CODEWITHSUNDEEP
pythonrecursion
Adam_G
Adam_G

Reputation: 7879

Python: Explanation of a Reversal Function

My professor posted the function below. I do not fully understand how it's working. Could someone explain it?

def rev(a):
    if a == []:
        return []
    else:
        return rev(a[1:]) + [a[0]]

Upvotes: 0

Views: 85

Answers (2)

Gareth Latty
Gareth Latty

Reputation: 88977

What this does is recursively reverses a list. The easiest way to see how it works is to follow through the execution.

The function takes the string and solves it by returning the reversed version of all but the first item (a[1:]) with the first item appended to the end.

Note that this is a bad way to do this in a real situation (I am presuming your professor is just showing the idea of recursion), as Python isn't optimized for recursion. Instead, use the reversed() builtin.

Also, it isn't particularly Pythonic code. If one had to have a recursive solution, instead of using the efficient, effective, well-tested, easy-to-use built-in, consider:

def rev(seq):
    return rev(seq[1:]) + [seq[0]] if seq else []
  • We use the ternary operator to condense the if/else.
  • Replacing a with seq makes the function clearer - Python doesn't have strict typing, so using names that give a clue to what the function takes (in this case, a sequence), makes it clearer.
  • We also replace the a == [] by simply checking seq. As lists evaluate to False when empty, there is no need to compare to an empty list.

Upvotes: 2

kiriloff
kiriloff

Reputation: 26333

a is a list. If a is the empty list, you return the empty list. If not, you apply (recursively) your 'reverse' function to the list but the first element, and you append the first element. This way, at each recursive call for reverse, you build your reversed list beginning with its rightmost element.

This is an example:

l=[1,4,6,7]

rev(l) returns rev([4,6,7])+[1]
rev([4,6,7]) returns rev([6,7])+[4]
...

and in the end you have rev([]) returning the empty list and terminating the recursive calls.

BTW, to reverse a list l, just use

l[::-1]

Upvotes: 2

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