# FAQ: List Comprehension - Double

This community-built FAQ covers the “Double” exercise from the lesson “List Comprehension”.

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This exercise can be found in the following Codecademy content:
Data Science

## FAQs on the exercise Double

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How to multiplying each number in `nums` by two

``````my_list = [1, 2, 3]
index = 1

my_list[index] = my_list[index] * 2

print(my_list)

'''Output:
[1, 4, 3]
'''
``````

I don’t understand the index bit. (doesn’t help that the learn python 3 course doesn’t really have an index bit)

There is no index thing but as in the explanation:

nums = [4, 8, 15, 16, 23, 42]
double_nums = [num*2 for num in nums]

Hey!

I was just wondering… the item in the whole line code works as a variable exclusively to modify the list? And is it even a variable? Because I can name it anything and somehow Python understands that I am talking about the items in my list. But I don’t define it anywhere else in the code …
In that case, I guess I cannot use item for something else outside the [operation…] because it’s not going to be seen as defined, is that right?

Thanks a lot!

From that we can only ascertain that `item` is a variable declared in a `for` statement.

``````for item in a_list:
``````

Or is this an incorrect presumption?

Okay so we declare a variable for that list only?

Yes, since that is the list we are iterating. The variable is there so actions in the code block have an identifiable object from that list.

Say we intend to iterate over every value in an iterable…

``````for temporary_value in iterable_object:
``````

Both variables above are references, one to the iterable, and one to the loop instance. One reference is to the object, and the other is to an object IN the object.

1 Like

I liked the guy matt from codecademy from ohio , he explained it perfectly for me this is how he explains it , excellent imho , go matt

[operation for item in list]

so if you have a list already then follow this for more explantion for example

operation is where you would put number times to do multiply by 2
then replace the word “item” with number or whatever is stored in variable for example number or string replace the word “list” with the name of the variable i hope this makes sense

so for example

[operation for item in list] instead of this you would get

so “operation” would be replace by number * 2
so replace “item” would be number or string or whatever is stored in the variable which is number in this case
replace list with the variable name before the = sign

so for example this

operation for item in list

double_nums = [number * 2 for number in nums]

print(double_nums)

hope this makes sense if not then please let me know thanks Ian