The following steps cover how to write and run your application:

1. From your Terminal/console application, create a new directory called ~/projects/go-programming-cookbook/chapter3/math.
2. Navigate to this directory.

3. Run the following command:

$ go mod init github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter3/math

You should see a file called go.mod that contains the following:

module github.com/PacktPublishing/Go-Programming-Cookbook-Second-Edition/chapter3/math    

4. Copy tests from ~/projects/go-programming-cookbook-original/chapter3/math or use this as an exercise to write some of your own code!
5. Create a file called fib.go with the following content:

        package math

        import "math/big"

        // global to memoize fib
        var memoize map[int]*big.Int

        func init() {
            // initialize the map
            memoize = make(map[int]*big.Int)
        }

        // Fib prints the nth digit of the fibonacci sequence
        // it will return 1 for anything < 0 as well...
        // it's calculated recursively and use big.Int since
        // int64 will quickly overflow
        func Fib(n int) *big.Int {
            if n < 0 {
                return big.NewInt(1)
            }

            // base case
            if n < 2 {
                memoize[n] = big.NewInt(1)
            }

            // check if we stored it before
            // if so return with no calculation
            if val, ok := memoize[n]; ok {
                return val
            }

            // initialize map then add previous 2 fib values
            memoize[n] = big.NewInt(0)
            memoize[n].Add(memoize[n], Fib(n-1))
            memoize[n].Add(memoize[n], Fib(n-2))

            // return result
            return memoize[n]
        }

6. Create a file called math.go with the following content:

package math

import (
  "fmt"
  "math"
)

// Examples demonstrates some of the functions
// in the math package
func Examples() {
  //sqrt Examples
  i := 25

  // i is an int, so convert
  result := math.Sqrt(float64(i))

  // sqrt of 25 == 5
  fmt.Println(result)

  // ceil rounds up
  result = math.Ceil(9.5)
  fmt.Println(result)

  // floor rounds down
  result = math.Floor(9.5)
  fmt.Println(result)

  // math also stores some consts:
  fmt.Println("Pi:", math.Pi, "E:", math.E)
}

7. Create a new directory named example and navigate to it.
8. Create a file named main.go with the following content:

        package main

        import (
            "fmt"

            "github.com/PacktPublishing/
             Go-Programming-Cookbook-Second-Edition/
             chapter3/math"
        )

        func main() {
            math.Examples()

            for i := 0; i < 10; i++ {
                fmt.Printf("%v ", math.Fib(i))
            }
            fmt.Println()
        }

9. Run go run main.go. You could also run the following:

$ go build
$ ./example

You should see the following output:

$ go run main.go
5
10
9
Pi: 3.141592653589793 E: 2.718281828459045
1 1 2 3 5 8 13 21 34 55

10. If you copied or wrote your own tests, go up one directory and run go test. Ensure that all tests pass.