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Date: Wed, 24 Oct 2012 18:05:43 +0100
From: MRAB <python@mrabarnett.plus.com>
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Subject: Re: turn list of letters into an array of integers
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On 2012-10-24 07:07, Chris Rebert wrote:
> 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 you're using .split() then you don't need to use .strip() as well:

     result = [[letter2number[letter] for letter in line.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.
>