
>         1
>         2
>         ...
>         8
>  etc.
>
>    Referencing list[0] returns the first tuple; list[0][0] returns the first
>  item in the first tuple; but I cannot cycle through the three first items
>  extracting the second items associated with each.
>
>    I've looked at my books and previous threads here, but I'm just not
>  getting how this is done. I've played with list comprehensions, zip, and
>  other tools without getting the proper syntax or results I need. Don't know
>  why the answer keeps eluding me, but it does.
>
>  Rich
>
>  --
>  Richard B. Shepard, Ph.D.               |  Integrity            Credibility
>  Applied Ecosystem Services, Inc.        |            Innovation
>  <http://www.appl-ecosys.com>     Voice: 503-667-4517      Fax: 503-667-8863
>  _______________________________________________
>  Portland mailing list
>  Portland at python.org
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>
```

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