How to do it...

These steps cover writing and running your application:

1. From your terminal/console application, create and navigate to the chapter3/math directory.
2. Copy tests from https://github.com/agtorre/go-cookbook/tree/master/chapter3/math or use this as an exercise to write some of your own code.
3. Create a file called math.go with the following contents:

        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)
        }

1. Create a file called fib.go with the following contents:

        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 nil
            }

            // 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]
        }

1. Create a new directory named example.
2. Navigate to example.

1. Create a file main.go with the following contents; be sure to modify the math import to use the path you set up in step 2:

        package main

        import (
            "fmt"

            "github.com/agtorre/go-cookbook/chapter3/math"
        )

        func main() {
            math.Examples()

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

1. Run go run main.go.
2. You could also run:

      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

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