Now, we're going to look at a program that has both input and output. So again, very critical. We frequently as a user, we're used to it. Let's say in an app, we're always entering information. Somehow, our program, our app has to capture that information, and then it makes use of it in delivering a response or computation. It's going to be a very simple one that we all know about, which is how to compute the area of a circle. We're going to do that in terms of meters. So let's look at this code again. We're just getting used to how to write a simple C program, some of the elements of it. Later, we're going to go into detail again about what each thing is and how it's constructed. So as before, there's our standard program comment. It's really for readability. In this case, it's telling us this program is to be used for computing area of a circle. Standard I/O. The pre-process command brings in both printf and now scanf. Scanf is a basic routine for doing input. By default, most cases we will come from our keyboard. In the very old days, this could have come from a card deck or a paper tape. So it depends what you default. In our case, standard input is defaulted to the computer keyboard. Here we see another directive. It's called a defined directive. The define directive does an interesting thing. We have, and this is by convention, a pre-processor token. In this case, we'll call it Pi, for the obvious reasons. Again, we want this to be documentation. We wanted to make the program readable. We will define Pi to six significant figures because that's what we get to store in a normal double. So it's 3.14159. Again, we introduce a program, which is main. We begin with what's called the open brace for a compound statement. The first statement is a declaration statement, in which we define area and radius. Area and radius are two identifiers. Identifiers can be ordinary words, unless they're keywords like double. So unless they are keyword, you can make use of whatever you want to think about. We initialize area to zero and radius to zero. We use 0.0 because that's a clear indicator that we're re-doing constants which are double. We could use zero, and that would be allowed. We then ask the user, "Please enter a radius." Otherwise, if we just didn't do anything, and we had omitted this, and try omitting it, you'll see that your screen stops, and you won't know what to do. So this is what we call a prompt. So it prompts the user to enter radius. This says, "Entered the radius as a double long float." We need the special operator, ampersand here. We'll explain that later. But for the moment, trust me. Just use ampersand of whatever that variable is. We're going to explain that much later in detail in the course because that's really a parameter. That's the address of radius. We'll have to understand things like address, memory, pointer to really understand what's going on. Here's our area calculation. Everybody should remember that from grade school or high school, Pi times radius times radius. We don't have a simple way to say square. So there isn't a notation like star star two. So we use repeated multiplication, and then we print our answer. Return with zero. So let's make sure we can use that program. I've already compiled it. Now, I'm going to execute it. So I compiled it into code. So we call dot exe. It says, "Enter a radius." Let's try the radius 1.0. Oh yes, 3.1459. Well, that makes sense. That's a simple test. When we go to the bug programs, we want data for which we know the answer. We want simple cases first. We can ask for a more complicated case. Let's repeat that. So it's saying, "Enter the radius again." Why don't we do 2.5? In this case, it's 19.634937. That's correct as well. So that shows you how not only to do simple output which we've studied in a few cases, but also simple input. The little secret there is outside of using a format string. Where you want the input, you have a percent with the appropriate format. In this case, doubles, so it's lf. If it was an integer, it would be percent D. Then, instead of using the ordinary variable name, we have to use the address of that variable.
