Go introduction: why Go, where it's used, setup, packages, variables and types, control flow, functions, errors and the go tool

mkdir hello && cd hello
go mod init example.com/hello      # creates go.mod (module definition)
go run .                           # compile and run
go build                           # create an executable

package main

import "fmt"

func main() {
	fmt.Println("Habari, Kenya!")
	fmt.Println("Learning Go on the Marzley learning hub.")
}

package main

import "fmt"

func main() {
	var school string = "Moi Forces Academy"
	var students int = 45
	fee := 12500.50        // short declaration: type inferred (float64), only inside functions
	paid := true
	const vat = 0.16

	fmt.Println(school, "has", students, "students")
	fmt.Printf("Fee: KSh %.2f, paid: %t, VAT: %.0f%%\n", fee, paid, vat*100)

	var count int          // zero value: 0
	var name string        // zero value: ""
	var ok bool            // zero value: false
	fmt.Printf("count=%d name=%q ok=%t\n", count, name, ok)

	a, b := 10, 3
	fmt.Println(a/b, a%b, float64(a)/float64(b))   // integer division, remainder, float division
}

package main

import (
	"fmt"
	"strconv"
	"strings"
)

func main() {
	name := "Wanjiku Kamau"
	fmt.Println(strings.ToUpper(name))
	fmt.Println(strings.Contains(name, "Kamau"), len(name))
	fmt.Println(strings.Split("Nairobi,Mombasa,Kisumu", ","))
	fmt.Println(strings.Replace("0712345678", "0", "254", 1))

	n, err := strconv.Atoi("1500")      // string -> int, with an error
	fmt.Println(n+500, err)
	_, err = strconv.Atoi("abc")
	fmt.Println("Converting 'abc':", err)

	msg := fmt.Sprintf("%-10s|%8.2f", "Unga", 195.0)
	fmt.Println(msg)
}

package main

import "fmt"

func grade(mark int) string {
	if mark >= 80 {
		return "A"
	} else if mark >= 65 {
		return "B"
	} else if mark >= 50 {
		return "C"
	}
	return "D/E"
}

func main() {
	fmt.Println("72 ->", grade(72))

	if balance := 0.0; balance <= 0 {      // if with a short statement
		fmt.Println("Fees cleared")
	}

	status := "shipped"
	switch status {                         // no break needed
	case "pending":
		fmt.Println("Waiting for payment")
	case "paid", "processing":
		fmt.Println("Preparing order")
	case "shipped":
		fmt.Println("On the way")
	default:
		fmt.Println("Unknown")
	}

	for i := 1; i <= 3; i++ {               // classic for
		fmt.Println("7 x", i, "=", 7*i)
	}

	balance, months := 0.0, 0
	for balance < 50000 {                   // for as a while loop
		balance = balance*1.01 + 4500
		months++
	}
	fmt.Println("Months to save 50,000:", months)

	towns := []string{"Nairobi", "Mombasa", "Kisumu"}
	for i, t := range towns {               // range over a slice
		fmt.Println(i, t)
	}
}

package main

import (
	"errors"
	"fmt"
)

func vat(amount float64) float64 {
	return amount * 0.16
}

func divide(a, b float64) (float64, error) {      // returns a result AND an error
	if b == 0 {
		return 0, errors.New("cannot divide by zero")
	}
	return a / b, nil
}

func minMax(values []int) (min, max int) {          // named results
	min, max = values[0], values[0]
	for _, v := range values {
		if v < min {
			min = v
		}
		if v > max {
			max = v
		}
	}
	return
}

func main() {
	fmt.Println(vat(1000))

	if result, err := divide(10, 4); err == nil {
		fmt.Println("Result:", result)
	}
	if _, err := divide(1, 0); err != nil {
		fmt.Println("Error:", err)
	}

	lo, hi := minMax([]int{67, 82, 45, 90})
	fmt.Println("Lowest", lo, "highest", hi)
}

data, err := os.ReadFile("students.csv")
if err != nil {
    log.Fatalf("could not read file: %v", err)
}

package main

import "fmt"

func process() {
	fmt.Println("Open connection")
	defer fmt.Println("Close connection")     // runs last, even if we return early
	fmt.Println("Process payments")
}

func main() {
	process()
}