Rounding Down

00:00 In the previous lesson, I showed you the algorithm for rounding up. In this lesson, I’ll cover the obvious next choice rounding down. Rounding down uses a similar approach to rounding up, but instead of going to the next integer, greater than or equal to, it goes down to the previous integer.

00:19 Consider 12.345 once more. This time, rounding down. Rounding to the tens place drops the two to a zero. The ones gets rid of everything to the right of the decimal no matter what that is. Tenths, and then hundredths does the same.

00:36 Let’s look at some code to implement rounding down. To round up, I used the ceiling function from the math library. The companion to ceil() is logically enough the floor. floor()` of 1.2 is one, and of negative 0.5 is negative one.

01:00 For positive numbers, the floor() is the same as truncation, but for negative numbers, it’s not. Let’s use floor() to write a function that rounds down.

01:12 The approach here is very similar to rounding up, except the ceil() method gets replaced with a call to floor(). Let’s try this out. Importing it

01:28 for 1.5, you get one.

01:34 1.37 for the tenths is 1.3,

01:40 and because I’m flooring, -0.5 adds left, which is negative one. Okay, you’ve seen a couple of algorithms now. Before proceeding to some more, let’s take a bit of a detour to talk about how to evaluate the different approaches to rounding.