For the last practical example, we can try speeding up Mr. Smith's SlowCode from Chapter 1, About Performance. Here, we immediately ran into a problem. To fix or change an algorithm we must understand what the code does. This happens a lot in practice, especially when you inherit some code. Reading and understanding code that you didn't write is an important skill.

Let's try to understand the first part of SlowCode. The for loop in SlowMethod starts counting with 2. Then it calls ElementInDataDivides, which does nothing as the data list is empty. Next, SlowMethod adds 2 to the list.

Next, i takes the value of 3. ElementInDataDivides checks if 3 is divisible by 2. It is not, so SlowMethod adds 3 to the list.

In the next step, i = 4, it is divisible by 2, and 4 is not added to the list. 5 is then added to the list (it is not divisible by 2 or 3), 6 is not (divisible by 2), 7 is added (not divisible by 2, 3, or 5), 8 is not (divisible by 2), 9 is not (divisible by 3), and...