messy_list = [5,2,1,3,4,7,8,0,9,-1]
silly_list = messy_list[12:0:-3]

Question

messylist = [5,2,1,3,4,7,8,0,9,-1]
so create a variable named sillylist that is suppossed to 
create the same list in reverse and take every three out of that list
sillylist = messylist[12 :0:-3]
returns [-1, 8, 3]
why isn't this right in the console

Jeff Wilton · Answer

Classic case on how to overthink the problem (happens to me all the time) :)
But it brings up an important lesson on Python slicing and indexing.
First the solution:
As you probably know, you can copy an entire list by doing this:
python
list_2 = list_1[:]

We didn't have to specify the start and stop indexes since we want the whole list. Same thing is true in reverse:
python
silly_list = messy_list[::-3]
print silly_list
[-1, 8, 3, 5]

We could be done there since you now have the solution, but let me explain some of what is happening automatically there.
In the above case [::-3], think of it like this:
[a : b : -3]
a is by default -1
b is by default -(len+1)
Thus the statement could be explicitly stated as follows:
python
silly_list = messy_list[-1:-11:-3]
print silly_list
[-1, 8, 3, 5]

I'm sure they set this assignment up this way intentionally to help people understand how Python slicing works. Hope this helps!

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Thomas Katalenas

Thomas Katalenas

messy_list = [5,2,1,3,4,7,8,0,9,-1] silly_list = messy_list[12:0:-3]

1 Answer

Jeff Wilton

Jeff Wilton

First the solution: