Prevent memory error in itertools.permutation

问题

Firstly I would like to mention that i have a 3 gb ram.

I am working on an algorithm that is exponential in time on the nodes so for it I have in the code

perm = list( itertools.permutations(list(graph.Nodes))) # graph.Nodes is a tuple of 1 , 2 , ... n integers

which generates all the combinations of vertices in a list and then i can work on one of the permutation.

However when i run the program for 40 vertices , it gives a memory error.

Is there any simpler way in implementation via which i can generate all the combinations of the vertices and not have this error.

回答1:

Try to use the iterator generated by the permutations instead of recreating a list with it :

perm_iterator = itertools.permutations(list(graph.Nodes))

for item in perm_iterator:
   do_the_stuff(item)

by doing this, python will keep in memory only the currently used permutation, not all the permutations (in term of memory usage, it is really better ;) )

On the other side, once the memory problem solved, the time to treat all the permutations will be growing exponentially with the number of vertices....

回答2:

It won't work. Looping over an iterator won't work either. You see, if the code in the for-loop takes 1 microsecond to run, it will take 2.587×10^34 years to run completely. (See http://www.wolframalpha.com/input/?i=40%21+microseconds+in+years)

来源：https://stackoverflow.com/questions/6503388/prevent-memory-error-in-itertools-permutation