ゼロ値を使おう #golang ref: https://qiita.com/tenntenn/items/c55095585af64ca28ab5

file0.go

package main

import "fmt"

func main() {
  var n int
  var f float64
  var s string
  var b bool
  fmt.Printf("%#v %#v %#v %#v", n, f, s, b)
}

file1.txt

0 0 "" false

file10.go

package main

import "fmt"

func main() {
  var f func()
  var ptr *int
  fmt.Printf("%#v %#v", f, ptr)
}

file11.txt

(func())(nil) (*int)(nil)

file12.txt

prog.go:8: Printf format %#v arg f is a func value, not called

file13.go

package main

import "fmt"

func main() {
  count := 0
  count++
  count++
  fmt.Println(count)
}

file14.go

package main

import "fmt"

func main() {
  var count int
  count++
  count++
  fmt.Println(count)
}

file15.go

package main

import "fmt"

func main() {
  words := []string{"dog", "cat", "dog", "fish", "cat"}
  wc := map[string]int{}
  for _, w := range words {
    wc[w]++
  }
  fmt.Println(wc)
}

file16.txt

map[dog:2 cat:2 fish:1]

file17.go

package main

import "fmt"

func main() {
  words := []string{"dog", "cat", "dog", "fish", "cat"}
  wc := map[string]bool{}
  for _, w := range words {
    wc[w] = true
  }

  for _, w := range []string{"dog", "pig"} {
    if wc[w] {
      fmt.Println(w, "は存在する")
    } else {
      fmt.Println(w, "は存在しない")
    }
  }
}

file18.go

package main

import "fmt"

func main() {
  var ns []int
  fmt.Printf("%#v %d %d\n", ns, len(ns), cap(ns))
  ns = append(ns, 10, 20)
  fmt.Println(ns)
}

file19.txt

[]int(nil) 0 0
[10 20]

file2.go

package main

import "fmt"

func main() {
  var ns [3]int
  fmt.Println(ns)
}

file20.go

package main

import "sync"

func main() {
  var mu sync.Mutex
  mu.Lock()
  mu.Unlock()
}

file21.go

package main

import (
  "fmt"
  "sync"
)

func main() {
  var wg sync.WaitGroup

  wg.Add(2)

  go func() {
    defer wg.Done()
    fmt.Println("done 1")
  }()

  go func() {
    defer wg.Done()
    fmt.Println("done 1")
  }()

  wg.Wait()
  fmt.Println("main done")
}

file22.go

package main

import "fmt"

type WordCounter struct {
  m map[string]int
}

func NewWordCounter() *WordCounter {
  return &WordCounter{
    m: map[string]int{},
  }
}

func (wc *WordCounter) Count(s string) int {
  wc.m[s]++
  return wc.m[s]
}

func (wc *WordCounter) Get(s string) int {
  return wc.m[s]
}

func main() {
  wc := NewWordCounter()
  words := []string{"dog", "cat", "dog", "fish", "cat"}
  for _, w := range words {
    wc.Count(w)
  }
  fmt.Println("dog", wc.Get("dog"))
}

file23.txt

dog 2

file24.go

func (wc *WordCounter) Count(s string) int {
  if wc.m == nil {
    wc.m = map[string]int{}
  }
  wc.m[s]++
  return wc.m[s]
}

file25.go

package main

import (
  "fmt"
  "sync"
)

type WordCounter struct {
  initOnce sync.Once
  m        map[string]int
}

func (wc *WordCounter) init() {
  wc.m = map[string]int{}
}

func (wc *WordCounter) Count(s string) int {
  wc.initOnce.Do(wc.init)
  wc.m[s]++
  return wc.m[s]
}

func (wc *WordCounter) Get(s string) int {
  wc.initOnce.Do(wc.init)
  return wc.m[s]
}

func main() {
  var wc WordCounter
  words := []string{"dog", "cat", "dog", "fish", "cat"}
  for _, w := range words {
    wc.Count(w)
  }
  fmt.Println("dog", wc.Get("dog"))
}

file26.go

func main() {
  var wc WordCounter
  words := []string{"dog", "cat", "dog", "fish", "cat"}
  for _, w := range words {
    wc.Count(w) // (&wc).Count(w)と同様
  }
  fmt.Println("dog", wc.Get("dog")) // (&wc).Get("dog")と同様
}

file27.go

var buf bytes.Buffer

// bufはio.Writerを実装していないからコンパイルエラー
fmt.Fprintf(buf, "Hello")

// &bufはio.Writerを実装している
fmt.Fprintf(&buf, "Hello")

file3.txt

[0 0 0]

file4.go

package main

import "fmt"

func main() {
  type Hoge struct {
    N  int
    NS [3]int
  }

  var h Hoge
  fmt.Printf("%#v", h)
}

file5.txt

main.Hoge{N:0, NS:[3]int{0, 0, 0}}

file6.go

package main

import "fmt"

func main() {
  type Hoge struct {
    N int
  }

  type Fuga struct {
    int
    Hoge
  }

  var f Fuga
  fmt.Printf("%#v", f)
}

file7.txt

main.Fuga{int:0, Hoge:main.Hoge{N:0}}

file8.go

package main

import "fmt"

func main() {
  var ch chan int
  var ns []int
  var m map[string]int
  fmt.Printf("%#v %#v %#v", ch, ns, m)
}

file9.txt

(chan int)(nil) []int(nil) map[string]int(nil)