# error_demo1.py3  all floating point 64 bits on modern computer

def add(v, ans):  # should force store
  sum = v + ans
  return sum

def main():
  v = [0.0 for i in range(3)] 
  ans = 0.0
  print("error_demo1.py3 1.0e16*((1.0+1.2e-16)-1.0)")
  print("only about 16 digits accuracy, power of 10 not exact")
  v[0] = 1.0  # same as 1.0 double constant into float memory
  v[1] = 1.2e-16
  v[2] = -1.0
  for i in range(3):
    ans = ans + v[i]  #  should be kept in register 

  ans = 1.0e16 * ans
  print("ans=", ans)
  print(" ")
  print("force store, break optimization, no difference here")
  ans = 0.0
  ans = add(v[0], ans)
  ans = add(v[1], ans)
  ans = add(v[2], ans)
  ans = 1.0e16 * ans
  print("ans=", ans)
  print(" ")
  print("expression with double ans=", 1.0e10*((1.0+1.0e-16)-1.0))
  print(" ")
  print("error_demo1.py3 (1.0+1.0e-16)+1.0  too small")
  v[1] = 1.0e-16
  ans = 0.0
  for i in range(3):
    ans = ans + v[i]
  
  ans = 1.0e10 * ans
  print("ans=", ans)
  print(" ")
  print("force store, break optimization, no difference here")
  ans = 0.0
  ans = add(v[0], ans)
  ans = add(v[1], ans)
  ans = add(v[2], ans)
  ans = 1.0e16 * ans
  print("ans=", ans)

if __name__ == '__main__':
  main()

# end error_demo.py3