Count or add list elements without using python built-in functions

Question

I have these 2 simple functions to sum and count list elements without using standard python commands like sum() and len() .

#sum the elements of a list 
def sum_list(x):
  n=0
  for i in x:
      n= n+i
  return n

ex. if x=[1,5] - should return 6

#count the number of elements in a list 
def count_list(x):
  n=0
  for i in x:
      n= n+1
  return n

ex. if x=[1,5] - should return 2

The problem is I cant understand how they work. More specifically:

1) What does n=0 represents before the loop? Is it some kind of starting point?

2) After that, I can understand what the for-loop does, but I cant get what the n= n+i and n=n+1 actually means. What is "n" in this case, and why if I add "i" (n+i) gives the sum while adding "1" (n+1) gives the length of the list?

Answer 1

Make use of print statements to help you understand how the loop is working. (copy paste the code and run) To answer you questions

n -> is the variable that holds the values of n+i and n+1 , so value of n+i and n+1 gets stored in n (The code moves left to right). You can think of it as a starting point

so n+i or n+1 does the computation needed and stores the number in n and then returns it in the next step.

To answer you question "why if I add "i" (n+i) gives the sum while adding "1" (n+1) gives the length of the list?" : i will keep changing with every loop iteration but in n+1 , only n changes everytime. Therefore sum function gives you the sum and the count function gives you the length.

You can use len to get the number of elements in a list

Hope this helps.

I have added more print statements to help you understand it better, run the code and see if it helps.

def sum_list(x):
    print("SUM FUNCTION START")    
    n = 0
    for i in x:
        print("n: ",n)
        print("i: ",i)        
        n = n + i
        print("n =",n,",","i= ",i)
        print("n + i = ",n)
        print()
    print("SUM FUNCTION END")
    print()
    return n

def count_list(x):
    print()
    print("COUNT FUNCTION START")
    n = 0
    for i in x:

        print("n: ",n)
        print("i: ",i)        
        print("n =",n,",","i= ",i)
        print("n + 1 = ",n)
        n = n + 1
        print()
    print("COUNT FUNCTION END")                
    return n

#EASIER WAY TO COMPUTER LENGTH OF THE LIST USING len FUNCTION        
def count_list2(x):
    return len(x)

def main():
    x = [1,2,3,4,5]
    answer = sum_list(x)
    print("Sum : ", answer)
    elements = count_list(x)
    elements2 = count_list2(x)
    print()
    print("There are", elements, "elements in the list")
    print()
    print("There are", elements2, "elements in the list")
main()