Python - Intersectiing strings

Question

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

Answer 1

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'

Answer 2

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))

Answer 3

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'])