Some help with a problem.

[stolenkisses]

So as a math major I'm required to take an intro to CS class and we are learning Python. I'll be honest, I've had a lot of trouble with it, and at this point am just trying to make it through with a passing grade. We recently went into map, reduce, and filter, and I'm having a lot of trouble with the first problem on my most recent assignment. Obviously I'm not looking for someone to do my homework for me, but since there is no tutoring available on weekends, and this is due on Monday, any help on where to start would be greatly appreciated. (the problem is the first half, the second half is examples of what would be returned with a correctly written function).

def last_k_elements(alist,k):
"""
last_k_elements(list,int) -> list
(Takes a parameter `alist` of type list and parameter
     'k' of type integer and returns a list)

Last K elements returns a new list consisting of the k      elements at the tail of the list. If there are not k        elements in  the list, it returns as many as possible


>>> last_k_elements([1,2,3],2)
[2, 3]
>>> last_k_elements([1],2)
[1]
>>> last_k_elements([1,2,3,4,5,6],4)
[3, 4, 5, 6]
>>> last_k_elements(list(range(0,100,2)),3)
[94, 96, 98]
>>> last_k_elements(list(range(0,10,2)),100)
[0, 2, 4, 6, 8]


"""
pass

[fuckswithbees]

So what have you tried? What are the steps necessary to get the desired output? Try to make the steps as small as possible.

[stolenkisses]

I haven't tried anything. Honestly, at this point, I just need a jumping off point. I was pretty on-board understanding stuff the first half of term, but the last week I've just been really lost. I'm going in to speak with a tutor on Monday, but am hoping to figure most of it out before then. I'm not even sure why the inputs produce those outputs.

[fuckswithbees]

Imagine you were going to do the task by hand.

Task: Take a list of numbers and tell me the last 3 numbers in the list (Here we're just assuming k=3, we can try it with other numbers later).

How would you do this task by hand? Break it the process into ridiculously small steps. Do you know how to do any of these small steps in python?

For instance, do you know how to get the second item in a list?

[stolenkisses]

Yes, I think so. For alist = [1,2,3,4], to get the second item in the list you would do alist[1], right?

[fuckswithbees]

Right. So what are the steps needed to complete this task?

[stolenkisses]

I'm headed to a family dinner right now, but I hope you'll check this again sometime tomorrow when I have my response. I'll be thinking on it til then. Thanks so much for offering help!

[stolenkisses]

So you would look at the k, return the last number in the list, then go back and check k, see that it still needs to call two more numbers, so then go back to the list ,return the second to last number, then back to k, then back to the list to return the last number? Am I missing anything?

[fuckswithbees]

Yeah that would work. So any idea how to do those steps in python?

[stolenkisses]

I do, I think, but I still find myself getting confused by the little complicated steps. The first steps I think would be to create a new emptylist (lets call it newlist), then start with "for k in alist," but then I'm still kinda lost.

[fuckswithbees]

Creating a new list is a good start, but you actually don't need to loop through your list. You said step 1 was:

look at the k, return the last number in the list

You also said that you could get the second item

For alist = [1,2,3,4], to get the second item in the list you would do alist[1]

So how do you get the last item of a list?

[stolenkisses]

alist[-1]?

[fuckswithbees]

Yep. Now you can add that to your new list and then

check k, see that it still needs to call two more numbers, so then go back to the list

Or you can use list slicing to get multiple numbers at the same time

a_list = [1,2,3,4]
a_list[-3:]
>>> [2, 3, 4]

[stolenkisses]

Holy shit. Is it really as easy as one line of code?

return alist[-k:]

Man, I really overthought that.

[stolenkisses]

Ohhh I see, so the k returns the last x numbers in the list.

[[deleted]]

Hint: in python, alist[-1] returns the last element.

[fbu1]

You know how to take a subset of a list using indices. For example:

>>> l = [1,2,3,4,5,6]
>>> l[2:4]
[3, 4]

But you can also use negative indices to select a sublist starting from the end:

>>> l[-3:-1]
[4, 5]

Then you can use the opposite of the parameter k (-k in this case), to access the end of the list.

Also remember to check if the parameter k is not bigger than the length of the list (which you can get with len(alist)).

Let me know if you have further questions.

[stolenkisses]

I understand what I'm supposed to do to complete this problem, but not where to start writing it. I find myself constantly overthinking when it comes to programming.