extract digits in a simple way from a python string

29
votes

I have a string that stores a number and a unit for example 

x= '$120'
y = ' 90 Degrees F'
banana = '200 kgm'
orange = '300 gm'
total_weight = banana + orange/1000

and for example I want to add the weights 

total_weight  = 200 + 300/1000

Thanks!

I'm trying to extract the numbers only to do some operations with these... any idea of what the simplest way to do this? I'm only dealing with these two formats i.e. digits are at the begining or at the end of the string...

5
python
I would suggest you to have a look at re module. Regular expressions are meant for extracting structured data from corpus. - Sushant Gupta
Your own example shows the issue here. banana is in kgm and orange is in gm, surely that means that the weights are 200000 and 300, not 200 and 300, which adds more complexity to the problem. Does that matter to you?. - Gareth Latty
He divides orange by 1000 for that exact reason - jamylak
@jamylak Exactly, is that fixed? Or is that something that could change? - Gareth Latty
I presumed that was not part of the question since it was hard-coded in. - jamylak

5 Answers

65
votes

The simplest way to extract a number from a string is to use regular expressions and findall. 

>>> import re
>>> s = '300 gm'
>>> re.findall('\d+', s)
['300']
>>> s = '300 gm 200 kgm some more stuff a number: 439843'
>>> re.findall('\d+', s)
['300', '200', '439843']

It might be that you need something more complex, but this is a good first step.

Note that you'll still have to call int on the result to get a proper numeric type (rather than another string):

>>> map(int, re.findall('\d+', s))
[300, 200, 439843]
31
votes

Without using regex, you can just do:

def get_num(x):
    return int(''.join(ele for ele in x if ele.isdigit()))

Result:

>>> get_num(x)
120
>>> get_num(y)
90
>>> get_num(banana)
200
>>> get_num(orange)
300

EDIT :

Answering the follow up question.

If we know that the only period in a given string is the decimal point, extracting a float is quite easy:

def get_num(x):
    return float(''.join(ele for ele in x if ele.isdigit() or ele == '.'))

Result:

>>> get_num('dfgd 45.678fjfjf')
45.678
4
votes

This regular expression handles floats as well

import re
re_float = re.compile(r'\d*\.?\d+')

You could also add a group to the expression that catches your weight units.

re_banana = re.compile(r'(?P<number>\d*\.?\d+)\s?(?P<uni>[a-zA-Z]+)')

You can access the named groups like this re_banana.match("200 kgm").group('number').

I think this should help you getting started.

2
votes
>>> x='$120'
>>> import string
>>> a=string.maketrans('','')
>>> ch=a.translate(a, string.digits)
>>> int(x.translate(a, ch))
120
0
votes

If you're doing some sort of math with the numbers you might also want to know the units. Given your input restrictions (that the input string contains unit and value only), this should correctly return both (you'll just need to figure out how to convert units into common units for your math).

def unit_value(str):
    m = re.match(r'([^\d]*)(\d*\.?\d+)([^\d]*)', str)
    if m:
        g = m.groups()
        return ' '.join((g[0], g[2])).strip(), float(g[1])
    else:
        return int(str)