matrix multiplication (1d to 2d mapping) lost!!!

[hello12]

hello,
i have 2 upper triangular matrices A and B.. the values are stored in efficient format in the text file. I wanted to use those values for matrix multiplication and display the result C on the screen.
I am supposed to use 1d arrays. SO, basically i have 2 --> 1d arrays which i am supposed to multiply to get C[i].C[i] must not store zeroes(i.e it must also be in efficient format).

I am having trouble in the multiplication part... i am getting answers totally out of range for C[i].
int i,j,k,m=0,n=0;
for(i=row;i>0;i--){
for(j=m;j<sum;j++){
for(k=0;k<=j;k++){
matC[j] += matA[k]*matB[j];

}

} m++;
printf("\n");
}
/**** 'sum' is the total slots in the C[i] array****/
I am having trouble developing the equation to put within the for loop.
for ex: A[i]={1,2,3,4,5,6} /*efficient format*/
B[i]={1,2,3,4,5,6}
I need to calculate
C[i] ={ }... i am stuck.. i hope the question is clear
any help would be great.
thanks in advance

[D_C]

I have some code that treats a 1D upper triangular matrix as 2D. I'll post the relevant parts of that code, and hopefully it will help you. Originally, the code was to find N x N symmetric matrices with a zero diagonal, such that each row and column summed to the same value. When N = 5, the matrix took the form

0 A B C D
A 0 E F G
B E 0 H I
C F H 0 J
D G I J 0

Only the upper triangular half was stored, or A - J, was stored.

Notice that each diagonal entry is 0, and not necessary. That's why for N = 5, there are only 1+2+3+4 = N(N-1) / 2 = 10 entries instead of 1+2+3+4+5 = N(N+1) / 2 = 15 entries. If you need 15 entries, possibly just change N-1 to N each time in int row(int) and int col(int).

static const int LENGTH = (N*(N-1))/2; 
int col(int c) // 0=>(0,1), N-1=>(0,N), and LENGTH=>(N,N)
{              
  int dim = N-1; 
  int add = 1;
  while(c >= dim)
  {
    c -= dim;
    dim--;
    add++;
  }
  return c+add;
}
 
int row(int c)
{
  int dim = N-1;
  int add = 0;
  while(c >= dim)
  {
    c -= dim;
    dim--;
    add++;
  }
  return add;
}