Sal's Shipping - confused about elif/if

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1

Hi all, I am confused by the elif/if code in the solution to Sal’s Shipping Project
https://www.codecademy.com/courses/learn-python-3/projects/sals-shipping

For the final function, to find the cheapest shipping option and return text that says which option is cheapest and how much it will cost for a given weight, here is my code (very similar to the solution given in the video tutorial walkthrough:

def print_cheapest_method(weight):
  ground = ground_shipping(weight)
  premium = premium_ground_shipping
  drone = drone_shipping(weight)

  if ground < premium and ground < drone:
    method = "ground shipping"
    cost = ground
  
  if premium < ground and premium < drone:
    method = "premium shipping"
    cost = premium

  else:
    method = "drone shipping"
    cost = drone

  print(
    "the cheapest shipping method is %s, it will cost you $%.2f"
  % (method, cost))```

I used if/if/else for my control flow.

When I answer the question: "What is the cheapest method of shipping a 4.8 pound package and how much would it cost?" 

My function returns the incorrect answer:
"the cheapest shipping method is drone shipping, it will cost you $43.20"

But if I change the control flow to if/elif/else, the function returns the correct answer:
"the cheapest shipping method is ground shipping, it will cost you $34.40" 

I really can't figure out why this is the case, like why the elif is necessary here. Especially since the correct answer is ground shipping, which comes first. If anyone can explain I'd appreciate it, thanks!
2

Hello! The reason you get the wrong answer when using if…if…else is that the else will run unless the second if is True:

if ground < premium and ground < drone:
#if this condition is True, the following code will run:
    method = "ground shipping"
    cost = ground
  if premium < ground and premium < drone:
#if this condition is True, the following code will run:
    method = "premium shipping"
    cost = premium
  else:
#If the above if statement evaluates to False-even if the first if statement evaluates to true, the following code will run:
    method = "drone shipping"
    cost = drone
1 Like
3

Thanks! That does make sense, that the “else” condition is triggered because the condition immediately before it is false, but wouldn’t that mean that it would also return the answer ground shipping (the first condition) because that is true? It seems like that is what happened here in this similar question thread (Sal's Shipping Exercise - why use elif?), where the fellow asking the question got two shipping methods returned. But in my case it only returned the final “else” condition (drone) even though the first condition (ground) should have registered as true.

4

Consider where the output is. You won’t get it until you ask for it and if your values change before then you’ll only get the current state. For a very over-simplified example-

x = 3
x = 4
print(x)
Out: 4
1 Like
5

Interesting thanks. I hope more examples come up like this in the exercise, I’d like to get more of a feel for it.