I wrote a code for linear search in python(recursive). Can somebody tell me why it isn't working? ERROR- maximum recursion depth exceeded

Question

lister=[4,5,1,2,3,6]

i=0
def Search(arr):
    if arr[i]==3:
        return i
    else:
        if i>=0 and i<=(len(arr)-2):          
            i+1
            return Search(arr)
        else:
            return -1

print(Search(lister))

Linear Search using recursion in python.

I don't know why its not working.

Does this answer your question? [What is the maximum recursion depth in Python, and how to increase it?](https://stackoverflow.com/questions/3323001/what-is-the-maximum-recursion-depth-in-python-and-how-to-increase-it) — Domenico, Jan 23 '23 at 08:02
This may be interesting from a purely academic perspective but is not a good technique for performing a linear search. The maximum recursion depth in Python is ~1000. Therefore if your list is longer than the recursion maxima **and** the item is not found before that limit is reached then it will always fail — DarkKnight, Jan 23 '23 at 08:19

score 2 · Accepted Answer · answered Jan 23 '23 at 08:02

There are 2 main issues.
The first is that you're not incrementing i anywhere. i+1 doesn't modify i, just returns the value of i+1 which you're not placing anywhere. To increment i you should use i += 1.
The second is you're not using the externally defined i variable in your Search function.

To fix this you could either declare it as global in the start of your method, or pass it as a parameter to your function.

Global method (less preferable):

lister=[4,5,1,2,3,6]

i=0
def Search(arr):
    global i
    if arr[i]==3:
        return i
    else:
        if i>=0 and i<=(len(arr)-2):          
            i+=1
            return Search(arr)
        else:
            return -1

print(Search(lister))

And the parameter method (which would be preferable):

lister=[4,5,1,2,3,6]

def Search(arr, i=0):
    if arr[i]==3:
        return i
    else:
        if i>=0 and i<=(len(arr)-2):          
            i+=1
            return Search(arr, i)
        else:
            return -1

print(Search(lister))

Now this is the complete answer imho +1. Global is a bad idea (it will work, but strongly discouraged), the 2nd solution is the one to go with. To the OP you could read about `assignment` and scopes `LEGB` in python (if you still had doubts and wanted to find out in depth why your solution didn't work) — Gameplay, Jan 23 '23 at 08:05

score 0 · Answer 2 · answered Jan 23 '23 at 08:01

0

You are not incrementing i variable. You have i+1 but you don't assign it to i. You should have i=i+1 or i+=1 for short. This recursive call function is looping infinitely, since i doesn't change.

answered Jan 23 '23 at 08:01

Alexsen

136
1
6

score 0 · Answer 3 · answered Jan 23 '23 at 08:30

Rather than explicitly testing that the index is in range, why not use exception handling? You can also manage the any potential recursion "overflow" in this manner. For example:

def rfind(_list, val, idx=0):
    try:
        if _list[idx] == val:
            return idx
        return rfind(_list, val, idx+1)
    except (IndexError, RecursionError):
        return -1


print(rfind([1, 2, 3], 2))
print(rfind([1, 2, 3], 4)) # will return None due to IndexError
print(rfind([], 4)) # will return None due to IndexError
print(rfind([0]*2000, 1)) # will return None due to RecursionError

Output:

1
-1
-1
-1

I wrote a code for linear search in python(recursive). Can somebody tell me why it isn't working? ERROR- maximum recursion depth exceeded

3 Answers3