Recursive Binary Search Python

Question

Hi can someone tell me what is wrong with my code? It returns false, false as opposed to false, true

I know the problem is under the 3rd "else", problem is solved when I change the following

return recursive_binary_search(list[:midpoint-1], target)

into this line

return recursive_binary_search(list[:midpoint], target)

However i don't understand why the list cant start from 0 all the way to midpoint - 1

Thanks in advance

def recursive_binary_search(list, target): if len(list) == 0: return False
else: midpoint = (len(list))//2

if list[midpoint] == target:
  return True
else:
  if list[midpoint] < target:
    return recursive_binary_search(list[midpoint+1:], target)
  else:
    return recursive_binary_search(list[:midpoint-1], target)

def verify(result): print("Target found: ", result)

numbers = [1,2,3,4,5,6,7,8] result = recursive_binary_search(numbers, 12) verify(result)

result = recursive_binary_search(numbers, 6) verify(result)

Answer 1 · 2025-11-03T16:44:09Z

November 3, 2025 4:44pm

Remember that in Python, the stop index of a slice is where the slice ends, but the slice will not include the value at the index itself. So a slice made using :midpoint will contain elements up to but not including the midpoint.

If you slice using :midpoint-1 instead, that slice will also discard the item directly before the midpoint, which (depending on the order of the data items) might just be exactly the item you are looking for.

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Eva Feng

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Recursive Binary Search Python

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Steven Parker

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