Question

I’m trying to construct the following 14*14 array i C: [I 0; 0 -I], that is a 7*7 identity matrix upper left, minus the identity lower right and zeros otherwise.

This is the method:

#define DIM 7

 double S[2*DIM][2*DIM];

 for(i = 0; i < DIM; i++){
     for(j = 0; j < DIM; j++){
         if(i == j){
             S[i][j] = 1.0;
             S[i+7][j+7] = -1.0;
         }
         else{
             S[i][j] = 0.0;
         }
     } 
 } 

This works fine for all the diagonal elements; however, some elements of the array get initialized to crazy values; for example, 13,6 gets initialized to

68111186113812079535019899599437200576833320031036694798491976301968333351950125611739840800974137748034248687763243996679617222196278187875968953700681881752083957666277350377710107236511681624408064.000000

This seems to be happening consistently (at least thrice) to entries 11,13, 12,9, 12,10, 13,12 and 13,6.

Can anybody tell me what’s at play here or provide an alternative solution?

Cheers!

EDIT: The weird entries aren’t consistent.

EDIT2: Typo: 13,12, not 13,15

Answer 1

Your loop only covers the upper left quadrant, so the off-diagonal elements in the other quadrants are not initialized and contain garbage. Your loop should go up to 2*DIM for each dimension, so that the off-diagonal elements are zeroed, and then your conditional for the diagonal elements should just be a little more complex to decide which value to set the diagonal element to.

Note that [13, 15] is entirely outside of this array!