Question on Iteration Code for This Section

This is perhaps a stupid question, but I am trying to parse why it is in this mission:

We check content ratings and add it to age ratings, we have to:

for row in apps_data[1:]:
c_rating = row[10]
if c_rating in content_ratings:
content_ratings[c_rating] += 1

As opposed to:

for c_rating[10] in apps_data[1:]:
if c_rating[10] in content_ratings:
content_ratings[c_rating[10]] += 1

I know the second method will return an error when we try to run the code, and I understand the first method is what I need to do in order to achieve the desire result, I am just trying to determine why this needs to be done.

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This is more to do with how Python works internally.

If I give you a list, as an example -

a = [5, 19, 17, 30]

We know that we can access a particular value at a given index easily using something like -


The above will give us 17.

We can also do something like -

for item in a:

Based on how Python works internally, that particular for loop, knows that what we want to do is check each value/element in that list a, and print out that value.

However, something like the following -

for item[2] in a:

will throw an error, just like you would have gotten in your code. In this case, Python does not know what item[2] is. item[2] does not exist as per Python. a[2] exists since a is defined and stored somewhere in memory. But there is no underlying implementation in Python for it to figure out what item[2] is.

I am being vague here because this is somewhat related to some advanced Python concepts which we might not even learn through DataQuest, I think.

It is related to a concept called Iterators in Python. Our list a is iterable, that is we can iterate over it. That’s why for item in a is possible.

That item is in turn called an Iterator which, as I mentioned, is an advanced concept related to Python. item gets defined as an iterator, and based on that we can easily have that for loop.

But Python does not create an iterator like item[2]. Because that’s essentially mixing two Python operations - creating an iterator, and indexing. Which is really not possible at the same time because Iterators don’t really “store” all the items from that list at one time. They have to iterate through it. This is just a very high-level overview of what they do.

So, you have to separate out the two steps.

If you are interested in learning a bit more about this, then I would suggest this resource as a decent starting point. If it seems confusing, then don’t worry. Just come back to it later as you get more and more comfortable with Python.


I think then I could say:

I cant do
for c_rating[10] in apps_data

Until I first make a separate variable, in the mission example, row, and then assign it to the values counted first in the row variable?

Also I like the username, favorite Doctor? Mine is the Sixth. :smile:

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Yup, that’s correct! Just like the first set of code you shared.

You can do the following though -

for row in apps_data[1:]:
    if row[10] in content_ratings:
        content_ratings[row[10]] += 1

Once you have row “created”, you can then use indexing on that for other operations.

favorite Doctor? Mine is the Sixth

Oh, wow. That’s a tough one to answer! I have only seen NuWho, though. I am mostly torn between 11 and 12! :smiley:

1 Like

Hmmmm, ok yeah that makes sense then!

I knew the method in the solution is what I had to do, I just like to try and understand the why

Definitely a tough one to answer! 11 is either my second or third favorite depending on the day, 12 was good but I think he sadly was a little hamstrung by some less than stellar episode writing.

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Any specific episodes for 12 that you thought were quite bad? In terms of writing or otherwise?