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

Step 3: Check the Value at the Middle Index

Now that we have the middle index, we want to see if the value at that given index is the number that we’re looking for. If it is, then we want to return True:

11| mid = len(aList) // 2 # two slashes ... ◽◽◽
13| # step 3: check the value at middle index, if it is equal to num return True
14| if aList[mid] == num:
15| return True
17| print( sorted(nums) ) # for debugging purposes ◽◽◽

Go ahead and run the cell. You’ll get an output of either True or None, depending on the list that was randomly generated for you. If the number 3 appears at index 5, then your output will be True as our condition on line 14 is True and will run the return statement.

Step 4: Check if Value Is Greater

If the number that we’re looking for isn’t at the middle index, then we need to figure out which half of the list to remove. Let’s first check if the value at the middle index is greater than the number we’re searching for. If it is, we need cut off the right half of the list:

15| return True ◽◽◽
17| # step 4: check if value is greater, if so, cut off right half of list using slicing
18| elif aList[mid] > num:
19| aList = aList[ : mid ]
21| print(aList) # remove after working properly
23| print( sorted(nums) ) # for debugging purposes ◽◽◽

Go ahead and run the cell. On line 18 we check to see if the value at the middle index of the list is greater than the argument that we passed in during the function call. Line 19 is where the magic of Binary Search occurs though. Using slicing, we’re able to re-declare the value of aList to the beginning half of the list.
Note Remember that slicing allows you to input the start, stop, and step. If you don’t input a number like earlier, it implies that you are using default values.
default values are start = 0, stop = len(list), and step = 1.
We imply that we want to keep the all items from index zero up to the middle index.
Remove line 21 after you’re done, as it will simply output the result of our new aList.