返回如何在 Python 中的递归 function 中工作？

Question

I am having issues when returning the values from a recursive function. For example, in a very simple function called 'RBinSearch', I am trying to find the index of the key in an array. Unfortunately, I just cannot able to return the value (answer should be 4) based on my understanding.

For comparison, I have used a normal function with a loop to test the return (in a function called 'BinarySearch' here), which is working as expected. Can anyone please explain how the return behave inside a recursive function and where my understanding is incorrect? Thanks!

import math
import sys

def BinarySearch(array, key):
    low=0
    high=len(array)-1
    while(low<=high):
        mid=math.floor((low+high)/2)
        if(key==array[mid]):
            return mid
        elif(key<array[mid]):
            high=mid-1
        elif(key>array[mid]):
            low=mid+1
            
    return -1

def RBinSearch(array,low,high,key):
    if(low<=high):
        mid=math.floor((low+high)/2)
        print("Value of mid: ",mid)
        if(key==array[mid]):
            print("Found the value: ",mid)
            return mid
            sys.exit()
                                   
        elif(key<array[mid]):
            RBinSearch(array, low, mid-1, key)
        else:
            RBinSearch(array, mid+1, high, key)
    return -1
arr=[4,8,10,15,18,21,24,27,29,33,34,37,39,41,43]
print("Index from while Bin search: ",BinarySearch(arr, 18))
print("The index found using the Binary search is: ",RBinSearch(arr, 0,len(arr)-1,18))
 

Output

Answer 1

You need to return RBinSearch result in your RBinSearch function:

def RBinSearch(array, low, high, key):
    if low <= high:
        mid = math.floor((low + high) / 2)
        print("Value of mid: ", mid)
        if key == array[mid]:
            print("Found the value: ", mid)
            return mid

        elif key < array[mid]:
            return RBinSearch(array, low, mid - 1, key)
        else:
            return RBinSearch(array, mid + 1, high, key)
    return -1

Also you shouldn't use sys.exit() .