I'm using Python 3.5.
As part of a problem, I'm trying to design a function that takes a list as input and reverts it. So if x = [a, b, c] the function would make x = [c, b, a].
The problem is, I'm not allowed to use any built-in functions, and it has got me stuck. My initial thought was the following loop inside a function:
for revert in range(1, len(x) + 1):
y.append(x[-revert])
And it works. But the problem is I'm using len(x), which I believe is a built-in function, correct?
So I searched around and have made the following very simple code:
y = x[::-1]
Which does exactly what I wanted, but it just seems almost too simple/easy and I'm not sure whether "::" counts as a function.
So I was wondering if anyone had any hints/ideas how to manually design said function? It just seems really hard when you can't use any built-in functions and it has me stuck for quite some time now.
range and len are both built-in functions. Since list methods are accepted, you could do this with insert. It is reeaallyy slow* but it does the job for small lists without using any built-ins:
def rev(l):
r = []
for i in l:
r.insert(0, i)
return r
By continuously inserting at the zero-th position you end up with a reversed version of the input list:
>>> print(rev([1, 2, 3, 4]))
[4, 3, 2, 1]
Doing:
def rev(l):
return l[::-1]
could also be considered a solution. ::-1 (:: has a different result) isn't a function (it's a slice) and [] is, again, a list method. Also, contrasting insert, it is faster and way more readable; just make sure you're able to understand and explain it. A nice explanation of how it works can be found in this S.O answer.
*Reeaaalllyyyy slow, see juanpa.arrivillaga's answer for cool plot and append with pop and take a look at in-place reverse on lists as done in Yoav Glazner's answer.