Unyeilding a permutation generator

[Steve Holden]

sillyhat at yahoo.com wrote:
> Hello, can someone please help.
> 
> I found the following code at http://code.activestate.com/recipes/252178/
> 
> def all_perms(str):
>     if len(str) <=1:
>         yield str
>     else:
>         for perm in all_perms(str[1:]):
>             for i in range(len(perm)+1):
>                 #nb str[0:1] works in both string and list contexts
>                 yield perm[:i] + str[0:1] + perm[i:]
> 
> which allows me to do things like
> 
> for x in  all_permx("ABCD"):
>   print x
> 
> I believe it is making use of the plain changes / Johnson Trotter
> algorithm.
> Can someone please confirm?
> 
> Anyway what I want to do is experiment with code similar to this (i.e.
> same algorithm and keep the recursion) in other languages,
> particularly vbscript and wondered what it would look like if it was
> rewritten to NOT use the yield statement - or at least if it was
> amended so that it can be easily translated to other languages that
> dont have python's yield statement. I think the statement "for perm in
> all_perms(str[1:]):"  will be hardest to replicate in a recursive
> vbscript program for example.
> 
There are various approaches you could use, the simplest of which is to
provide a "callback function" as an additional argument to the all_perms
function, and call it with the permutation as an argument in place of
the yield statement.

regards
 Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
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