Why Am I Getting An Infinite For Loop?

For this python problem I am taking in one argument an int, this is the max length of a list I am going to be appending to. Starting with an int value of 1, I want to iterate throu

Solution 1:

You are adding to the list as you loop over it. A list iterator simply increments a position counter and returns the value at that index until the index doesn't exist. It doesn't keep track of the length of the list up-front. In your case, by adding more elements to the end, the iterator never reaches an index that doesn't exist.

You have several options:

• Create a copy of the list first to iterate over; that copy won't grow:

  defdbl_linear(n):
      lst_u = [1]
      whilelen(lst_u)<=n:
          for i in lst_u[:]:  # [:] returns a shallow copy
              lst_u.append(2*i+1)
              lst_u.append(3*i+1)
      returnsorted(lst_u)
  

• Append to a separate, new list and extend the original after the loop:

  defdbl_linear(n):
      lst_u = [1]
      whilelen(lst_u)<=n:
          new_values = []
          for i in lst_u:
              new_values.append(2*i+1)
              new_values.append(3*i+1)
          lst_u.extend(new_values)
      returnsorted(lst_u)
  

• Use a range() to produce indices; this is based on taking the length once:

  defdbl_linear(n):
      lst_u = [1]
      whilelen(lst_u)<=n:
          for idx inrange(len(lst_u)):
              i = lst_u[idx]
              lst_u.append(2*i+1)
              lst_u.append(3*i+1)
      returnsorted(lst_u)