Separate number from unit in a string in Python

问题

I have strings containing numbers with their units, e.g. 2GB, 17ft, etc. I would like to separate the number from the unit and create 2 different strings. Sometimes, there is a whitespace between them (e.g. 2 GB) and it's easy to do it using split(' ').

When they are together (e.g. 2GB), I would test every character until I find a letter, instead of a number.

s='17GB'
number=''
unit=''
for c in s:
    if c.isdigit():
        number+=c
    else:
        unit+=c

Is there a better way to do it?

Thanks

回答1:

s='17GB'
for i,c in enumerate(s):
    if not c.isdigit():
        break
number=int(s[:i])
unit=s[i:]

回答2:

You can break out of the loop when you find the first non-digit character

for i,c in enumerate(s):
    if not c.isdigit():
        break
number = s[:i]
unit = s[i:].lstrip()

If you have negative and decimals:

numeric = '0123456789-.'
for i,c in enumerate(s):
    if c not in numeric:
        break
number = s[:i]
unit = s[i:].lstrip()

回答3:

You could use a regular expression to divide the string into groups:

>>> import re
>>> p = re.compile('(\d+)\s*(\w+)')
>>> p.match('2GB').groups()
('2', 'GB')
>>> p.match('17 ft').groups()
('17', 'ft')

回答4:

tokenize can help:

>>> import StringIO
>>> s = StringIO.StringIO('27GB')
>>> for token in tokenize.generate_tokens(s.readline):
...   print token
... 
(2, '27', (1, 0), (1, 2), '27GB')
(1, 'GB', (1, 2), (1, 4), '27GB')
(0, '', (2, 0), (2, 0), '')

回答5:

You should use regular expressions, grouping together what you want to find out:

import re
s = "17GB"
match = re.match(r"^([1-9][0-9]*)\s*(GB|MB|KB|B)$", s)
if match:
  print "Number: %d, unit: %s" % (int(match.group(1)), match.group(2))

Change the regex according to what you want to parse. If you're unfamiliar with regular expressions, here's a great tutorial site.

回答6:

>>> s="17GB"
>>> ind=map(str.isalpha,s).index(True)
>>> num,suffix=s[:ind],s[ind:]
>>> print num+":"+suffix
17:GB

回答7:

This uses an approach which should be a bit more forgiving than regexes. Note: this is not as performant as the other solutions posted.

def split_units(value):
    """
    >>> split_units("2GB")
    (2.0, 'GB')
    >>> split_units("17 ft")
    (17.0, 'ft')
    >>> split_units("   3.4e-27 frobnitzem ")
    (3.4e-27, 'frobnitzem')
    >>> split_units("9001")
    (9001.0, '')
    >>> split_units("spam sandwhiches")
    (0, 'spam sandwhiches')
    >>> split_units("")
    (0, '')
    """
    units = ""
    number = 0
    while value:
        try:
            number = float(value)
            break
        except ValueError:
            units = value[-1:] + units
            value = value[:-1]
    return number, units.strip()

回答8:

How about using a regular expression

http://python.org/doc/1.6/lib/module-regsub.html

回答9:

For this task, I would definitely use a regular expression:

import re
there = re.compile(r'\s*(\d+)\s*(\S+)')
thematch = there.match(s)
if thematch:
  number, unit = thematch.groups()
else:
  raise ValueError('String %r not in the expected format' % s)

In the RE pattern, \s means "whitespace", \d means "digit", \S means non-whitespace; * means "0 or more of the preceding", + means "1 or more of the preceding, and the parentheses enclose "capturing groups" which are then returned by the groups() call on the match-object. (thematch is None if the given string doesn't correspond to the pattern: optional whitespace, then one or more digits, then optional whitespace, then one or more non-whitespace characters).

回答10:

A regular expression.

import re

m = re.match(r'\s*(?P<n>[-+]?[.0-9])\s*(?P<u>.*)', s)
if m is None:
  raise ValueError("not a number with units")
number = m.group("n")
unit = m.group("u")

This will give you a number (integer or fixed point; too hard to disambiguate scientific notation's "e" from a unit prefix) with an optional sign, followed by the units, with optional whitespace.

You can use re.compile() if you're going to be doing a lot of matches.

回答11:

SCIENTIFIC NOTATION This regex is working well for me to parse numbers that may be in scientific notation, and is based on the recent python documentation about scanf: https://docs.python.org/3/library/re.html#simulating-scanf

units_pattern = re.compile("([-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?|\s*[a-zA-Z]+\s*$)")
number_with_units = list(match.group(0) for match in units_pattern.finditer("+2.0e-1 mm"))
print(number_with_units)
>>>['+2.0e-1', ' mm']

n, u = number_with_units
print(float(n), u.strip())
>>>0.2 mm

回答12:

try the regex pattern below. the first group (the scanf() tokens for a number any which way) is lifted directly from the python docs for the re module.

import re
SCANF_MEASUREMENT = re.compile(
    r'''(                      # group match like scanf() token %e, %E, %f, %g
    [-+]?                      # +/- or nothing for positive
    (\d+(\.\d*)?|\.\d+)        # match numbers: 1, 1., 1.1, .1
    ([eE][-+]?\d+)?            # scientific notation: e(+/-)2 (*10^2)
    )
    (\s*)                      # separator: white space or nothing
    (                          # unit of measure: like GB. also works for no units
    \S*)''',    re.VERBOSE)
'''
:var SCANF_MEASUREMENT:
    regular expression object that will match a measurement

    **measurement** is the value of a quantity of something. most complicated example::

        -666.6e-100 units
'''

def parse_measurement(value_sep_units):
    measurement = re.match(SCANF_MEASUREMENT, value_sep_units)
    try:
        value = float(measurement[0])
    except ValueError:
        print 'doesn't start with a number', value_sep_units
    units = measurement[5]

    return value, units

来源：https://stackoverflow.com/questions/2240303/separate-number-from-unit-in-a-string-in-python

标签

python

string

units-of-measurement