From where this 2 came in output? Fibonacci series using python Lists

If you do not want to read this text, I have explained the problem in this Youtube video: https://youtu.be/Ekkkgjf0F_s

Following is the code that I am using to generate Fibonacci series using python
lists.

list1 = [0, 1]
x=1
while x <=2:
length = len(list1)
first =list1[length-2]
second =list1[length-1]
third = first + second
list1.append(third)
x+=1
print list1


When the while loop runs for the 1st iteration, it generates the upcoming element in series and stores in list exactly what it should do.
The list will now become:

list1 = [0,1,1]


But what confusing me is when the second iteration is made by while loop.
If you dry run the code, you will see that the code is outputting 2 (according to fibonacci series sequence, it is right)
but if we dry run the code, the 4th element should be 3 instead of 2

2nd interation, length = 3>>Dry RUN below:

3-2=1
3-1=2
1+2=3
list1 should be: [0,1,1,3]


But i am getting the output as:
list1=[0,1,1,2]

list1=[0,1,1,2]

I can't understand , how this 2 came in output.

2 Answers
2

your list has len() of 3, and therefore your algorithm adds element 1 and 2 (which are both 1) together. that's why you get returned the 2.

len()

EDIT: which is exactly how the Fibonacci-Series goes....

Code with comments as follows:

length = len(list1) #3
first =list1[length-2] #list on index 1 is value 1
second =list1[length-1] #list on index 2 is value 1
third = first + second # 1+1 = 2
list1.append(third) # 2 is appended
x+=1
print list1


you probably confuse the value on list index [1] with the actual difference between len of list 3 and 1.

When you list1 becomes [0,1,1], the len of list1 is 3.

[0,1,1]

Now try running the following:

length = len(list1) = 3
first = list1[length-2] = list1[3-2] = list1[1] = 1
second = list1[length-1] = list1[3-1] = list1[2] = 1
third = first + second = 1 + 1 = 2


Hence, it adds 2 to the list.

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