Python runtime error

Question

why isn't mcnt a global variable???
.........................................
Below is the recursive
algorithm to preform
the towers of hanoi. For n rings move n-1 rings to the temperary peg, then move the nth ring form source peg to the final peg.
Now move n-1 rings from the temperary peg to the to the final peg '''
#global mcnt
mcnt = 0
def hanoi(n, a, b, c):
#mcnt += 1
if n <= 0: return
hanoi(n-1, a, c, b)
report(n, a, b)
hanoi(n-1, c, b, a)

def report(n, a, b):
pna=['a', 'b', 'c']
x, y = pna[a], pna[b]
#mcnt = 1
mcnt += 1
printf("Move# %d ring %d from peg %c to peg %c\n", mcnt,n, x, y );

………………………………………………………….

File "./Playground/file0.py", line 40, in report mcnt += 1 UnboundLocalError: local variable 'mcnt' referenced before assignment
………………………………………………………………

in report mcnt += 1 UnboundLocalError: local variable 'mcnt'

Answer 1

If you want to mutate/change the value of a global variable, you need to bring it in with "global" keyword like this:

mcnt = 0
def hanoi(n, a, b, c):
#mcnt += 1
if n <= 0: return
hanoi(n-1, a, c, b)
report(n, a, b)
hanoi(n-1, c, b, a)

def report(n, a, b):
global mcnt # indicate that report may change the value of global variable mcnt
pna=['a', 'b', 'c']
x, y = pna[a], pna[b]
#mcnt = 1
mcnt += 1
print("Move# %d ring %d from peg %c to peg %c\n" % (mcnt,n, x, y) )

Also I think you want print instead of printf.

Answer 2

thanks Josh