Binary Search

Instructions

Your task is to implement a binary search algorithm.

A binary search algorithm finds an item in a list by repeatedly splitting it in half, only keeping the half which contains the item we're looking for. It allows us to quickly narrow down the possible locations of our item until we find it, or until we've eliminated all possible locations.

The algorithm looks like this:

• Find the middle element of a sorted list and compare it with the item we're looking for.
• If the middle element is our item, then we're done!
• If the middle element is greater than our item, we can eliminate that element and all the elements after it.
• If the middle element is less than our item, we can eliminate that element and all the elements before it.
• If every element of the list has been eliminated then the item is not in the list.
• Otherwise, repeat the process on the part of the list that has not been eliminated.

Here's an example:

Let's say we're looking for the number 23 in the following sorted list: [4, 8, 12, 16, 23, 28, 32].

• We start by comparing 23 with the middle element, 16.
• Since 23 is greater than 16, we can eliminate the left half of the list, leaving us with [23, 28, 32].
• We then compare 23 with the new middle element, 28.
• Since 23 is less than 28, we can eliminate the right half of the list: [23].
• We've found our item.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.

This particular exercise requires that you use the raise statement to "throw" a ValueError when the given value is not found within the array. The tests will only pass if you both raise the exception and include a message with it.

To raise a ValueError with a message, write the message as an argument to the exception type:

# example when value is not found in the array.
raise ValueError("value not in array")

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Source

Wikipedia