[MUSIC]

[SOUND] Before we go on,

I want to introduce you to another tool we have available to us in the console.

So far, we've used the mono C# compiler to compile our code.

The compiler turns our code, which may be contained in multiple files,

into a single file that can be run as a program.

It also converts the human readable C# code,

into code that the computer can more easily understand.

Our code must follow all the rules of the C# language.

Otherwise the compiler can't understand our code well enough to perform

this conversion.

This is why we occasionally get compiler errors.

Compilers convert code files into programs.

But what if we just wanna run a few lines of C# code without creating a file,

writing a class and a method?

Well, the Mono C# compiler comes with a tool for doing just that.

This type of program is called a REPL.

Just like MSC, it also runs right in the console.

REPL stands for Read, Evaluate, Print, Loop,

which describes what it does each time we give it a line of code to run.

First, it reads our code from the console.

Then is evaluates it, which means it runs it.

Then it prints something back to the console.

The final step, Loop, in coding terms, means to repeat.

So, it starts all over again and waits for us to enter another line of code.

I think it's best to show you how it works.

To launch the C# REPL, just go to the console in workspaces and type csharp.

Now we can write code like we would inside of a method.

For instance let's declare a variable and assign it a value.

Now we can see what the value of the truth variable contains just by typing it and

pressing Enter.

Just like at the command prompt on the console,

you can press the up arrow to see what you've previously run.

Let's run the code again.

Finally you can type quit when you want to exit the REPL.

For the rest of this course, I'll use the C# REPL, as a teaching tool to demonstrate

some of the features of the C# programming language.

We'll also continue to code up our Fitness Frog app.

If you've ever used a language like Ruby or Python,

then you've probably used a REPL program before.

Oddly enough, Most C# programmers don't use REPLs.

You wouldn't ever use a REPL to actually run a program.

i find it incredibly useful though because it's an excellent way to learn

a programming language through experimentation.

Without the REPL, I find myself making lots of small programs and

compiling them just so that I can experiment with some code.