Subj : Re: how to generate a sequence?
To   : comp.programming
From : Thad Smith
Date : Thu Sep 08 2005 04:36 pm

Neo wrote:

> >> I have problem, I want to generate all possible (unique) combinations of
> >> the
> >> following sequence:
> >>
> >>
> >> 123456789
> >> 213456789
> >> 312456789
> >> 321456789

What you describe is all possible permutations, not combinations.

> Well, for 4-5 symbols it was quick enough, but to ma surprise when I
> increase symbols to 9 damm.... it took 01 min 28.0954414 sec to exec    and
> it process's mem usage was 216 MB !!!!!!!!!!!!!!!!!!!!!

That's because you kept each generated permutation in memory.

> BUT THIS IS REALY REALYYYYY CRAZY... This not absolutely not
> acceptable!!!!!!!
> Think about this.. if this sequence is to be generated in an embedded system
> where maximum ram available is 32 KB only!!!!!!!!!!!

The questions to ask are: 
1. Why do you want to generate all permutations?  
2. What do you do with a permutation once generated?
3. How long do you need to keep each permutation in memory?

Typically, you don't need all permutations at once.

Thad

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