Unable to perform if conditional in list comprehension using a list in the if conditional section

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#1

I am unable to perform a if conditional where a list is < 30. It returns an error everytime. I know this isn’t right but I’m not sure how to assign this to a variable within a list comprehension. Here is the question:

Carly thinks she can bring in more customers by advertising all of the haircuts she has that are under 30 dollars.Use a list comprehension to create a list called cuts_under_30 that has the entry hairstyles[i] for each i for which new_prices[i] is less than 30 .You can use range() in your list comprehension to make i go from 0 to len(new_prices) - 1 .

Here is my code:

cuts_under_30 = hairstyles[i] for i in range(len(new_prices - 1)) if new_prices < 30]
#2

What do you hope to accomplish by this condition: list < 30?
You answer is what changes what you do with list in regards to that condition.

#Case 1
if len(list) < 30 
#checks if number of elements in list is less than 30

#Case 2
if reduce(lambda a, b: a+b, list) < 30
#checks if sum of elements in list is less than 30

These are just some things you can do with that.
Also, your cuts_under_30 list comprehension isn’t closed. Add a [ at the start.

Can you post a link to the exercise?

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#3

DEBUG DEBUG look carefully :wink:

1 Like
#4

https://www.codecademy.com/paths/computer-science/tracks/cspath-flow-data-iteration/modules/dspath-python-loops/projects/carlys-clippers

Here is the link. I guess I’m trying to see if each number within the new_prices list is < 30.

#5

@h1lo

I’m not sure what lambda does.

#6

Found the missing [ but there is still an issue with the list and I believe it lies within the if conditional.

#7

Don’t worry about lambda for now, glad you fixed your missing [ .
I find that whenever I have an error, I go back and reconstruct my code.
Ok so it seems like you are trying to get a hairstyle if it’s price is less than 30.
Think about the steps you would take to solve this.

  1. Maybe you could zip together the lists (hairstyles and new_prices)? Could lead to possible indexing… WINK WINK
#8

@h1lo

Does any type of manipulation take place before the for statement in list comprehension? Trying to see where would be the best part to zip.

#9

Well, from what I can see, your list comprehension is handling 2 different lists right now. I guess you could do something like:

cuts_under_30 = [hair[0] for hair in zip(hairstyles, new_prices) if hair[1] < 30]

Something a bit closer to your original code:

cuts_under_30 = [hairstyles[i] for i in range(len(new_prices))-1 if new_prices[i] < 30]
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#10

I think that your statement is trying to loop through your range(len()), but it can’t find what it’s looking for

range(len(new_prices - 1))

is trying to iterate through the range of items you have in a variable named “new_prices - 1”

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#11

Gosh I was so close. So are you saying that I needed to have the - 1 outside of the parenthesis?

#12

I’m kinda new to this myself, but maybe… I think it still needs to be within the range(len()) somewhere though…

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#13

Err sorry. You want to subtract from len(new_prices) right? Then:

range(len(new_prices)-1)
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#14

If I didn’t have the - 1, how would that affect the loop? I tried it both ways and it returned the same results.

#15

I really only added that because the question pretty much stated to do it.

#16 
lst = [1, 2, 4, 6, 7, 8, 9, 10]
len(lst)#is 8
for i in range(len(lst)-1):
  print(i)
0
1
2
3
4
5
6

for i in range(len(lst)):
  print(i)
0
1
2
3
4
5
6
7

Hmm… wouldn’t that throw an IndexError though?

#17

The first loop wouldn’t print the full size of the list however the second one would. The first loop just leaves off the last value.

#18

Yeah, so I’m wondering then why you got the same result. You wouldn’t be able to reach the last element of new_prices right?

#19

That’s correct. Fortunately the last item in new_prices is 30.

#20

LOL! Ok, then. In the future, I suggest not just blindly going along with what the answer is. If you take steps to solve a problem you can find an alternative solution. Try changing the last element in new_prices to 25, and run your program again both ways, with the -1 and without and see if you get the same answer this time.