Exercise - Use variables to store data

3 minutes

In this exercise, you learn how to create programs that output code to the console. You build the start of a calculator program, and you also analyze a program that has errors in it and learn how to fix the errors.

Create a project

Let's start by creating a project. You create a project from the command line by using the dotnet executable.

Run the command dotnet new in the console.
```
dotnet new console --language F# -o Variables
cd Variables
```
This command creates a new F# project in the directory Variables.
Replace the code in Program.fs with the following code:
```
[<EntryPoint>]
let main argv =
    printfn "Welcome to the calculator program"
    // read input from the console and assign to `sum`
    let sum = 0
    printfn "The sum is %i" sum
    0
```
The first printfn statement welcomes the user of the program. On the second line, there's a comment that starts with // showing code that you'll implement in the next exercise unit. Then you have let sum = 0. Finally, you create a sum variable that holds the calculation result.
Run the program by running dotnet run.
```
dotnet run
```
You should see the following output:
```
Welcome to the calculator program
The sum is 0
```

Congratulations! You managed to scaffold a project, start on a larger project, and get it to run.

Correct the fault

Part of learning to program in any language is learning how to recognize compilation errors and fix them. For this exercise, you must scaffold a new project. Make sure you're one level up in the directory structure before you continue.

Scaffold a new F# project by running dotnet new.

dotnet new console --language F# -o Errors
cd Errors

Replace the code in Program.fs with the following code:
```
[<EntryPoint>]
let main argv =
    let aNumber = 0
    printfn "Here's a number %s" aNumber
    0 // return an integer exit code
```
Tip

Pause for a second, and read the preceding code. Do you think the code will compile? Why or why not?
Run the project by running dotnet run.
```
dotnet run
```
An error message displays that's similar to this text:
```
/<path>/Errors/Program.fs(8,34): error FS0001: This expression was expected to have type    'string'    but here has type    'int' [/<path>/Errors/Errors.fsproj]

The build failed. Fix the build errors and run again.
```
The error message is saying "expected string but here was type int." Do you understand what the problem is? The problem is the use of the formatter %s instead of %i because you're giving the printfn function the variable aNumber that's of type int. Let's correct the code.
Locate the row with the code "printfn "Here's a number %s" aNumber" and change it to:
```
printfn "Here's a number %i" aNumber
```
Save the file, and run it again with dotnet run. You should now see the following output:
```
Here's a number 0
```

Congratulations! You debugged a program by reading the code, understanding what was wrong, and fixing it. Recognizing error messages and knowing how to correct the errors are essential skills to have as a developer.

Exercise - Use variables to store data

Create a project

Correct the fault

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