add: json encoding/decoding quiz and exercises

24-structs/exercises/04-encode/main.go

@@ -0,0 +1,65 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+// ---------------------------------------------------------
+// EXERCISE: Encode
+//
+//  Add a new command: "save". Encode the games to json, and
+//  print it, then terminate the loop.
+//
+//  1. Create a new struct type with exported fields: ID, Name, Genre and Price.
+//
+//  2. Create a new slice using the new struct type.
+//
+//  3. Save the games into the new slice.
+//
+//  4. Encode the new slice.
+//
+//
+// RESTRICTION
+//  Do not export the fields of the game struct.
+//
+//
+// EXPECTED OUTPUT
+//  Inanc's game store has 3 games.
+//
+//    > list   : lists all the games
+//    > id N   : queries a game by id
+//    > save   : exports the data to json and quits
+//    > quit   : quits
+//
+//  save
+//
+//  [
+//          {
+//                  "id": 1,
+//                  "name": "god of war",
+//                  "genre": "action adventure",
+//                  "price": 50
+//          },
+//          {
+//                  "id": 2,
+//                  "name": "x-com 2",
+//                  "genre": "strategy",
+//                  "price": 40
+//          },
+//          {
+//                  "id": 3,
+//                  "name": "minecraft",
+//                  "genre": "sandbox",
+//                  "price": 20
+//          }
+//  ]
+//
+// ---------------------------------------------------------
+
+func main() {
+	// use your solution from the previous exercise
+
+}

24-structs/exercises/04-encode/solution/main.go

@@ -0,0 +1,126 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	type item struct {
+		id    int
+		name  string
+		price int
+	}
+
+	type game struct {
+		item
+		genre string
+	}
+
+	games := []game{
+		{
+			item:  item{id: 1, name: "god of war", price: 50},
+			genre: "action adventure",
+		},
+		{item: item{id: 2, name: "x-com 2", price: 40}, genre: "strategy"},
+		{item: item{id: 3, name: "minecraft", price: 20}, genre: "sandbox"},
+	}
+
+	// index the games by id
+	byID := make(map[int]game)
+	for _, g := range games {
+		byID[g.id] = g
+	}
+
+	fmt.Printf("Inanc's game store has %d games.\n", len(games))
+
+	in := bufio.NewScanner(os.Stdin)
+	for {
+		fmt.Printf(`
+  > list   : lists all the games
+  > id N   : queries a game by id
+  > save   : exports the data to json and quits
+  > quit   : quits
+
+`)
+
+		if !in.Scan() {
+			break
+		}
+
+		fmt.Println()
+
+		cmd := strings.Fields(in.Text())
+		if len(cmd) == 0 {
+			continue
+		}
+
+		switch cmd[0] {
+		case "quit":
+			fmt.Println("bye!")
+			return
+
+		case "list":
+			for _, g := range games {
+				fmt.Printf("#%d: %-15q %-20s $%d\n",
+					g.id, g.name, "("+g.genre+")", g.price)
+			}
+
+		case "id":
+			if len(cmd) != 2 {
+				fmt.Println("wrong id")
+				continue
+			}
+
+			id, err := strconv.Atoi(cmd[1])
+			if err != nil {
+				fmt.Println("wrong id")
+				continue
+			}
+
+			g, ok := byID[id]
+			if !ok {
+				fmt.Println("sorry. i don't have the game")
+				continue
+			}
+
+			fmt.Printf("#%d: %-15q %-20s $%d\n",
+				g.id, g.name, "("+g.genre+")", g.price)
+
+		case "save":
+			type jsonGame struct {
+				ID    int    `json:"id"`
+				Name  string `json:"name"`
+				Genre string `json:"genre"`
+				Price int    `json:"price"`
+			}
+
+			// load the data into the encodable game values
+			var encodable []jsonGame
+			for _, g := range games {
+				encodable = append(encodable,
+					jsonGame{g.id, g.name, g.genre, g.price})
+			}
+
+			out, err := json.MarshalIndent(encodable, "", "\t")
+			if err != nil {
+				fmt.Println("Sorry:", err)
+				continue
+			}
+
+			fmt.Println(string(out))
+			return
+		}
+	}
+}

24-structs/exercises/05-decode/main.go

@@ -0,0 +1,57 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+// ---------------------------------------------------------
+// EXERCISE: Decode
+//
+//  At the beginning of the file:
+//
+//  1. Load the initial data to the game store from json.
+//     (see the data constant below)
+//
+//  2. Load the decoded values into the usual `game` values (to the games slice as well).
+//
+//     So the rest of the program can work intact.
+//
+//
+// HINT
+//
+//  Move the jsonGame type to the top and reuse it both when
+//  loading the initial data, and in the "save" command.
+//
+//
+// EXPECTED OUTPUT
+//  Please run the solution to see the output.
+// ---------------------------------------------------------
+
+const data = `
+[
+        {
+                "id": 1,
+                "name": "god of war",
+                "genre": "action adventure",
+                "price": 50
+        },
+        {
+                "id": 2,
+                "name": "x-com 2",
+                "genre": "strategy",
+                "price": 40
+        },
+        {
+                "id": 3,
+                "name": "minecraft",
+                "genre": "sandbox",
+                "price": 20
+        }
+]`
+
+func main() {
+	// use your solution from the previous exercise
+}

