input: Write a function to calculate the harmonic sum of n-1.
  assert harmonic_sum(7) == 2.5928571428571425
  assert harmonic_sum(4) == 2.083333333333333
  assert harmonic_sum(19) == 3.547739657143682
  
output: def harmonic_sum(n):
      sum = 0
      for i in range(1, n):
          sum += 1 / i
      return sum