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reading numbers from file

Neelakantan Krishnaswami neelk at alum.mit.edu
Thu Dec 21 21:40:31 EST 2000 
On 22 Dec 2000 00:27:06 GMT, Jacek Pop³awski <jp at ulgo.koti.com.pl> wrote:
>I have file with numbers:
>
>23 41 24 0 2 42 532 53 234
>34 234 53 23 4 323 53 (etc...)
>
>1.How to read one line of this file and write it to list of numbers?
>  (like: a=[23,41,24...])

This code will put a big list of numbers into the array allnums

import string

def string_to_numbers(line):
    return map(int, string.split(line))

allnums = []
file = open("foo.txt")
for line in file.readlines():
    numbers_on_line = string_to_numbers(line)
    allnums.extend(numbers_on_line)

The function string_to_numbers can also be written in two other
ways:

def string_to_numbers(line):
    return [int(x) for x in string.split(line)]

def string_to_numbers(line):
    lst = []
    for x in string.split(line):
        lst.append(int(x))
    return lst

Any of these is fine -- use whichever is most clear to you.

> 2.How to read only 5 numbers, write it to list, then another 5, and
> write it to second list?

lst1 = []
lst2 = []
file = open("foo.txt")

current = lst1
count = 1
for line in file.readlines():
    numbers_on_line = string_to_numbers(line)
    #
    # Each number should be written to either lst1 or lst2. We'll
    # track the number of numbers written to both lists and flip
    # to the other one each time we hit a multiple of 5.
    #
    for n in numbers_on_line:
    	current.append(n)
    	count = count + 1
    	if (count % 5) == 0:
    	    if current == lst1:
    		current = lst2
    	    else:
    		current = lst1

> 3.What will change if I want to add comments (#) and empty lines to
> my file?

You'll need to change the function string_to_numbers() so that it can
account for these two changes to your format. string.split() already
returns an empty list if it receives a line of whitespace, so it
already works on blank lines. To handle '#' to the end of line
comments, we can either use regexps or string.split() again. I'll do
both.

The basic idea is that if we do string.split on '#' then we get a list
of elements that are divided by the '#'. We only need to look at the
first element of that list since it is the string that precedes the
first '#' on the line.

def string_to_numbers(line):
    comment_free_line = string.split(line, "#")[0]
    return map(int, string.split(comment_free_line))

# And then the prior solutions follow.

Using regexps, the solution might look like this:

import re

comment_free = re.compile("^([^#]*).*$")

def string_to_numbers(line):
    m = comment_free.match(line)
    comment_free_line = m.group(1)
    return map(int, string.split(comment_free_line))

I hope this helped.


Neel
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