Python: Use list.index with regular expression

Question

I have lists of strings of which i want to extract a certain value:

["bla","blabla","blablabla","time taken to build model: 5.1 seconds", "blabla"]

Normally I would look for the index of the element I am looking for by

list.index("time taken")

But since the time changes, I think of using a regular expression. I just can't figure out how to do this.

So how can I find out the index of a list element that matches a certain regex like e.g. re.match()? (Without iterating through the list, this would take to long)

Do you really think you can get away without iteration? Even list.index is an iteration. If you need that much performance, use a dictionary with known keys, rather than searching through a list. — Piotr Kalinowski
– Piotr Kalinowski, Commented Oct 25, 2013 at 13:37
Are there multiple items in the list that have 'time taken' in them? If so, are you trying to find a specific item based on the time. Do you need to preserve the index of the item in the list? — wwii
– wwii, Commented Oct 25, 2013 at 13:41
possible duplicate of Python: get list indexes using regular expression? — evuez
– evuez, Commented Oct 25, 2013 at 13:41
Ok lets say it would be preferable if the operation didn't take minutes but if there is no other way I can also iterate. But the idea with a dictionary is good. I am not that deep in python to know that list.index is also iterating. Actually I asked the question, to learn for future tasks and to learn elegant coding. — aldorado
– aldorado, Commented Oct 25, 2013 at 13:45
@evuez - In principle you are right. It is a duplicate, if there is no other way to do this without iterating. Sorry for that then, I did not find it when searching for an answer. — aldorado
– aldorado, Commented Oct 25, 2013 at 13:48

Srikar Appalaraju · Accepted Answer · 2016-05-11 04:01:20Z

5

Not sure if there is a built in method but its easy to do this with list comprehensions in O(n) time.

With regular expressions:

import re
your_list = ["bla","blabla","blablabla","time taken to build model: 5.1 seconds", "blabla"]
regex = re.compile("^time taken")
idxs = [i for i, item in enumerate(your_list) if re.search(regex, item)]

And without regular expressions:

your_list = ["bla","blabla","blablabla","time taken to build model: 5.1 seconds", "blabla"]
query_term = 'time taken'
idxs = [i for i, item in enumerate(your_list) if item.startswith(query_term)]

You can make it return the first found index or last found index depending or parameterise it in a method to provide flexibility.

answered May 11, 2016 at 4:01

Srikar Appalaraju

74.1k55 gold badges221 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Aseem Yadav · Accepted Answer · 2019-04-03 14:10:07Z

1

If your priority is to get first match in the sequence , then only index() is useful. That's how you do it, if you want to use regex in index() method

lst=["bla","blabla","blablabla","time taken to build model: 5.1 seconds", "blabla"]

lst.index([i for i in lst if re.findall(r'^time taken', i)][0])

answered Apr 3, 2019 at 14:10

Aseem Yadav

7787 silver badges16 bronze badges

Comments

fryday · Accepted Answer · 2013-10-25 13:51:04Z

0

Regex solution need iterate through sequence. If you want get strings with some prefix or suffix, you should implement Trie it's the fastest solution of a problem. Also you can implement solution with cycled hashes of different lengths, but in some cases it will be uneffciient.

answered Oct 25, 2013 at 13:51

fryday

3972 silver badges14 bronze badges

Comments

tk. · Accepted Answer · 2013-10-25 13:43:33Z

-1

To find an element in a list, unless you have extra information (such as order of elements), you have to iterate through it. If you really want to go faster, change the structure, use a database or use another language.

answered Oct 25, 2013 at 13:43

tk.

6245 silver badges14 bronze badges

1 Comment

aldorado Over a year ago

Thank you for the info. So since this seemed to be asked before and if no other suggestion will appear, I will delete my question.

Collectives™ on Stack Overflow

Python: Use list.index with regular expression

4 Answers 4

Comments

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related