Question

I’m somewhat of a noob to python but I’m trying to create a recursive function which works just like the built in range function:

def Range (lo, hi):
    if lo >= hi:
        return []
    else:
        return [lo, Range (lo+1,hi)]

but its returning multiple lists.

Instead of [3,4,5,6], which is what I want, its returning [3,[4,[5,[6,[]]]]]
Why is this and how do I fix it?

Answer 1

When you recurse like that, Range returns a list each time:

Range(3,7)
# translates to
[3, Range(4,7)]
# which translates to
[3, [4, Range(5,7)]]
# etc.

In order to avoid this, add your lists together:

def Range (lo, hi):
    if lo >= hi:
        return []
    else:
        return [lo] + Range(lo+1, hi)

---

EDIT:

As @delnan points out, this function is very inefficient – it both recurses in a language without tail-call optimization^* and it generates two (possibly three) new lists for each level of recursion. @mipadi’s answer is more performant because it creates only one list (the acc or accumulator argument) and passes it as it recurses.

^{* This may not be true for the Python language, but I’m 99% sure it is true for the most common implementation of Python, namely CPython.}