When the for loop runs, why does it print all permutations of ABC instead of all 'A's?
def perm(l, n, str_a):
if len(str_a) == n:
print str_a
else:
for c in l:
perm(l, n, str_a+c)
perm("ABC", 3, "")
Prints:
AAA AAB AAC ABA ABB ABC ACA ACB ACC BAA BAB BAC BBA BBB BBC BCA BCB...
perm("ABC", 3, "")
, the else
clause is executed (because len("") != 3
). perm("ABC", 3, "A")
, perm("ABC", 3, "B")
and perm("ABC", 3, "C")
. Let's see what happens with the first one: else
is executed, resulting in the function calls perm("ABC", 3, "AA")
, perm("ABC", 3, "AB")
and perm("ABC", 3, "AC")
. perm("ABC", 3, "AA")
: When called, the else
is executed yet again --> perm("ABC", 3, "AAA")
, perm("ABC", 3, "AAB")
and perm("ABC", 3, "AAC")
. len(str_a)
finally is == 3
, which means that str_a
will be printed. CCC
. It does not keep printing 'A's, because, after 3 recursions, it will have formed the string "AAA". Then, the line print str_a
will be executed, as the condition len(str_a) == n
will be verified.
After that, the execution will go back to the callee function, which was inside the c
loop. c
had value "A". At the following iteration, c
will get value "B", and perm("ABC", 3, "AAB")
will be invoked, printing "AAB", and so on.
Maybe the recursion graph could clearen things up (it's incomplete, because it's big)
I have no idea what you are trying to do, but maybe a little bit of debug output would help you figure it out. Try this:
def perm(iter, l, n, str_a):
print "starting iteration {0}: l='{1}', n='{2}', str_a='{3}'".format(
iter, l, n, str_a)
if len(str_a) == n:
print str_a
else:
for c in l:
perm(iter+1, l, n, str_a+c)
perm(1, "ABC", 3, "")
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