Unable to understand the "if" and "elif" section of this program

Q&A
, ,
#1

Hi,
I understood most of the program, but I’m stuck at the “if” and “elif” section. Please explain it.

laptop1 = None
laptop2 = None
# look for a solution
for row in rows:
    price = int(row[2])
    # this laptop's cost is price, so if there is a laptop that 
    # costs 5000 - price then their total price is exactly 5000
    if price == 2500 and len(price_to_name[2500]) >= 2:
        laptop1 = price_to_name[2500][0]
        laptop2 = price_to_name[2500][1]
    elif 5000 - price in price_to_name:
        laptop1 = price_to_name[price][0]
        laptop2 = price_to_name[5000 - price][0]
        print(price)
# print the solution
print(laptop1)
print(laptop2)

https://app.dataquest.io/m/1011/python-fundamentals-practice-problems/7/laptop-purchase

Thank you very much

Python Fundamentals Practice Problem #7
#2

Hi @vtwinhead, welcome to the community!

The way that the price_to_name dictionary was created in the solution ends up giving you dictionary items where the key is the price and the value of the key is a list that contains one or more laptop names. I’ll print a few of them just for visual aids, because I think it’s important to understand how the dictionary looks to understand the if/elif statements:

{174: ['C740-C9QX (3205U/2GB/32GB/Chrome'],
 192: ['Vivobook E200HA'],
 196: ['V131 (X5-Z8350/4GB/32GB/FHD/W10)'],
 199: ['E402WA-GA010T (E2-6110/2GB/32GB/W10)', 'Chromebook C910-C2ST'],
 203: ['K146 (N3350/4GB/32GB/W10)'],
 209: ['Chromebook 15', 'Stream 11-Y000na'],
...

Let’s look at the 209 key. price_to_name[209] has the value of ['Chromebook 15', 'Stream 11-Y000na']. If I wanted to access just one of these laptop names, I need to use the index value. That means:
price_to_name[209][0] = 'Chromebook 15'
and
price_to_name[209][1] = 'Stream 11-Y000na'

Additionally, this would mean that len(price_to_name[209]) = 2, because there were 2 items in the list.

For key 192, price_to_name[192] has a value of ['Vivobook E200HA']. This means that price_to_name[192][0] = 'Vivobook E200HA'.

So now let’s look at the if/elif statements.

The first scenario is the possibility that we have 2 laptops that both have a price of 2500. The if statement is checking for 2 things: first, that the price is 2500 for that row in the dataset, and secondly if there are 2 or more laptops at that price from the dictionary. If both conditions pass, then we can assign the laptop variables to those values.

The second scenario would be to find 2 laptops with different prices but add to 5000. Let’s say price = 2300 – then we’re looking in the dictionary to see if the key 2700 (5000-2300) exists in the dictionary. If it does, then that means laptop1 = price_to_name[2300][0] and laptop2 = price_to_name[2700][0]. (We use [0] for both, because even if there are 2 laptops at either price, we only need one of them and every key has at least 1 item in the list.)

I hope that helps!

2 Likes
#3

It definitely helps! Thank you so much.

#4

Hi @april.g , just following up with your explanation. I have difficulty to understand the solution to create the dictionary. Why we can’t just use the code to create the dictionary like we did in the previous questions?

price_to_name = {}
for row in rows:
price = int(row[2])
name = row[1]
price_to_name[price] = name

1 Like