如何从字符串列表中提取数字？

Question

How should I extract numbers only from

a = ['1 2 3', '4 5 6', 'invalid']

I have tried:

mynewlist = [s for s in a if s.isdigit()]
print mynewlist

and

for strn in a:
    values = map(float, strn.split())
print values

Both failed because there is a space between the numbers.

Note: I am trying to achieve output as:

[1, 2, 3, 4, 5, 6]

Answer 1

I think you need to process each item in the list as a split string on whitespace.

a = ['1 2 3', '4 5 6', 'invalid']
numbers = []
for item in a:
    for subitem in item.split():
        if(subitem.isdigit()):
            numbers.append(subitem)
print(numbers)

['1', '2', '3', '4', '5', '6']

Or in a neat and tidy comprehension:

[item for subitem in a for item in subitem.split() if item.isdigit()]

Answer 2

That should do for your particular case since you include a string within list. Therefore you need to flatten it:

new_list = [int(item) for sublist in a for item in sublist if item.isdigit()]

Answer 3

假设列表只是字符串：

[int(word) for sublist in map(str.split, a) for word in sublist if word.isdigit()]

Answer 4

With the help of sets you can do:

>>> a = ['1 2 3', '4 5 6', 'invalid']
>>> valid = set(" 0123456789")
>>> [int(y) for x in a if set(x) <= valid for y in x.split()]
[1, 2, 3, 4, 5, 6]

This will include the numbers from a string only if the string consists of characters from the valid set.

Answer 5

mynewlist = [s for s in a if s.isdigit()]
print mynewlist

doesnt work because you are iterating on the content of the array, which is made of three string:

1. '1 2 3'
2. '4 5 6'
3. 'invalid'

that means that you have to iterate again on each of those strings.

you can try something like

mynewlist = []
for s in a:
    mynewlist += [digit for digit in s if digit.isdigit()] 

Answer 6

一种衬垫解决方案：

new_list = [int(m) for n in a for m in n if m in '0123456789']

Answer 7

There are lots of option to extract numbers from a list of strings.

A general list of strings is assumed as follows:

input_list = ['abc.123def45, ghi67 890 12, jk345', '123, 456 78, 90', 'abc def, ghi'] * 10000

If the conversion into an integer is not considered,

def test_as_str(input_list):
    output_list = []
    
    for string in input_list:
        output_list += re.findall(r'\d+', string)
    
    return output_list

%timeit -n 10 -r 7 test_as_str(input_list)
> 37.6 ms ± 168 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_str(input_list):
    output_list = []
    
    [output_list.extend(re.findall(r'\d+', string)) for string in input_list]
    
    return output_list

%timeit -n 10 -r 7 test_as_str(input_list)
> 39.5 ms ± 118 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_str(input_list):
    return list(itertools.chain(*[re.findall(r'\d+', string) for string in input_list]))

%timeit -n 10 -r 7 test_as_str(input_list)
> 40.4 ms ± 202 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_str(input_list):
    return list(filter(None, [item for string in input_list for item in re.split('[^\d]+' , string)]))

%timeit -n 10 -r 7 test_as_str(input_list)
> 42.8 ms ± 372 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

The conversion into an integer can be also considered.

def test_as_int(input_list):
    output_list = []
    
    for string in input_list:
        output_list += re.findall(r'\d+', string)
    
    return list(map(int, output_list))

%timeit -n 10 -r 7 test_as_int(input_list)
> 44.7 ms ± 232 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_int(input_list):
    output_list = []
    
    for string in input_list:
        output_list += re.findall(r'\d+', string)
    
    return [int(item) for item in output_list]

%timeit -n 10 -r 7 test_as_int(input_list)
> 47.8 ms ± 198 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_int(input_list):
    return [int(item) for string in input_list for item in re.findall(r'\d+', string)]

%timeit -n 10 -r 7 test_as_int(input_list)
> 48.3 ms ± 101 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_int(input_list):
    return [int(item) for string in input_list for item in re.split('[^\d]+' , string) if item]

%timeit -n 10 -r 7 test_as_int(input_list)
> 51.4 ms ± 150 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_int(input_list):
    return [int(item) for string in input_list for item in re.split('[^\d]+' , string) if item.isdigit()]

%timeit -n 10 -r 7 test_as_int(input_list)
> 54.9 ms ± 210 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

def test_as_int(input_list):
    return [int(item) for string in input_list for item in re.split('[^\d]+' , string) if len(item)]

%timeit -n 10 -r 7 test_as_int(input_list)
> 55.5 ms ± 175 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

The performance test, which does not show much difference, is done on Windows OS, Python 3.8.8 virtual environment.