finding substring

Question

Thanks in advance. I have a string:

A = 'asdfghjklmn'

How can I get a substring having a maximum length which is a multiple of three?

You must elaborate what "multiple of three" means. Some examples would be useful. — Adam Matan, Oct 14 '09 at 16:56
the question is rather vague. Are you looking for a substring that's as long as possible while satisfying the restraint or simply any substring which is a multiple of 3? — AlbertoPL, Oct 14 '09 at 16:57

score 4 · Accepted Answer · edited May 23 '17 at 10:27

4

You can use slice notation and integer arithmetic.

>>> a = 'asdfghjklmn'
>>> a[:len(a)//3*3]
'asdfghjkl'   
>>> len(a)
11
>>> len(a[:len(a)//3*3])
9

In general, n//k*k will yield the largest multiple of k less than or equal to n.

edited May 23 '17 at 10:27

Community

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answered Oct 14 '09 at 17:03

Stephan202

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Thank you.useful for gene sequence analysis – Oct 14 '09 at 17:26

score 1 · Answer 2 · answered Oct 14 '09 at 17:02

It seems like you're looking for something like this:

>>> A = 'asdfghjklmn'
>>> mult, _ = divmod(len(A), 3)
>>> A[:mult*3]
'asdfghjkl'

here resulting string will have length which is multiple of three and it will be the longest possible substring of A with such length.

score 1 · Answer 3 · answered Oct 14 '09 at 17:06

1

Yet another example:

>>> A = '12345678'
>>> A[:len(A) - len(A)%3]
'123456'
>>>

answered Oct 14 '09 at 17:06

Nick Dandoulakis

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score 0 · Answer 4 · answered Oct 14 '09 at 17:00

0

Is this what you want?

A = 'asdfghjklmn'
A[0:(len(A)/3)*3]
'asdfghjkl'

answered Oct 14 '09 at 17:00

Lance Rushing

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does not run on python3: Traceback (most recent call last): File "", line 1, in TypeError: slice indices must be integers or None or have an index method. You should have used floor division (just as Stephan202 did). See http://python.org/dev/peps/pep-0238 – Oren S Oct 14 '09 at 17:33

Mark Rushakoff · Answer 5 · 2009-10-14T17:19:00.843

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With the foreword that it will never be as efficient as the ones that actually use math to find the longest multiple-of-3-substring, here's a way to do it using regular expressions:

>>> re.findall("^(?:.{3})*", "asdfghjklmn")[0]
'asdfghjkl'

Changing the 3 quantifier will allow you to get different multiples.

edited Oct 14 '09 at 17:19

answered Oct 14 '09 at 17:12

Mark Rushakoff

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finding substring

5 Answers5