Thread: Numbers combinations(possibilities)

>for example i get 1,2,3 so the combinaions is: 1,3,2 3,1,2 3,2,1
For future reference, combinations and permutations are very different. You want permutations.

>Do it recursively
It's just as easy to do it non-recursively. The factorial example is a good example of a bad way to use recursion.

>If you were to do it recursively, how would you have done it ?
I wouldn't have done it recursively, that's the point. A recursive factorial function doesn't buy you anything over a non-recursive function:

Code:

int factorial ( int n )
{
  int f = n != 0 ? n : 1;

  while ( --n > 1 )
    f *= n;

  return f;
}

It's a silly example that displays exactly none of the benefits of recursion and all of the problems.

>>A recursive factorial function doesn't buy you anything over a non-recursive function.
Sometimes it buy.
1.It makes the code short n sweet.
2.Sometimes its easy to find recursive than iterative soln for a novice like me.
Of course recursive soln have lots of overhead but who cares when it is short program .

>When I see a recursive function (i.e factorial etc) then I guess its
>natural for me to think along the lines of implementing it recursively.
True, but an elegant recursive description doesn't necessarily translate into an elegant recursive function. As a general rule, I avoid recursion unless it clearly solves a smaller version of the same problem, but also divides the problem roughly in half at each step. At least then I have a good chance of the function not being ghastly. As with any guideline, this doesn't always apply, but it fits for a lot of the algorithms work that I do.

>Anyway, to complete a simple task as the one outlined I dont see the harm.
Also true, but IMO it's akin to bubble sort. Sure, it's fine on the small problems that teachers mean it to be used for, but they neglect to mention that it's godawful slow for anything else. That's why you always see recursive solutions for the fibonacci series when those people really should know better. You said "Do it recursively", with no mention of the pitfalls. There's nothing wrong with using a suboptimal solution as long as you're aware that there are better choices and make an informed decision.