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Why can't I append to list inside of a list comprehension? (lemmy.world)
submitted 4 months ago* (last edited 4 months ago) by librejoe@lemmy.world to c/python@programming.dev
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I'm new to programming a bit, and am learning python so I can learn flask, using the python crash course book. I was learning about list comprehension but it briefly talks about it. If I do

list[list.append(value) for value in range(1, 20)]

it doesn't work. Would this be some sort of recursive expression that is not possible?

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[-] bitfucker@programming.dev 2 points 4 months ago

List comprehension is not whatever you're doing there. An example of list comprehension:

list = [value*2 for value in range(1, 20)]

See, list comprehension is used to make a list from an existing list. The value of the new list is defined by a function. In this case, the value of a will be 2,4,6, etc.

Your current syntax list[...], is trying to access an element of a list.

[-] librejoe@lemmy.world 1 points 4 months ago

So you cannot use methods inside a list comprehension, only binary operators and the function range?

[-] bitfucker@programming.dev 2 points 4 months ago* (last edited 4 months ago)

You can. Whatever the method returns will be the element of that list. So if for example I do this:

def mul(x):
  return x*2

list = [mul(value) for value in range(1,20)]

It will have the same effect. But this:

def mul(x):
  return

list = [mul(value) for value in range(1,20)]

Will just makes the list element all None

Edit to add more: List comprehension works not from the range function. Rather, the range function is returning a list. Hence the name, "list comprehension". You can use any old list for it.

What it did under the hood is that it iterates each element on the list that you specify (the in ...), and applies those to the function that you specify in the very first place. If you are familiar with the concept of Array.map in other languages, this is that. There is also a technical explanation for it if it helps, but it requires more time to explain. Just let me know if you would like to know it.

[-] librejoe@lemmy.world 1 points 4 months ago

Thanks for the response.

I am aware somewhat of what an array is, as i've dabbled with them in C, and know they can be multi-dimensional. Sorry if I'm being blind, but all I see are function calls in that list comprehension. I think what im asking is stupid, as the range function is returning a list populated.

[-] bitfucker@programming.dev 2 points 4 months ago* (last edited 4 months ago)

No problems. Learning a new concept is not stupid. So you are familiar with C. In C term, you are likely to do something like this:

int a[10] = {0}; // Just imagine this is 0,1,2,etc...
int b[10] = {0};
for (int i=0; i < 10; i++) {
  b[i] = a[i]*2;
}

A 1 to 1 correspondent might looks like ths:

a = range(10) # 0,1,2,etc...
b = []
for x in a:
  b.append(x*2)

However in python, you can then simplify to this:

a = range(10) # Same as before, 0,1,2,etc...
b = [x*2 for x in a]

# This is also works
b = [x*2 for x in [0,1,2,...]]

Remember that list comprehension is used to make a new list, not just iteration. If you want to do something other than making a list from another list, it is better to use iteration. List comprehension is just "syntactic sugar" so to speak. The concept comes from functional programming paradigm.

[-] librejoe@lemmy.world 1 points 4 months ago

Great explanation! I don't know too much C, just a bit here and there, and my dad's copy of K&R C he gave to me.

[-] decivex@yiffit.net 1 points 4 months ago* (last edited 4 months ago)

I know I'm being somewhat pedantic but range() returns an iterable range type, not a list, in python 3.

[-] bitfucker@programming.dev 1 points 4 months ago

Not at all. It is indeed helpful to differentiate between an iterable and literal list. After all, sometimes it will bite you in the ass when you don't differentiate between the two.

[-] onlinepersona@programming.dev 0 points 4 months ago

Sure you can. As others have said, a list comprehension returns a new list. See the documentation.

What are you trying to do though? Append a list comprehension to an existing list?

See a modified version of @eager_eagle@lemmy.world's code from their comment.

def double(x):
  return 2 * x
a = list(range(10))
a.extend(double(value) for value in range(5))

# a has 15 elements
print(a)

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this post was submitted on 25 Jun 2024
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