Data Types in Depth — Programuoki

Code

Refactor main.go — a validated numeric field

Both ideas from this lesson go into your program — as two new menu entries. The menu loop from lesson 5 stays; we only add to it.

Give Item a PagesRead field:

type Item struct {
	Title     string
	Pages     int
	PagesRead int // NEW: pages read so far
	Read      bool
}

Add two entries to the menu line and two cases to the switch:

		fmt.Println("\n1) Show  2) Set title  3) Set pages  4) Mark read  5) Pages read  6) Progress  q) Quit")

		case "5":
			// readInt (lesson 5) guarantees a whole number; we still check
			// the BOUNDS ourselves — 0..Pages.
			n := readInt("Pages read: ")
			if n < 0 || n > it.Pages {
				fmt.Println("Out of range (0 ..", it.Pages, ").")
				continue
			}
			it.PagesRead = n
			fmt.Println("Saved:", n, "pages.")
		case "6":
			fmt.Printf("Progress: %.1f%%\n", progressPercent(it))

Add the progressPercent function — with the float64 conversion before the division:

func progressPercent(it Item) float64 {
	if it.Pages == 0 {
		return 0
	}
	return float64(it.PagesRead) / float64(it.Pages) * 100
}

Finally, let formatItem show the read/total form — one changed line:

	return fmt.Sprintf("%s | pages: %d/%d | %s", it.Title, it.PagesRead, it.Pages, status)

Verify — type 6, 5 + 190, 6, 1, q:

> 6
Progress: 0.0%

> 5
Pages read: 190
Saved: 190 pages.

> 6
Progress: 50.0%

> 1
The Go Programming Language | pages: 190/380 | reading

> q
Bye!

The point shows in the second 6: 50.0%, not 0% — because both sides became float64 before the division.

Tip. Change float64(it.PagesRead) / float64(it.Pages) inside progressPercent to it.PagesRead / it.Pages and run again — watch it print 0%. Breaking it on purpose is the best way to remember why the conversion matters.

main.go MODIFY 28 lines changed

Edit this file in ./

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

const appName = "Book list"

// Item is one record of the list.
type Item struct {
	Title     string
	Pages     int
	PagesRead int
	Read      bool
}

// scanner is the ONE place the program reads keyboard input.
var scanner = bufio.NewScanner(os.Stdin)

func main() {
	it := Item{Title: "The Go Programming Language", Pages: 380}
	printHeader()

	for {
		fmt.Println("\n1) Show  2) Set title  3) Set pages  4) Mark read  5) Pages read  6) Progress  q) Quit")
		fmt.Print("> ")

		switch readLine() {
		case "1":
			printItem(it)
		case "2":
			fmt.Print("New title: ")
			it.Title = readLine() // a WHOLE line — spaces included
			fmt.Println("Title set to:", it.Title)
		case "3":
			it.Pages = readInt("Pages: ") // retries until a whole number
			fmt.Println("Pages set to:", it.Pages)
		case "4":
			it.Read = true
			fmt.Println("Marked as read.")
		case "5":
			// readInt (from lesson 5) guarantees a whole number; we still
			// check the BOUNDS ourselves — 0..Pages.
			n := readInt("Pages read: ")
			if n < 0 || n > it.Pages {
				fmt.Println("Out of range (0 ..", it.Pages, ").")
				continue
			}
			it.PagesRead = n
			fmt.Println("Saved:", n, "pages.")
		case "6":
			fmt.Printf("Progress: %.1f%%\n", progressPercent(it))
		case "q":
			fmt.Println("Bye!")
			return
		default:
			fmt.Println("Unknown choice — try again.")
		}
	}
}

// progressPercent returns how much of the book is read, in percent. Both sides
// are converted to float64 BEFORE dividing — integer division would silently
// truncate 190/380 to 0.
func progressPercent(it Item) float64 {
	if it.Pages == 0 {
		return 0
	}
	return float64(it.PagesRead) / float64(it.Pages) * 100
}

// readLine reads ONE whole line of input (spaces included) and trims it.
// Unlike fmt.Scanln, which stops at the first space, this reads a full title
// like "Buy milk" instead of just "Buy".
func readLine() string {
	scanner.Scan()
	return strings.TrimSpace(scanner.Text())
}

// readInt keeps asking until the user types a whole number — the retry loop.
func readInt(prompt string) int {
	for {
		fmt.Print(prompt)
		n, err := strconv.Atoi(readLine())
		if err == nil {
			return n
		}
		fmt.Println("Please enter a whole number.")
	}
}

// printHeader prints the app title line (from lesson 3).
func printHeader() {
	fmt.Println("App:", appName)
}

// formatItem builds the one-line summary for a single record (from lesson 3),
// now with the read status.
func formatItem(it Item) string {
	status := "reading"
	if it.Read {
		status = "done"
	}
	return fmt.Sprintf("%s | pages: %d/%d | %s", it.Title, it.PagesRead, it.Pages, status)
}

// printItem prints the summary built by formatItem.
func printItem(it Item) {
	fmt.Println(formatItem(it))
}