CODEWITHSUNDEEP
pythonstringintersection
bang
bang

Reputation: 181

Python - Intersectiing strings

Trying to write a for function that takes two strings and returns the characters that intersect in the order that they appear in the first string.

Here's what I tried:

def strIntersection(str1, str2):
    for i in str1:
        str3 = ''
        str3 = str3.join(i for i in str1 if i in str2 not in str3)
    return str3

str1 = 'asdfasdfasfd'
str2 = 'qazwsxedc'

strIntersection(str1,str2)

=> 'asdasdasd'

however I only want the the intersection characters to appear once and in order of the first string ie. 'asd'

Can anyone help?

I've found some similar problems on other forums but the solutions all seem to involve lists whereas I'd like my output to be a string

Upvotes: 8

Views: 28830

Answers (7)

Suat Atan PhD
Suat Atan PhD

Reputation: 1382

All answers help, however, you may need to ignore or not ignore the orders. It may be important, especially finding misspellings. Here more sophisticated version of the function for finding string intersection:

def str_intersection(str_left, str_right, ignore_order = True ):
    common_letters = set(str_left) & set(str_right)
    str_intersected = ''
    for c in str_left:
        if (c in common_letters) and (c not in str_intersected):
            str_intersected += c
    if ignore_order:
        pass
    else:
        if str_intersected in str_left and str_intersected in str_right:
            return str_intersected
        else:
            return None
    return str_intersected

If you run str_intersection("AB","XAB") result will be AB. But main handicap of this way is the result of str_intersection("god","dog") will be god. If you want to get nothing use this function like this: str_intersection("god", "dog", ignore_order=False) in that case result will be None because god and dog share the same characters but the order is different. I hope this sophisticated may help you.

Upvotes: 0

Rohit Malgaonkar
Rohit Malgaonkar

Reputation: 523

easiest is to use sets in python

>>> a='asdfasdfasfd'
>>> b='qazwsxedc'
>>> set(a).intersection(b)
set(['a', 's', 'd'])

Upvotes: 2

aquavitae
aquavitae

Reputation: 19114

It looks like your current script should do it if you fix the typo on the fourth line:

str3 = str3.join(i for i in str1 if i in str2 not in str3)

should be

str3 = str3.join(i for i in str1 if i in str2 and i not in str3)

I wouldn't recommend using a set for this simpy because they don't guarantee order. Your script is also likely to be faster.

Upvotes: 1

unwind
unwind

Reputation: 399871

Check for occurances the other way around to get the order under control, and don't emit characters you've already emitted:

def strIntersection(s1, s2):
  out = ""
  for c in s1:
    if c in s2 and not c in out:
      out += c
  return out

Sure you could re-write it to be a list comprehension, but I find this easier to understand.

For your test data, we get:

>>> strIntersection('asdfasdfasfd' , 'qazwsxedc')
'asd'

Upvotes: 7

vartec
vartec

Reputation: 134611

def str_intersection(str1, str2):
    common_letters = set(str1) & set(str2)
    str3 = ''
    for c in str1:
        if (c in common_letters) and (c not in str3):
            str3 += c
    return str3

Upvotes: 0

supakeen
supakeen

Reputation: 2914

You can use python sets http://docs.python.org/library/stdtypes.html#set to do this, like so:

>>> set("asdfasdfasfd") & set("qazwsxedc")
set(['a', 's', 'd'])

Upvotes: 9

Karl Knechtel
Karl Knechtel

Reputation: 61527

You want a string consisting of the unique characters that are common to str1 and str2, in the order they appear in str1.

Uniqueness and commonality imply set operations: that is, we're looking for the set of characters that appear in both str1 and str2. A set is fundamentally unordered, but we can re-order the data by sorting the characters according to their "index" of first occurrence in str1. Then it's a simple matter of creating a string from the sorted sequence.

Putting it all together, we get:

''.join(sorted(set(str1) & set(str2), key = str1.index))

Upvotes: 7

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