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# Unyeilding a permutation generator

 P: n/a 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. Thanks for any constructive help given. Hal Nov 2 '08 #1
13 Replies

 P: n/a si******@yahoo.com writes: 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 - Without the yield statement and keeping the same space complexity, you basically have to wrap the enumeration state in a data object that you can enumerate over explicitly. That in turn may mean you have to unwind the recursion into old fashioned stack operations. Nov 2 '08 #2

 P: n/a si******@yahoo.com wrote: 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 An obvious (though memory-inefficient) replacement is to accumulate a list and return the list. Initialize a result variable to an empty list, and instead of yielding elements, append them to the result variable. Then return the result variable at the end of the function. HTH, -- Carsten Haese http://informixdb.sourceforge.net Nov 2 '08 #3

 P: n/a On Nov 2, 3:34*pm, silly...@yahoo.com wrote: Hello, can someone please help. I found the following code athttp://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. Thanks for any constructive help given. Hal I think multi-threading is the "truest" to the original. You might develop a framework to set events when particular generators are to take a turn, place their "yields", per se, in a particular place, set the return event, etc., and reuse it. Of course, starting threads in VBS might be another matter. Nov 2 '08 #4

 P: n/a si******@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 Holden Web LLC http://www.holdenweb.com/ Nov 3 '08 #5

 P: n/a Steve Holden wrote: si******@yahoo.com wrote: >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 wasrewritten to NOT use the yield statement - or at least if it wasamended so that it can be easily translated to other languages thatdont have python's yield statement. I think the statement "for perm inall_perms(str[1:]):" will be hardest to replicate in a recursivevbscript 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. I had thought of three ways to avoid yield, that makes a fourth. Summary: 1. Think of generator function as an abbreviated iterator class and unabbreviate by writing a full iterator class with .__next__ (3.0). + Transparent to caller - Will often have to re-arrange code a bit to save locals values as attributes before returning. If there is an inner loop, efficiency may require copying attributes to locals. 2. Add to collection rather than yield, then return collection rather than raise StopIteration. + Transparent to caller; small change to function. - Returned collection can be arbitrarily large and take an arbitrarily long time to collect. 3. Add callback parameter and change 'yield value' to 'callback(value)'. + Minor change to generator function. - Minor to major change to caller to turn consumer code into a callback that somehow gets result of callback to where it is needed. (Avoiding the major changes sometimes needed was one of the motivations for generators.) 4. Combine caller and generator code by inlining one into or around the other. + Neither code will change much. - Loss benefit of modularity and reuse without cut-and-paste, which is subject to error. I have not thought through yet how well each of these work with recursive generators. Conclusion: Generators with yield make Python a great language for expressing combinatorial algorithms. Terry Jan Reedy Nov 3 '08 #6

 P: n/a On Nov 2, 10:34*pm, silly...@yahoo.com wrote: 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. Thanks for any constructive help given. Here is a solution which does not use yield, translittered from some Scheme code I have: def perm(lst): ll = len(lst) if ll == 0: return [] elif ll == 1: return [lst] else: return [[el] + ls for el in lst for ls in perm([e for e in lst if not e==el])] if __name__ == '__main__': print perm('abcd') Nov 3 '08 #7

 P: n/a On Sun, 2 Nov 2008 14:09:01 -0800 (PST), Aaron Brady for x in *all_permx("ABCD"):* print x .... I think multi-threading is the "truest" to the original. You might develop a framework to set events when particular generators are to take a turn, place their "yields", per se, in a particular place, set the return event, etc., and reuse it. Of course, starting threads in VBS might be another matter. Why multi-threading? I see no concurrency in the original algorithm. There is, in my mind, nothing concurrent about 'yield'. /Jorgen -- // Jorgen Grahn

 P: n/a On Mon, 03 Nov 2008 21:09:58 +0000, Jorgen Grahn wrote: Why multi-threading? I see no concurrency in the original algorithm. There is, in my mind, nothing concurrent about 'yield'. No "real" concurrency but a generator can be seen as independent thread of code where the generator code is allowed to run when `next()` is called and stops itself when it ``yield``\s an object. Sort of cooperative multitasking. The name "yield" is often used in concurrent code like Java's or Io's `yield()` methods. Ciao, Marc 'BlackJack' Rintsch Nov 3 '08 #9

 P: n/a On Nov 3, 4:13*pm, Marc 'BlackJack' Rintsch