remove element from a list by using recursive function python

January 11, 2022

I have a list for which I want to subtract element i+1 with element i.
If result is greater than 2, element i must to be remove.
I use a recursive function but I always get this error : IndexError: list index out of range

l = [10,11,12,20,21,22]  

def remove_Element():
    for i in range(len(l)-1):            
        if l[i+1]-l[i] > 2:
            l.remove(l[i])
            remove_Element()                
      
remove_Element()

Could someone help me to fix it ?

>Solution :

A few issues:

If you use recursion, you should not keep looping after the recursive call has been made. Moreover, since then the list has become shorter, the loop will go too far, and produce the error you got. So a simple solution is to add a break or return immediately after the recursive call
remove is not the method you want to use, since it will look for a certain value. If that value occurs elsewhere before the current index, it will be that one that will be removed. This is not right. Use .pop(i) instead.

With those two fixes the code becomes:

def remove_Element():
    for i in range(len(l)-1):            
        if l[i+1]-l[i] > 2:
            l.pop(i)
            remove_Element()
            break

Still some remarks:

Be aware that the check is being repeated from the start when you make the recursive call, which explains why the value 10 is eventually being removed (after 12 has been removed, 11 gets removed, and then 10). I assume this is what you want. If not, you should pass as argument from which index you want to continue the removal logic:
```
def remove_Element(start=0):
    for i in range(start, len(l)-1):            
        if l[i+1]-l[i] > 2:
            l.pop(i)
            remove_Element(i)
            break
```
Recursion is overkill here. This should better be done iteratively, also because the stack available for recursion is limited.
It may also be more appropriate not to mutate the given list, but create a new one that has the desired items.
Make the list a parameter to the function, so it is clear which list it is taking as input