In Python, what's the best way to get an unknown int from a string?

Question

SOLUTION: Ok guys I ended up using the following. It involves regular expressions. This is what I was trying to get at.

matches = re.findall(r'My favorite chili was number \d+"', line) # gets 1 match
    if matches: # if there are matches
        nums.append(int(re.findall(r'\d+',matches[0])[0])) # append the number

This isn't very elegant, but it is extremely robust. It also always works based on the format of the files I'm using. Thank you @The2ndSon for suggesting regular expressions. That's what I had vaguely heard of once and couldn't quite remember.

Original Question

In Python, say I am opening a file. In this file there are certain strings that I want. These strings have the format

"My favorite chili was chili number NUM"

where NUM is replaced by a natural number (a non-negative integer).

What is the best way to find lines containing this string, and store NUM? I have some code laid out, but it seems like there should be a more elegant solution. Are there built-ins made specifically for this purpose? It seems very similar to other string formatting stuff that Python has built-ins for.

I am using Python 2.7

Here is my code so far:

nums = []
with open('file.txt', 'r') as txtfile:
    for line in txtfile:
        indx = line.find('My favorite chili was number ')
            if indx != -1:
                nums.append(int(line[indx+30:indx+31]))

IMPORTANT EDIT: There can be more than one number on each line, and the numbers I want are NOT always necessarily at the end. The whole chili thing was just an example.

If you have working code, you could post it on http://codereview.stackexchange.com/ — pgreen2, Jul 22 '14 at 17:56
The short answer is regular expressions! Or just `str.startswith` and `str.split`. — jonrsharpe, Jul 22 '14 at 17:57
Yes, python has "string formatting stuff", but your question is about parsing. — shx2, Jul 22 '14 at 17:59
`There can be more than one number on each line, and the numbers I want are NOT always necessarily at the end. ` Then what number DO you want? — dawg, Jul 22 '14 at 18:11

The2ndSon · Accepted Answer · 2014-07-22T18:08:21.687

2

You could use a regular expression to extract digits from a string.

>>>import re
>>>re.findall(r'\d+', "My favorite chili was chili number 19")
['19']

edited Jul 22 '14 at 18:08

answered Jul 22 '14 at 17:58

The2ndSon

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2
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And if you have: "My favorite 3 chili was chili number 19" ? – Nir Alfasi Jul 22 '14 at 18:01
it would output `['3','19']`, this regular expression should be modified if the intent is to only grab the last element. – The2ndSon Jul 22 '14 at 18:02
See @KronoS method for grabbing the last element in a string. – The2ndSon Jul 22 '14 at 18:04
Sorry, I didn't provide enough info. I updated the post. I think I will using regular expressions somehow. – user3745189 Jul 22 '14 at 18:07

score 1 · Answer 2 · answered Jul 22 '14 at 17:59

1

Assuming that the number is always at the end you can do this:

test = "My favorite chili was chili number 123456"

num = int(test.split()[-1])
print(num)

answered Jul 22 '14 at 17:59

James Mertz

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and for lines that don't have a num as the last item - it will raise an exception... – Nir Alfasi Jul 22 '14 at 18:03
@alfasin well the OP didn't clarify this. – James Mertz Jul 22 '14 at 18:17

In Python, what's the best way to get an unknown int from a string?

Original Question

2 Answers2