[Tutor] Recursion
Glen Wheeler
wheelege@tsn.cc
Wed, 6 Jun 2001 18:41:18 +1000
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Hey all,
While fiddling around with maths, I thought it would be nice and cool =
to get all the possible permutations of a set, taken n at a time. I =
have been trying to get this to work nicely, but I'm just not familiar =
enough with recursion to get it to behave.
For example, if I wanted to get all the permutations of a set =
['a','b'] taken three at a time, I would write this :
>>> c =3D []
>>> s =3D ['a', 'b']
>>> for i1 in range(len(s)):
... for i2 in range(len(s)):
... for i3 in range(len(s)):
... c.append([s[i1], s[i2], s[i3]])
...=20
>>> for item in c: print item
...=20
['a', 'a', 'a']
['a', 'a', 'b']
['a', 'b', 'a']
['a', 'b', 'b']
['b', 'a', 'a']
['b', 'a', 'b']
['b', 'b', 'a']
['b', 'b', 'b']
Which is all well and good, but when I want to take them n at a time I =
need more for loops. Thus I thought 'Hmmm, I could get a function which =
calls itself n times...that would work!' - but I'm just too green with =
this sort of programming for it to happen for me.
Help?
Thanks,
Glen.
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<BODY bgColor=3D#ffffff>
<DIV> Hey all,</DIV>
<DIV> </DIV>
<DIV> While fiddling around with maths, I thought it would be nice =
and=20
cool to get all the possible permutations of a set, taken n at a =
time. I=20
have been trying to get this to work nicely, but I'm just not familiar =
enough=20
with recursion to get it to behave.</DIV>
<DIV> For example, if I wanted to get all the permutations of =
a set=20
['a','b'] taken three at a time, I would write this :</DIV>
<DIV> </DIV>
<DIV>>>> c =3D []<BR>>>> s =3D ['a', =
'b']<BR>>>> for i1=20
in range(len(s)):<BR>... for i2 in range(len(s)):<BR>... =
for=20
i3 in range(len(s)):<BR>... c.append([s[i1], s[i2],=20
s[i3]])<BR>... <BR>>>> for item in c: print item<BR>... =
<BR>['a', 'a',=20
'a']<BR>['a', 'a', 'b']<BR>['a', 'b', 'a']<BR>['a', 'b', 'b']<BR>['b', =
'a',=20
'a']<BR>['b', 'a', 'b']<BR>['b', 'b', 'a']<BR>['b', 'b', 'b']<BR></DIV>
<DIV> Which is all well and good, but when I want to take them n =
at a time=20
I need more for loops. Thus I thought 'Hmmm, I could get a =
function which=20
calls itself n times...that would work!' - but I'm just too green =
with this=20
sort of programming for it to happen for me.</DIV>
<DIV> Help?</DIV>
<DIV> </DIV>
<DIV> Thanks,</DIV>
<DIV> Glen.</DIV></BODY></HTML>
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