24-structs/exercises/05-decode/solution/main.go

@@ -0,0 +1,157 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"bufio"
+	"encoding/json"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+const data = `
+[
+        {
+                "id": 1,
+                "name": "god of war",
+                "genre": "action adventure",
+                "price": 50
+        },
+        {
+                "id": 2,
+                "name": "x-com 2",
+                "genre": "strategy",
+                "price": 40
+        },
+        {
+                "id": 3,
+                "name": "minecraft",
+                "genre": "sandbox",
+                "price": 20
+        }
+]`
+
+func main() {
+	type item struct {
+		id    int
+		name  string
+		price int
+	}
+
+	type game struct {
+		item
+		genre string
+	}
+
+	// ----------------------------------------------------------
+	// DECODING SOLUTION:
+
+	// encodable and decodable game type
+	type jsonGame struct {
+		ID    int    `json:"id"`
+		Name  string `json:"name"`
+		Genre string `json:"genre"`
+		Price int    `json:"price"`
+	}
+
+	// load the initial data from json
+	var decoded []jsonGame
+	if err := json.Unmarshal([]byte(data), &decoded); err != nil {
+		fmt.Println("Sorry, there is a problem:", err)
+		return
+	}
+
+	// load the data into usual game values
+	var games []game
+	for _, dg := range decoded {
+		games = append(games, game{item{dg.ID, dg.Name, dg.Price}, dg.Genre})
+	}
+	// ----------------------------------------------------------
+
+	// index the games by id
+	byID := make(map[int]game)
+	for _, g := range games {
+		byID[g.id] = g
+	}
+
+	fmt.Printf("Inanc's game store has %d games.\n", len(games))
+
+	in := bufio.NewScanner(os.Stdin)
+	for {
+		fmt.Printf(`
+  > list   : lists all the games
+  > id N   : queries a game by id
+  > save   : exports the data to json and quits
+  > quit   : quits
+
+`)
+
+		if !in.Scan() {
+			break
+		}
+
+		fmt.Println()
+
+		cmd := strings.Fields(in.Text())
+		if len(cmd) == 0 {
+			continue
+		}
+
+		switch cmd[0] {
+		case "quit":
+			fmt.Println("bye!")
+			return
+
+		case "list":
+			for _, g := range games {
+				fmt.Printf("#%d: %-15q %-20s $%d\n",
+					g.id, g.name, "("+g.genre+")", g.price)
+			}
+
+		case "id":
+			if len(cmd) != 2 {
+				fmt.Println("wrong id")
+				continue
+			}
+
+			id, err := strconv.Atoi(cmd[1])
+			if err != nil {
+				fmt.Println("wrong id")
+				continue
+			}
+
+			g, ok := byID[id]
+			if !ok {
+				fmt.Println("sorry. i don't have the game")
+				continue
+			}
+
+			fmt.Printf("#%d: %-15q %-20s $%d\n",
+				g.id, g.name, "("+g.genre+")", g.price)
+
+		case "save":
+			// load the data into the encodable game values
+			var encodable []jsonGame
+			for _, g := range games {
+				encodable = append(encodable,
+					jsonGame{g.id, g.name, g.genre, g.price})
+			}
+
+			out, err := json.MarshalIndent(encodable, "", "\t")
+			if err != nil {
+				fmt.Println("Sorry:", err)
+				continue
+			}
+
+			fmt.Println(string(out))
+			return
+		}
+	}
+}

24-structs/exercises/README.md

@@ -12,4 +12,12 @@ You'll build a queryable command-line game store.
 
 3. **[Query By Id](https://github.com/inancgumus/learngo/tree/master/24-structs/exercises/03-query-by-id)**
 
-    Add a new command: "id". So the users can query the games by id.
+    Add a new command: "id". So the users can query the games by id.
+
+4. **[Encode](https://github.com/inancgumus/learngo/tree/master/24-structs/exercises/04-encode)**
+
+    Add a new command: "save". Encode the games to json, and print it, then terminate the loop.
+
+5. **[Decode](https://github.com/inancgumus/learngo/tree/master/24-structs/exercises/05-decode)**
+
+    Load the initial data to the game store from json.

24-structs/questions/README.md

@@ -176,3 +176,55 @@ fmt.Println(m.title, "&", m.item.title)
 > **4:** Right! `m.title` returns "avengers: end game" because the outer type always takes priority. However, `m.item.title` returns "midnight in paris" because you explicitly get it from the inner type: item.
 >
 
+
+## What is a field tag?
+1. It allows Go to index struct fields more efficiently
+2. You can use it for documenting your code
+3. It's like a comment
+4. Associates metadata about the field *CORRECT*
+
+> **4:** Correct. For example, the json package can read and encode/decode depending on the associated metadata.
+
+
+## Which one is correct about a field tag?
+1. It needs to be typed according to some rules
+2. You can change it to a different value in runtime
+3. It's just a string value, and it doesn't have a meaning on its own *CORRECT*
+
+> **1:** This is true to some extent but it can have any value.
+> 
+> **2:** Fields tags are part of a struct type definition so you cannot change their value in runtime.
+> 
+> **3:** Right! It's just a string value. It's only meaningful when other code reads it. For example, the json package can read it and encode/decode depending on the field tag's value.
+> 
+
+
+## What is wrong with the following program?
+```go
+type movie struct {
+    title string `json:"title"`
+}
+
+m := movie{"black panthers"}
+encoded, _ := json.Marshal(m)
+
+fmt.Println(string(encoded))
+```
+1. `movie` is unexported so you cannot encode
+2. `title` is unexported so you cannot encode *CORRECT*
+3. Error handling is missing so you cannot encode
+
+> **1:** The json package can encode a struct even though its type is unexported.
+> 
+> **2:** Right! The json package can only encode exported fields.
+> 
+> **3:** It's better to handle errors but it's not the main problem here.
+> 
+
+
+## Why do you need to pass a pointer to the Unmarshal function?
+1. To make it work faster and efficient
+2. So it can update the value on memory *CORRECT*
+3. To prevent errors
+
+> **2:** Otherwise, it would not be able to update the given value. It's because, every value in Go is passed by value. So a function can only change the copy, not the original value. However, through a pointer, a function can change the original value.