Find maximum number in a string list with a specific format

Question

Using Python 3, I have a list of the form

ls = ["a", "b_0", "b_1", "b_2", "c", "d_0", "d_1"]

I have to find out the maximum number after "b_". In this case, the desired output is 2.

I came up with something like that:

import re
p = re.compile("b_(\d+)")
nums = [int(p.match(l).group(1)) for l in ls if p.match(l) is not None]
res_max = max(nums)

But I don't like the repetition of p.match two times.

What is the most pythonic way of achieving this?

The maximum number after d_ is not guaranteed to be less than the maximum number after b_

I specifically need to find out the maximum number after the "b_" prefix. I am not interested in the maximum number after any other prefixes

Most Pythonic way might be without using any modules or imports. — PCM
– PCM, Commented Sep 5, 2021 at 13:07
How about the first element in the list "a"? Or do you mean to just ignore anything that has no "b_" in the prefix? — Niel Godfrey P. Ponciano
– Niel Godfrey P. Ponciano, Commented Sep 5, 2021 at 13:09
@NielGodfreyPonciano ignore anything that has no b_ as a prefix — robertspierre
– robertspierre, Commented Sep 5, 2021 at 13:11

not_speshal · Accepted Answer · 2021-09-05 13:15:26Z

2

Another way:

>>> max(int(i.split("_")[-1]) for i in ls if i.startswith("b_"))
2

answered Sep 5, 2021 at 13:15

not_speshal

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Comments

Andrej Kesely · Accepted Answer · 2021-09-05 13:15:47Z

2

If you don't like repetition, you can use walrus operator :=

import re

ls = ["a", "b_0", "b_1", "b_2", "c", "d_0", "d_1"]

p = re.compile(r"b_(\d+)")

max_num = max(int(m[1]) for l in ls if (m := p.match(l)))
print(max_num)

Prints:

answered Sep 5, 2021 at 13:15

Andrej Kesely

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Comments

U13-Forward · Accepted Answer · 2021-09-05 13:06:54Z

1

Something like...

>>> max([int(i.split('_')[-1]) for i in ls if i.split('_')[-1].isdigit()])
2
>>>

answered Sep 5, 2021 at 13:06

U13-Forward

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4 Comments

not_speshal Over a year ago

I think you also need and i[0]=="b"

U13-Forward Over a year ago

@not_speshal It doesn't need to be I gues.

robertspierre Over a year ago

The number in d is not guaranteed to be less than the number in b

U13-Forward Over a year ago

@robertspierre Yeap! that's what my code does :P

Corralien · Accepted Answer · 2021-09-05 13:12:49Z

1

You can try:

>>> max([int(i.split('_')[1]) for i in ls if i.split('_')[0] == 'b'])
2

answered Sep 5, 2021 at 13:12

Corralien

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1 Comment

Corralien Over a year ago

I'm agree with you but I prefer use '_' as delimiter because I guess a, b, c, d are just example. I can use startswith

igoras1993 · Accepted Answer · 2021-09-05 13:15:42Z

1

In such case, expand the list comprehension into explicit list appending:

import re
p = re.compile("b_(\d+)")

nums = []
for l in ls:
    m = p.match(l)
    if m is not None:
        nums.append(int(m.group(1))

res_max = max(nums)

answered Sep 5, 2021 at 13:15

igoras1993

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Comments

PCM · Accepted Answer · 2021-09-05 13:17:33Z

1

You can try list comprehension-

ls = ["a", "b_0", "b_1", "b_2", "c", "d_0", "d_1"]

res = max([int(string.split('_')[1]) for string in ls if string.startswith('b_')])

print(res) # 2

answered Sep 5, 2021 at 13:17

PCM

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Comments

Sabil · Accepted Answer · 2021-09-05 13:17:51Z

1

This could another way

ls = ["a", "b_0", "b_1", "b_2", "c", "d_0", "d_1"]
res_max = max([int(i.split('_')[1]) for i in ls if i[0] == 'b'])
print(res_max)

Output: 2

answered Sep 5, 2021 at 13:17

Sabil

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Comments

Jiří Baum · Accepted Answer · 2021-09-05 14:09:04Z

0

One way would be nested comprehensions:

import re
p = re.compile("b_(\d+)")
nums = [
    int(m.group(1))
    for m in (p.match(l) for l in ls)
    if m is not None
]
res_max = max(nums)

answered Sep 5, 2021 at 14:09

Jiří Baum

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Comments

Diyeddin · Accepted Answer · 2021-09-05 14:56:45Z

Heres another way of doing it using re.search()

    import re

    ls = ["a", "b_0", "b_1", "b_2", "c", "d_0", "d_1"]
    pattern = r'b_(\d+)'

    lst = [res.group(1) for i in ls if (res := re.search(pattern, i))]
    print(max(lst))

Output:

In the 6th line it's checking every element in the list ls if it matches the pattern, then adds the first group of the matching string (which is the number) to the list lst.

The part res := re.search(pattern, i) is assigning the value of re.search(pattern, i) to the variable res and at the same time as its checking if it's None then the if statement won't activate, if it has a value then the if statement will get activated. This is thanks to the 'Walrus Operator' :=.

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