Backtracking in Python [2]

Question

I have just came up with this simple Python algorithm for generating all the possible permutations from 1 to n, but it does not seem to work. Here is the code:

def main ():
    n = 3
    x = [0] * 4
    k = 1
    while k:
        ok = True
        while x[k] < n and ok:
            for i in range (0,k-1):
                if x[i] == x[k]:
                    ok = False
                if ok:
                    x[k] += 1
            if x[k] < n:
                if k == n:
                    print x
                else:
                    k+=1
                    x[k] = 0
            else:
                k-=1
main()

When I run it, nothing happens. Can you please help me? I am also new to Python

Zero is treated as false. Body of while loop is not never run. — falsetru, May 26 '14 at 16:36
for n = 3: 1 2 3 [endline] 1 3 2 [endline] 2 1 3 [endline] 2 3 1 [endline] 3 1 2 [endline] 3 2 1 [endline] — ivanciprian, May 26 '14 at 17:16

score 0 · Answer 1 · edited May 23 '17 at 11:50

I have no idea why this is supposed to output permutations (also it prints newline every time, so in any case it will just print out a column of numbers even if it works). You really should take a debugger and investigate it yourself.

Just put this line in the beginning of the function:

import pdb; pdb.set_trace()

and you will be able to go through you program step-by-step. Here is a question with some links and tips on how to use it - Getting started with the Python Debugger pdb

If you know how to install packages, you can install ipdb and do

import ipdb; ipdb.set_trace()

to the same effect, but the debugger will have autocomplete and will generally be a little bit sexier.

Good luck in your studies!

Backtracking in Python [2]

1 Answers1