Dealing with errors

[old_soul]

Continuing the discussion from Multiple list elements:

Screen Link: https://app.dataquest.io/c/115/m/609/python-for-loops/8/dealing-with-errors
My Code:

opened_file = open('AppleStore.csv')
from csv import reader
read_file = reader(opened_file)
apps_data = list(read_file)

apps_data = apps_data[1:]

rating_sum = 0
for row in apps_data[1:]:
    rating = float(row[1:][7])
    rating_sum = rating_sum + rating
    avg_rating = rating_sum / 7197
(or)
  avg_rating = rating_sum / len(apps_data[1:])                           

What I expected to happen:

avg_rating
float (<class ‘float’>)
-+ 3.526955675976101

What actually happened:

• 3.253091565930249

Replace this line with the output/error
avg_rating is less than what we expected.

<!--Dividing the rating_sum and app_data(excluding header row) to find avg_rating.
Here, I thought  dividing using len() function or number of rows(7,197) will give same results. But I am getting the error that the avg_rating is less. where am i going wrong? Guide me pls : -->

[the_doctor]

I would highly recommend that you go through the content on lists and list indexing/slicing again. Because it doesn’t seem you have reached a level of intuitive understanding of that concept. Which is perfectly ok, you just need to review it a bit more.

There are a couple of issues in your code.

Firstly, you keep slicing apps_data.

apps_data = apps_data[1:]
for row in apps_data[1:]:
avg_rating = rating_sum / len(apps_data[1:])

If I gave you the list what would be the output of the following:

a = [1, 2, 3, 4]
a= a[1:]
a= a[1:]
print(a)

Think about the above and think more about how (and why) you are using apps_data[1:] in your code.

Secondly, the following

float(row[1:][7])

• Print out a single row, then
• print out row[1:], and then
• print out row[1:][7].

Do you get the value you need? Cross-check with the data and see if it matches up or not. Think about what each step above is doing and how you are slicing and indexing the list.

[old_soul]

To exclude the header, app_data is stored as app_data[1:].

apps_data = **apps_data[1:]**--> to define it 
for row in **apps_data[1:]**:---> for loop
avg_rating = rating_sum / len**(apps_data[1:]**) for finding avg_rating using len() ==> this is how i understood abt app_data[1:]  at each time it is used.

the example you have mentioned:
a = [1, 2, 3, 4]
a= a[1:]
a= a[1:]
print(a)

expected 
<<<[2, 3, 4]>>>

actual
<<<[3, 4]>>> why??


It's a good suggestion about going through indexing/slicing topic again to know how slicing varies at each step. Really, appreciate your time for replying.

[the_doctor]

@old_soul

<<<[3, 4]>>> why??

Because you are modifying a twice.

First time -

@old_soul

a = [1, 2, 3, 4]
a= a[1:]

You modify a and save the result back into a. So, now a contains [2,3,4]

Then, you modify a again with -

@old_soul

a= a[1:]
print(a)

So, now a contains [3, 4]. This becomes clearer if you add more print statements -

a = [1, 2, 3, 4]
print(a)

a= a[1:]
print(a)

a= a[1:]
print(a)

You are doing something similar with apps_data.