FAQ: Code Challenge: Lists - Append Sum


This community-built FAQ covers the “Append Sum” exercise from the lesson “Code Challenge: Lists”.

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This exercise can be found in the following Codecademy content:

Computer Science
Data Science

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This challenge was comparatively far easier than some of the other challenges in this section. I simply got the sum of the last two elements and appended the list. Rinsed and repeated until finished. This seemed to be a little inefficient. I’m sure this will be covered in later lessons but my curiosity is getting the best of me. How would I build a loop that terminates after a finite number of runs?


I was also curious, how to build a loop here, and this is what I came up with

def append_sum(lst):
  i = 0
  while i < 3:
    lst.append(lst[-2] + lst [-1])
    i += 1
  return lst

Changing the “3” will change the number of loops the code will run.


We can use the input to derive the number of iterations:

while i < len(lst):

This way the list may be any length. We can also clone the list and iterate over that…

for _ in lst[:]:

An equivalent to this line,

lst.append(lst[-2] + lst[-1])

would be,


Slightly off topic, what does this process remind one of (sum last two digits and append to list)? Hint: Fibonacci number sequence ring a bell?

def fibonacci_seq(n):
  fib = [1,1]
  for _ in ['']* (n - 2):
  return fib

print (fibonacci_seq(6))    # [1, 1, 2, 3, 5, 8]


But did this work? I used the same code but the although the loop runs three times it only adds one element to the list

ps: found the error: in my code the return lst command was not indented…