Replace multiple elements in string with str methods

Question

I am trying to write a function that takes a string of DNA and returns the compliment. I have been trying to solve this for a while now and looked through the Python documen

Answer 1

You can map each letter to another letter.

You probably need not create translation table with all possible combination.

>>> M = {'A':'T', 'T':'A', 'C':'G', 'G':'C'}
>>> STR = 'CGAATT'
>>> S = "".join([M.get(c,c) for c in STR])
>>> S
'GCTTAA'

How this works:

# this returns a list of char according to your dict M
>>> L = [M.get(c,c) for c in STR]  
>>> L
['G', 'C', 'T', 'T', 'A', 'A']

The method join() returns a string in which the string elements of sequence have been joined by str separator.

>>> str = "-"
>>> L = ['a','b','c']
>>> str.join(L)
'a-b-c'

Answer 2

For a problem like this, you can use string.maketrans (str.maketrans in Python 3) combined with str.translate:

import string
table = string.maketrans('CGAT', 'GCTA')
print 'GCTTAA'.translate(table)
# outputs CGAATT