I took around half an hour to come up with the following:
"Only 10% of programmers can write a binary search"
Binary search solves the problem [of searching within a pre-sorted
array] by keeping track of a range within the array in which T
[i.e. the sought value] must be if it is anywhere in the array.
Initially, the range is the entire array. The range is shrunk by
comparing its middle element to T and discarding half the range.
The process continues until T is discovered in the array, or until
the range in which it must lie is known to be empty. In an
N-element table, the search uses roughly log(2) N comparisons.
def binsearch(data, item):
if not data:
lo, hi = 0, len(data)-1
mid = (hi + lo) // 2
while item != data[mid] and lo < hi:
lo, mid, hi = ( (lo, (lo + mid) // 2, mid)
if item < data[mid] else
(mid+1, (hi + mid + 1) // 2, hi) )
return item == data[mid]
if __name__ == '__main__':
for dlimit in range(5):
data = list(range(dlimit))
for item in data:
assert binsearch(data, item)
for item in list(range(dlimit+1)):
assert not binsearch(data, item - 0.5)
What happens with an empty list, and the little plus ones were added after I put in the assert statements, and I was satisfied enough to go on and complete the reading of the blog.
The blog goes on to state that the rules of test are that no testing is to be done. A compiler can be used to ensure that their are no mechanical errors however.
Whoa! I was shocked and then secretly pleased. I use Python, and automatically added the tests as part of my development process. In no way did I think that I could do the task without passing some tests. I am still not saying that my implementation is correct, but I have tested for a lot of corner-cases such as:
- Lists of length zero, one and more than one.
- Items in every position of the list.
- Items outside the list: less than, greater than and between all members of the list.
All I can conclude is that if given the test and access to a compiler/interpreter of your choice then one should argue convincingly for testing any submission as you would never deign to submit any code as complete unless tested. If the testers remain unconvinced then it may be time to stick to your guns as you don't gain by lowering your standards, especially in an interview situation when you know that testing is right!
P.S. I have since had a look at the code of the Python bisect module and the algorithm given on Wikipedia, and they both seem more elegant than mine.