How do I return the index of the target element in a Python array?

Question

Currently, when I search for the element that is at the midpoint it returns the correct index, but for any other element it does not work for me.

I think I am making a mistake when I split the array:

    aList = [1,3,5,6,8,9,10,12,34,56,78,456]
    def recursiveBinarySearch(aList, target):
        #aList = sorted(aList)
        
        if len(aList) == 0:
            return False
        else:
            midpoint = len(aList) // 2
            if aList[midpoint] == target:
                return aList.index(target)
            else:
                if target < aList[midpoint]:
                    return recursiveBinarySearch(aList[:midpoint],target)
                else:
                    return recursiveBinarySearch(aList[midpoint+1:],target)
    
    print(recursiveBinarySearch(aList,9))

Answer 1

That is because every time you make a recursive call, a different modified list is passed and the index will change in every call. For example, if you search for a number in second half of the array, the final returned value will be less than len(aList)/2 because only this part of the array will be passed in the next iteration.

The workaround is to pass start and end points of the list instead of splitting the list.

aList = [1,3,5,6,8,9,10,12,34,56,78,456]
def recursiveBinarySearch(aList, target, start, end):
    #aList = sorted(aList)

    if end-start+1 <= 0:
        return False
    else:
        midpoint = start + (end - start) // 2
        if aList[midpoint] == target:
            return midpoint
        else:
            if target < aList[midpoint]:
                return recursiveBinarySearch(aList, target, start, midpoint-1)
            else:
                return recursiveBinarySearch(aList ,target, midpoint+1, end)

print(recursiveBinarySearch(aList,455, 0, len(aList)))