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Date: Tue, 23 Oct 2012 23:07:26 -0700
Subject: Re: turn list of letters into an array of integers
From: Chris Rebert <clp2@rebertia.com>
To: "Michael J. Krause" <michael.j.krause@gmail.com>
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On Tue, Oct 23, 2012 at 10:23 PM, seektime <michael.j.krause@gmail.com> wrote:
> Here's some example code. The input is a list which is a "matrix" of letters:
>    a  b  a
>    b  b  a
>
> and I'd like to turn this into a Python array:

You mean a Python list. The datatype Python calls an `array` is very
different and relatively uncommonly used.
Although, confusingly, Python's lists are implemented using C arrays
rather than linked lists.

>   1 2 1
>   2 2 1
>
> so 1 replaces a, and 2 replaces b. Here's the code I have so far:
>
>>>> L=['a b a\n','b b a\n']
<snip>
>>>> seq
> '1 2 1\n 2 2 1\n'
>
> My question is how can I turn "seq" into a python array?

I'd say you're asking the wrong question. The better question is "Why
wasn't the result a list in the first place?". Many transformations
are cumbersome to express over just strings, which is why the first
job of most programs is to parse their input into a more convenient
structure that is suited to their main task(s).

This (along with some other improvements) leads to a better, somewhat
different program/algorithm:

letter2number = {'a': 1, 'b': 2}
with open("path/to/file.txt", "r") as f:
    result = [[letter2number[letter] for letter in
line.strip().split()] for line in f]

If it's safe to assume that the correspondence between the letters and
numbers isn't completely arbitrary, some further improvements are also
possible.

Some relevant docs:
http://docs.python.org/library/stdtypes.html#string-methods
http://docs.python.org/tutorial/datastructures.html#list-comprehensions

Cheers,
Chris

P.S.: I'm guessing you obtained `L` from file.readlines() or similar;
it is worth noting for future reference that the readlines() method is
considered somewhat deprecated.