Two for loops written as a while loop for assign values to a symmetric matrix

Question

I know the logic two get into a symmetric matrix the values of an array

int k=0;
for (int i = 0; i < size; i++){
     for (int j = 0; j <= i; j++){                
          Q[i, j] = Q[j, i]= arr[k++];
     }
}

But How to do this if I can only use a while loop?

I was doing something like:

int i=0;
int j=0;
while (reader.Read())
{
   Q[i, j] = Q[j, i]=reader.GetDouble(1);
   if (j < i){
      j++;
   }else{
      j = 0;
      i++;
   }
}

Is the logic correct, How to improve this code?

Answer 1

I personally think the while loop looks less clean than the nested for loop. I'd think just adding an extra conditional like this would work:

int k=0;
for (int i = 0; i < size; i++){
     for (int j = 0; j <= i && reader.Read(); j++){                
          Q[i, j] = Q[j, i]= reader.GetDouble(1);
     }
}

If reader returns false before your matrix is full it will spend some cycles going through the i loop, but it should be simple to improve if that's a concern.

Answer 2

First look at my answer on how to represent a symmetric matrix with a 1D array here:

https://stackoverflow.com/a/9040526/380384

So you can assign the values like this

int k = 0;
while (reader.Read())
{
   A[k++] = reader.GetDouble(1);
}
int size = (int)(Math.Sqrt(2*k+0.25)-0.5);

given that only size*(size+1) values are needed to be entered.