Re: the script is failing without a useful report

[MRAB]

On 04/09/2010 22:22, nvictor wrote:

hi all,

need your help. i get a traceback that doesn't tell much about the
actual error in my code:

Traceback (most recent call last):
   File ".\eightqueens.py", line 32, in<module>
     generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)
   File ".\eightqueens.py", line 17, in generate
     else: generate(n, x, col, up, down)

here is the full source:

def generate(n, x, col, up, down):
     for h in range(8):
         # place a queen
         if col[h] and up[n-h] and down[n+h]:
             x[n] = h
             col[h] = False
             up[n-h] = False
             down[n+h] = False
             n += 1

         # 8 queen placed?
         if n == 8: print x
         else: generate(n, x, col, up, down)

         # displace a queen
         n -= 1
         col[h] = True
         up[n-h] = True
         down[n+h] = True


if __name__ == '__main__':
     n = 0
     x = [None]*8
     col = [True]*8
     up = [True]*15
     down = [True]*15
     generate(n, x, col, up, down)

The traceback ends with:

    RuntimeError: maximum recursion depth exceeded

I think what's happening is that if the 'if' condition just after the
"# place a queen" comment is false then 'n' won't be incremented, and
therefore 'generate' will call itself with the exactly same values
which were passed in, resulting in infinite recursion (or at least
until the limit is reached).
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