Question

I need to write an expression and I’m completely stuck. I have part of the code that I think I have written correctly but I’m stick on the rest of it. I need the code to return a new list containing every 3rd element in the list, starting at index 0.

For example: if I have the list [0, 1, 2, 3, 4, 5] I need it to return [0, 3]

The code I have so far is:

result = []
i = 0
while i < len(L):
    result.append(L[i])
    i = 
return result

Can someone please help me figure out what I need the i = expression to be for this code to work.

Answer 1

First of all, you can make use of extended slice notation to make things easier:

In [1]: l = [0, 1, 2, 3, 4, 5]

In [2]: l[::3]
Out[2]: [0, 3]

From the docs:

Some sequences also support “extended slicing” with a third “step” parameter: a[i:j:k] selects all items of a with index x where x = i + n*k, n >= 0 and i <= x < j.

As for your code sample, you probably need i = i + 3 or i += 3 there.