int i, x, a[2];

void q(int b)
{
  // here you have: a = [1, 2], and b = 1 from the call
  
  a[0] = 2; // now: a = [2, 2]
  
  i = 1;
  
  x = b+a[i]; // x = 1 + a[1] = 1 + 2 = 3
  
  b = a[0]; // b = 2 but forgot because it's local to the function
}

void main()
{
  i = 0;
  a[0] = 1;
  a[1] = 2;
  
  // a[i] = a[0] = 1, so: q(1)
  q(a[i]);
}