How to use Python slice to extract a list of positions in a string

I have two strings:

a='1234512345'

b='abcdefghik'

I would like to search string "a" for occurrences of '1' and then print the positions of "b" that correspon开发者_开发技巧d to that index, i.e.

'af'

I can use

import re

starts=[match.start() for match in re.finditer(re.escape('1'), a)]

to find that '1' occurs in positions [0,5]. How would I use this info to extract 'af' from string "b"

You could do something like this:

''.join(b[x] for x in starts)

But I would recommend this instead:

a='1234512345'
b='abcdefghik'

''.join(y for (x, y) in zip(a, b) if x == '1')

>>> a='1234512345'
>>> b='abcdefghik'
>>> [ j for i,j in zip(a,b) if i=="1" ]
['a', 'f']

In [11]: a='1234512345'    
In [12]: b='abcdefghik'

In [16]: ''.join(b[i] for i,num in enumerate(a) if num=='1')
Out[16]: 'af'

or, if you really want to use regex:

In [21]: ''.join(b[match.start()] for match in re.finditer(re.escape('1'), a))
Out[21]: 'af'

import re
a='1234512345'

b='abcdefghik'

starts= [ b[i] for i in [ match.start() for match in re.finditer(re.escape('1'), a)]]
print ''.join(starts)

"".join(b[i] for i in range(min(len(a), len(b))) if a[i] == "1")

like this?

a='1234512345'

b='abcdefghik'

for char in a:
    n = -1
    for subc in a:
        n=n+1
        if subc == char:
            print b[n],
    print

produces:

a f
b g
c h
d i
e k
a f
b g
c h
d i
e k

If you have to repeat this for a few values of a, it will be more efficient ((O(n)) to build a dictionary than to loop through a and b repeatedly (O(n*n))

>>> a='1234512345'
>>> b='abcdefghik'
>>> from collections import defaultdict
>>> D=defaultdict(str)
>>> for i,j in zip(a,b):
...  D[i]+=j
... 
>>> D['1']
'af'
>>> D['2']
'bg'