Python Projects for Beginners a ten-Week Bootcamp Approach to Python Programming

Step 5: Check if Value Is Less

This step is the exact same as step 4 but with the opposite condition. If the value at the middle index is less than the number we’re looking for, we want to remove the left half:

19| aList = aList[ : mid ] ◽◽◽
21| # step 5: check if value is less, if so, cut off left half of list using slicing
21| elif aList[mid] < num:
22| aList = aList[ mid + 1 : ]
23| print(aList) # remove after working properly
25| print( sorted(nums) ) # for debugging purposes ◽◽◽

Go ahead and run the cell. On line 22 we perform the opposite slice from step 4. This time we declare “mid + 1” because we don’t want to include the middle index, as it’s already been checked. The logic has now been implemented for our Binary Search. All that’s left is to set up a loop to repeat steps 2 through 5 and return False if we don’t find what we’re looking for.

Step 6: Set Up a Loop to Repeat Steps

We’ll need to loop until the argument is found, or until the list is empty. This sounds like a great case for a while loop. After creating the while statement, we need to make sure we execute the code for steps 2 through 5 within the loop:

8| aList.sort( ) ◽◽◽
10| # step 6: setup a loop to repeat steps 2 through 6 until list is empty 11| while aList:
12| mid = len(aList) // 2 14| if aList[mid] == num:
15| return True 16| elif aList[mid] > num:
17| aList = aList[ : mid ] 18| elif aList[mid] < num:
19| aList = aList[ mid + 1 : ]
21| print(aList) # remove after working properly
21| print( sorted(nums) ) # for debugging purposes ◽◽◽

Go ahead and run the cell. Our Binary Search is now performing all the necessary steps to either return True when the argument is found or create an empty list, in which case the loop will end. Remember that our preceding while statement is the same as
“while len(aList) > 0:”. All that’s left is to return False if the loop ends, as that means that the list does not contain our number.