How to find required elements among multiple items in one list?

Question

I have a list which contains multiple filename:

example_list = ['2021-08-09_01.csv', '2021-08-09_02.csv', '2021-08-10_12.csv',
                '2021-08-10_03.csv']

I want to return a new list which includes the days that are inside the example_list:

new_list = ['09','10']

Regardless the number after the day string, what's the easiest way to achieve this?

create new empy list, add all the files name splitted (use split() twice or see here https://stackoverflow.com/questions/56718450/how-can-i-split-between-two-characters-in-a-list-element-in-python for re.split) to get the day and then use set() to get each day just once — pippo1980, Aug 10 '21 at 17:21

score 4 · Accepted Answer · answered Aug 10 '21 at 17:19

4

You can use a set comprehension where you slice the needed part from the string elements, and then (if actually needed) convert the created set into a list:

example_list = ['2021-08-09_01.csv', '2021-08-09_02.csv', '2021-08-10_12.csv', '2021-08-10_03.csv']

new_list = list({elem[8:10] for elem in example_list})

print(new_list)

Output:
['09', '10']

answered Aug 10 '21 at 17:19

ruohola

21,987
6
62
97

thanks for teaching me that: Alternately, a set can be defined with curly braces ({}): – pippo1980 Aug 10 '21 at 18:09
@pippo1980 This *is* using curly braces? – ruohola Aug 11 '21 at 06:04
yep : {elem[8:10] for elem in example_list} , I always thought about set just as set() , while {} to me, meant only empty dictionary – pippo1980 Aug 11 '21 at 10:27
@pippo1980 Ah right right, now I get that I though you that :D – ruohola Aug 11 '21 at 11:21
@pippo1980 Remember that an **empty** set must be defined with `set()`, otherwise it would clash with empty dict, only a non-empty set can use the `{"elem"}` syntax. – ruohola Aug 11 '21 at 11:22

score 0 · Answer 2 · answered Aug 10 '21 at 17:19

0

You can use str.split to get the day string and then use set() to remove duplicates:

new_list = list(set(s.split("-")[-1].split("_")[0] for s in example_list))
print(new_list)

Prints:

['10', '09']

answered Aug 10 '21 at 17:19

Andrej Kesely

168,389
15
48
91

score 0 · Answer 3 · answered Aug 10 '21 at 17:40

You could try like this.

Split the sub-string (i[8:]) at _ and select the first item. Use a set to remove duplicates.

example_list = ['2021-08-09_01.csv', '2021-08-09_02.csv', '2021-08-10_12.csv', '2021-08-10_03.csv']

s = {i[8:].split('_')[0] for i in example_list}
print(list(s))

['09', '10']

How to find required elements among multiple items in one list?

3 Answers3