Trick to visit all 4 adj. blocks in a matrix

int di[]={1,-1,0,0}; // <-----THIS IS THE TRICK---|

int dj[]={0,0,1,-1}; // <------------------------------|

for(int i=0;i<4;i++){ // ALL FOUR ADJ. BLOCKS

    I=a+di[i];    
    J=b+dj[i];  
  if(I>=0 && I<r && J>=0 && J<c) //validation for borders
                       //CODE;
}