Determine prefix from a set of (similar) strings

user_396732 0
Viewed 46962

I have a set of strings, e.g.

my_prefix_what_ever
my_prefix_what_so_ever
my_prefix_doesnt_matter

I simply want to find the longest common portion of these strings, here the prefix. In the above the result should be

my_prefix_

The strings

my_prefix_what_ever
my_prefix_what_so_ever
my_doesnt_matter

should result in the prefix

my_

Is there a relatively painless way in Python to determine the prefix (without having to iterate over each character manually)?

PS: I'm using Python 2.6.3.

prefix python string
11 Answers

I had a slight variation of the problem and google sends me here, so I think it will be useful to document:

I have a list like:

  • my_prefix_what_ever
  • my_prefix_what_so_ever
  • my_prefix_doesnt_matter
  • some_noise
  • some_other_noise

So I would expect my_prefix to be returned. That can be done with:

from collections import Counter

def get_longest_common_prefix(values, min_length):
    substrings = [value[0: i-1] for value in values for i in range(min_length, len(value))]
    counter = Counter(substrings)
    # remove count of 1
    counter -= Counter(set(substrings))
    return max(counter, key=len)
user_9048870

In one line without using itertools, for no particular reason, although it does iterate through each character:

''.join([z[0] for z in zip(*(list(s) for s in strings)) if all(x==z[0] for x in z)])
user_28279710

Find the common prefix in all words from the given input string, if there is no common prefix print -1

stringList = ['my_prefix_what_ever', 'my_prefix_what_so_ever', 'my_prefix_doesnt_matter']
len2 = len( stringList )
if len2 != 0:
    # let shortest word is prefix
    prefix = min( stringList )
    for i in range( len2 ):
        word = stringList[ i ]
        len1 = len( prefix )
        # slicing each word as lenght of prefix
        word = word[ 0:len1 ]
        for j in range( len1 ):
            # comparing each letter of word and prefix
            if word[ j ] != prefix[ j ]:
                # if letter does not match slice the prefix
                prefix = prefix[ :j ]
                break # after getting comman prefix move to next word
    if len( prefix ) != 0:
        print("common prefix: ",prefix)
    else:
        print("-1")
else:
     print("string List is empty")
user_119773910
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