Thread: Simplification of nested loops in C

This would allow you to choose a K value without having to edit the code each time.

Code:

    i[0]= i[1] = i[2] = 0;
    do {
      int x;
      printf("%d %d %d\n", i[0], i[1], i[2] ); // for example, if K = 3
      for ( x = K-1 ; x >= 0 ; x-- ) {
        if ( ++i[x] == N[x] ) {
          if ( x > 0 ) i[x] = 0;
        } else {
          break;
        }
      }
    } while ( i[0]<N[0] );

> How I can reduce the loops in an optimal way to speed up my program.
You need to tell us what you're trying to do if you want us to suggest a better way than try every possible combination.

Rewriting the code as above won't make it any quicker. All the loops are still there, they're just less obvious.

Thank you.

Suppose that I have a matrix M of dimension K and I want to calculate the partial derivation for each dimension of this matrix.
Then for dimension K I have to use K loops for each partial derivative




If K = 3:

Code:

for(i2=0 ; i2<N[2] ; i2++){
    for(i1=0 ; i1<N[1] ; i1++){
        for(i0=1 ; i0<N[0] ; i0++){
            Dx0[i0-1][i1][i2] = M[i0][i1][i2] -M[i0-1][i1][i2]  ;
        }
    }
}

Code:

for(i0=0 ; i0<N[0] ; i0++){
    for(i2=0 ; i2<N[2] ; i2++){
        for(i1=1 ; i1<N[1] ; i1++){
            Dx1[i0][i1-1][i2] = M[i0][i1][i2] -M[i0][i1-1][i2]  ;
        }
    }
}

Code:

for(i1=0 ; i1<N[1] ; i1++){
    for(i0=0 ; i0<N[0] ; i0++){
        for(i2=1 ; i2<N[2] ; i2++){
            Dx2[i0][i1][i2-1] = M[i0][i1][i2] -M[i0][i1][i2-1]  ;
        }
    }
}

I'm looking for a general method to decrease the amount of code.

I suppose I could suggest you use a code-generator to automatically generate the large amount of nested loops and nasty looking matrix indexing? Best I got...

Basically, instead of writing all that nasty stuff, write a program that would generate all that, given whatever parameters there are.. like matrix dimensions, whatever... sort of like a web page template system... then compile the output of that. It will still be ugly but at least you won't have to deal with writing it by hand!

Also, if there are any optimizations your program could make, do it. Then you're on you're way to writing a real type of compiler. First get it working correctly of course...