Modify Binary Search in C++: fails when target value is immediately before midpoint of array

The C++ Binary Search example fails for any input where the target value is immediately before the midpoint of array. (When the value is at v[v.size()/2-1]).

Here are some example inputs that fail (returning false when true would be expected):

• "1, 2" "1"
• "1, 2, 3" "1"
• "1, 2, 3, 4" "2"
• "1, 2, 3, 4, 5" "2"
• "1, 2, 3, 4, 5, 6" "3"
• "1, 2, 3, 4, 5, 6, 7" "3"

For the given example “1, 2” “1”, the code does the following:

• before the while loop:
  • start = 0
  • end = 2
• while loop condition start < end is true, so we enter the while loop:
  • mid = 1
  • v[1] is 2, which greater than target num==1, so we set end=mid-1, which is end==0
• while loop condition start < end is now false, because start==1 and end==1, so we exit the while loop
• the search target was not found, so we return “false”

To fix it:

end is an exclusive range limit, not inclusive, so setting end=mid-1 effectively skips an index.

line 94 ought to be end=mid and not end=mid-1.

https://github.com/TheRenegadeCoder/sample-programs/blob/beec51fe10378beacb422a62a774eb7a34075125/archive/c/c-plus-plus/binary-search.cpp#L88-L101

I’d recommend adding one of these failing example inputs as a test case for all binary search implementations in all languages, since this is an algorithmic issue that could appear in any language.

[AjayMaheshwari23]

Can you guide me a bit how to fix the testing part or add new test cases . provide any article link or something which I can refer .

[AjayMaheshwari23]

@PennyMew i can make the necessary changes , can you assign it to me .