Why do I get an empty list?

Question

I want to creat a new list by operating some elements of a price list, but the new list is empty. What am I doing wrong?

prices = [2, 50, 70, 30, 555]
new_prices =[]

if int in prices > 20 and int in prices <= 50: 
       new_price.append(int * 0.4)
elif int in prices > 50: 
       new_price.append(int * 0.6)
elif int in prices == 5: 
        new_price.append(int * 0)

print(new_prices)

What do you think `if int in prices > 20 and int in prices <= 50` does? This is a decent attempt at writing some code but illustrates a fundamental misunderstanding of python. I suggest you back up a little bit and learn how to take an action on each item in a list. We do this with a `for` loop. — Code-Apprentice, Sep 20 '21 at 19:56
And the specific thing you need to learn for your current problem is called a "for loop". — Code-Apprentice, Sep 21 '21 at 20:02

BTables · Accepted Answer · 2021-09-20T20:08:42.737

2

I believe you're a little confused on the specific syntax of Python. One thing to note is that while using int works here, it should not be done. In Python int is a built-in type that your code is overwriting. You should also have a loop to actually iterate over every element of the list. Take this code for example:

prices = [2, 50, 70, 30, 555]
new_prices =[]
for price in prices:
    if price > 20 and price <= 50:
        new_prices.append(price * 0.4)
    elif price > 50:
        new_prices.append(price * 0.6)
    elif price == 5:
        new_prices.append(0)

edited Sep 20 '21 at 20:08

answered Sep 20 '21 at 20:02

BTables

4,413
2
11
30

2

To be technical, ```int``` is a value too, and ```int in prices``` is perfectly valid syntax, it's just not useful. – sj95126 Sep 20 '21 at 20:03
@sj95126 you are correct. I've edited my answer to reflect that. – BTables Sep 20 '21 at 20:11
Where are they overwriting `int`? – no comment Sep 20 '21 at 20:47
@MountQ I suggest you step away from the computer and get a pencil and paper. Write out in words very detailed steps how you would do this by hand. Then you can translate those words into python code. – Code-Apprentice Sep 21 '21 at 20:07

score 1 · Answer 2 · answered Sep 21 '21 at 00:57

Your problem seems to be a confusion between how to iterate through values in python and how if statements work.

If statements simply check whether a given condition is true or not and execute a specific piece of code in the case it is.

So in the first if statement, what you are doing is checking whether the variable int is inside the list prices is greater than 20... As you can see this sentence doesn't make sense grammatically, because you are trying to check two conditions at the same time without any relation (connectors) between them. On one side you want to know if int is in prices (which already raises the problem that int hasn't been defined) and on the other if 'something' is greater than 20.

My impression of what you wanted to do is something in the lines of: for every value in prices do something if the value is greater than 20. Here you actually would need to go through or iterate for every value within the list prices. This can be achieved by using a for loop, a while loop with a counter or list comprehensions. A good example of how to solve this problem with a for loop can be found in @BTables answer.

score 0 · Answer 3 · answered Sep 20 '21 at 19:59

0

There seems to be a typo: first you initiate “new_prices” but you append stuff to “new_price”

Furthermore, I would write the if condition like this

for price in prices:
    if price > 20 and price <= 50:
…

answered Sep 20 '21 at 19:59

MattiH

554
5
9

score -2 · Answer 4 · answered Sep 20 '21 at 20:00

-2

prices = [2, 50, 70, 30, 555]
new_prices =[]
for int in prices:
    if int > 20 and int <= 50: 
           new_prices.append(int * 0.4)
    elif int > 50: 
           new_prices.append(int * 0.6)
    elif int == 5: 
            new_prices.append(int * 0)

print(new_prices)

You probably want something like the above. Note your original code had new_price and new_prices.

Output

[20.0, 42.0, 12.0, 333.0]

answered Sep 20 '21 at 20:00

randomer64

70
6

1

While this works you're overwriting a built-in type. This will *technically* work, but it's very bad practice and should not be done. – BTables Sep 20 '21 at 20:04
2

@not_speshal int is not a reserved keyword. https://www.tutorialspoint.com/What-are-Reserved-Keywords-in-Python – randomer64 Sep 20 '21 at 20:07
@BTables int() is a function. int is a valid value. – randomer64 Sep 20 '21 at 20:10
[In general, using variable names that eclipse a keyword or built-in function in any language is a bad idea, even if it is allowed.](https://stackoverflow.com/questions/77552/id-is-a-bad-variable-name-in-python) – not_speshal Sep 20 '21 at 20:12
1

@randomer64 `int` is a valid variable name only in the technical sense here. It's a built-in type and should absolutely not be used in this context. – BTables Sep 20 '21 at 20:15

Why do I get an empty list?

4 Answers4