[Tutor] substitute function

Alan G alan.gauld at freenet.co.uk
Mon Aug 1 00:07:01 CEST 2005

> Is there a 'substitute' function in python.

Assuming we are dealing with strings then there are two.
The string objects provide a replace() method and the 
regulatr expression module re provides substitute.

> I want to substitute A with T and G with C and vice
> versa
> A -> T
> G -> C
> T -> A
> c -> G

But the vice-versa creates an awkward sequencing problem.
Whichever one you do first messes you up for the reverse 
mapping. Youi need to do multiple swaps at once.

ISTR a module being highlighted in the Python challenge 
thing that was muxch discussed a few weeks ago that might 
help here, otherwise I'd create a map table and just brute 
force scan each character and replace where the character 
is in the table.

--------- untested pseudo code --------
swaps = {'A':'T', 'G':'C',...}

chars = list(longstring)
for index in len(chars):
   if chars[index] in swaps:
      chars[index] = swaps[chars[index]]

longstring = ''.join(chars)

Not very fast but it would do it. If the strings are very 
long then I might try some clever regex stuff, or look up 
the old thread to find that clever module..


Alan G.


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