Generalizing a nested loop

[nanotech]

I have a nested loop which generates ordered pairs, triplets, ... of integers from 0 to a maximum N.

for ( a[0]=0;a[0]<N;a[0]++)
   for( a[1]=0;a[1]<a[0];a[1]++)
      for( a[2]=0;a[2]<a[1];a[2]++)
 

I would like to generalize this for a[n] for given n, or find some more creative way of generating these sets.
Any help is much appreciated.

[willakawill]

I have a nested loop which generates ordered pairs, triplets, ... of integers from 0 to a maximum N.

for ( a[0]=0;a[0]<N;a[0]++)
   for( a[1]=0;a[1]<a[0];a[1]++)
      for( a[2]=0;a[2]<a[1];a[2]++)
 

I would like to generalize this for a[n] for given n, or find some more creative way of generating these sets.
Any help is much appreciated.

Hi are n and N different values?

[nanotech]

Yes they are different sorry for the confusion. N is just the maximum value allowed and n is the number of elements taken at a time 2,3,4... this is what I would like to make general.

[willakawill]

Yes they are different sorry for the confusion. N is just the maximum value allowed and n is the number of elements taken at a time 2,3,4... this is what I would like to make general.

OK. here you go

   const int n = 3;
   int a[n];
   int N = 100;
 
   for (int i = 0; i < n; i++)
      a[i] = N - (i + 1);

[nanotech]

Thank you for your reply but that is not quite what I am looking for. I need all possiblities, as produced by this code for n=3 and N=5. If I want n=4 then I need to add an additional loop
for ( a[3]=0;a[3]<a[2];a[3]++)
and having to change this for different n or n>10 i would like to avoid.

#include<iostream>
int main()
{
  const int n=3;
  int N=5,a[n];
 
  for ( a[0]=0;a[0]<N;a[0]++)
    for( a[1]=0;a[1]<a[0];a[1]++)
      for( a[2]=0;a[2]<a[1];a[2]++)
        std::cout<<a[0]<<','<<a[1]<<','<<a[2]<<'\n';
 
  return 0;
}
 

[willakawill]

Thank you for your reply but that is not quite what I am looking for. I need all possiblities, as produced by this code for n=3 and N=5. If I want n=4 then I need to add an additional loop
for ( a[3]=0;a[3]<a[2];a[3]++)
and having to change this for different n or n>10 i would like to avoid.

#include<iostream>
int main()
{
  const int n=3;
  int N=5,a[n];
 
  for ( a[0]=0;a[0]<N;a[0]++)
    for( a[1]=0;a[1]<a[0];a[1]++)
      for( a[2]=0;a[2]<a[1];a[2]++)
        std::cout<<a[0]<<','<<a[1]<<','<<a[2]<<'\n';
 
  return 0;
}
 

You have just added a line of code that completely changes your question

[nanotech]

I should have posted this code to begin, I need to make use of the values in each loop, not just the final loop.

[nanotech]

Well, I found a solution in a recursive function, the only thing I am not thrilled about is that instructions are now in the recursive function 'loop' after the else. Thanks willakawill for replying.

#include<iostream>
 
void loop(int a[], int n,int i){
  for (a[i]=0;a[i]<a[i-1];a[i]++){
    if ((i+1)<n) loop( a, n, i+1);
    else std::cout<<a[0]<<','<<a[1]<<','<<a[2]<<'\n';
  }
}
 
int main()
{
  const int n=3;
  int N=5,a[n];  
 
  for ( a[0]=0;a[0]<N;a[0]++)
    loop(a,n,1);
 
  return 0;
}