This is a Python program to implement binary search with recursion.

The program takes a list and key as input and finds the index of the key in the list using binary search.

1. Create a function binary_search that takes a list and the variables start, end and key as arguments. The function searches for the key in the range [start… end – 1].

2. The base case consists of testing whether start is less than end. If not, -1 is returned.

3. mid is calculated as the floor of the average of start and end.

4. If the element at index mid is less than key, binary_search is called again wit start=mid + 1 and if it is more than key, it is called with end=mid. Otherwise, mid is returned as the index of the found element.

Here is the source code of a Python program to implement binary search using recursion. The program output is shown below.

def binary_search(alist, start, end, key): """Search key in alist[start... end - 1].""" if not start < end: return -1 mid = (start + end)//2 if alist[mid] < key: return binary_search(alist, mid + 1, end, key) elif alist[mid] > key: return binary_search(alist, start, mid, key) else: return mid alist = input('Enter the sorted list of numbers: ') alist = alist.split() alist = [int(x) for x in alist] key = int(input('The number to search for: ')) index = binary_search(alist, 0, len(alist), key) if index < 0: print('{} was not found.'.format(key)) else: print('{} was found at index {}.'.format(key, index))

1. The user is prompted to enter a list of numbers.

2. The user is then asked to enter a key to search for.

3. The list and key is passed to binary_search with start=0 and end=length of the list.

4. If the return value is -1, the key is not found and a message is displayed, otherwise the index of the found item is displayed.

Case 1: Enter the sorted list of numbers: 4 5 6 7 8 9 10 The number to search for: 9 9 was found at index 5. Case 2: Enter the sorted list of numbers: 3 4 5 10 The number to search for: 8 8 was not found. Case 3: Enter the sorted list of numbers: 7 The number to search for: 7 7 was found at index 0.

**Sanfoundry Global Education & Learning Series – Python Programs.**

To practice all Python programs, __here is complete set of 150+ Python Problems and Solutions__.