Python: the for loop inside the fib function doesn't make sense to me

Question

In my Python course I have an exercise about Fibonacci numbers. Below you encounter the complete text of chapter 4.1.5.7.

My question is about in the function itself. Particularly I am puzzled what is happening inside the loop:

for i in range(3, n + 1):
    sum = elem1 + elem2
    elem1, elem2 = elem2, sum
return sum

In the third line you see elem1 comma elem2 is equal to elem2 comma sum. What does it mean?

Can someone please explain step by step how I can interpret the content of this loop?

Complete text of chapter 4.1.5.7:

Some simple functions: Fibonacci numbers Are you familiar with Fibonacci numbers?

They are a sequence of integer numbers built using a very simple rule:

the first element of the sequence is equal to one (Fib1 = 1) the second is also equal to one (Fib2 = 1) every subsequent number is the sum of the two preceding numbers (Fibi = Fibi-1 + Fibi-2) Here are some of the first Fibonacci numbers:

fib1 = 1
fib2 = 1
fib3 = 1 + 1 = 2
fib4 = 1 + 2 = 3
fib5 = 2 + 3 = 5
fib6 = 3 + 5 = 8
fib7 = 5 + 8 = 13

What do you think about implementing it as a function?

Let's create our fib function and test it. Here it is:

def fib(n):
    if n < 1:
         return None
    if n < 3:
        return 1

    elem1 = elem2 = 1
    sum = 0
    for i in range(3, n + 1):
        sum = elem1 + elem2
        elem1, elem2 = elem2, sum
    return sum

for n in range(1, 10): # testing
    print(n, "->", fib(n))

Analyze the for loop body carefully, and find out how we move the elem1 and elem2 variables through the subsequent Fibonacci numbers.

The test part of the code produces the following output:

1 -> 1
2 -> 1
3 -> 2
4 -> 3
5 -> 5
6 -> 8
7 -> 13
8 -> 21
9 -> 34

Source available on Cisco Networking Academy - Course name: PCAP - Programming Essentials in Python - module 4, chapter 4.1.5.7 titled "Creating functions | Fibonacci numbers".

"elem1 comma elem2 is equal to elem2 comma sum" More accurately, `=` should be verbalized as "is assigned". This will help you avoid confusion between `=` and `==` in programming. — Code-Apprentice, Jun 22 '20 at 19:58

score 5 · Answer 1 · answered Jun 22 '20 at 18:36

5

In python, a, b = c, d means "set a=c and b=d"

answered Jun 22 '20 at 18:36

Igor Rivin

4,632
2
23
35

score 4 · Answer 2 · answered Jun 22 '20 at 18:39

elem1, elem2 = elem2, sum

This line is ALMOST the same as saying

elem1 = elem2
elem2 =  sum

The only difference is they happen "at the same time". That can be important. For example, if I write:

x, y = y, x

it swaps the elements, whereas

x = y
y = x

does not.

score 2 · Answer 3 · answered Jun 22 '20 at 18:41

2

For your code elem1, elem2 = elem2, sum means: elem1=elem2 and elem2=sum, or you can interpret this as i,j =1,10 where i=1 and j=10

answered Jun 22 '20 at 18:41

Md Fantacher Islam

82
5

score 1 · Answer 4 · answered Jun 22 '20 at 18:45

Hopefully, this will help you understand:

elem1 = elem2 = 1 # Set the first two numbers of the fib sequence to 1
sum = 0
for i in range(3, n + 1): # if n is 3, we get -- for i in range(3,4), where i will only be 3 -- if n is 4, i will be 3, then 4 etc. 
    sum = elem1 + elem2 # First find the third number of the sequence, and during other iteration, the third will be shifted to the fourth, fifth, etc.
    elem1, elem2 = elem2, sum # Now shift the variables so the first number is the second, and the second is the sum(third) During each loop, we shift them again
return sum

Python: the for loop inside the fib function doesn't make sense to me

4 Answers4