"A tour of Go" study notes (b)

Foreword

This article, " A tour of Go " Language Guide study notes.

surroundings

macOS

Control flow

for

Go only one loop structure forcycle. forNo statement in parentheses.

package main

import "fmt"

func () {
    sum := 0
    for i := 0; i < 10; i++ {
        sum += i
    }
    fmt.Println(sum)
}

Front and rear statement may be omitted.

package main

import "fmt"

func () {
    sum := 1
    for ; sum < 1000; {
        sum += sum
    }
    fmt.Println(sum)
}

// 1024

forStatement may be in another language whilestatement.

package main

import "fmt"

func () {
    sum := 1
    for sum < 1000 {
        sum += sum
    }
    fmt.Println(sum)
}

// 1024

Infinite loop the loop condition is omitted.

package main

func () {
    for {
    }
}

// process took too long

if

if No statement in parentheses.

package main

import (
    "fmt"
    "math"
)

func sqrt(x float64) string {
    if x < 0 {
        return sqrt(-x) + "i"
    }
    return fmt.Sprint(math.Sqrt(x))
}

func () {
    fmt.Println(sqrt(2), sqrt(-4))
}

// 1.4142135623730951 2i

math.Sqrt() 函式用來取得平方根。

if 語句可以在條件之前執行一個簡單的語句，由這個語句定義的變數，其作用域只限於在該 if 語句之內。

package main

import (
    "fmt"
    "math"
)

func pow(x, n, lim float64) float64 {
    if v := math.Pow(x, n); v < lim {
        return v
    }
    return lim
}

func main() {
    fmt.Println(
        pow(3, 2, 10),
        pow(3, 3, 20),
    )
}

// 9 20

在 if 的便捷語句定義的變數可以在對應的 else 區塊中使用。

package main

import (
    "fmt"
    "math"
)

func pow(x, n, lim float64) float64 {
    if v := math.Pow(x, n); v < lim {
        return v
    } else {
        fmt.Printf("%g >= %gn", v, lim)
    }
    // can't use v here, though
    return lim
}

func main() {
    fmt.Println(
        pow(3, 2, 10),
        pow(3, 3, 20),
    )
}

// 27 >= 20
// 9 20

switch

switch 語句的條件從上到下執行。

package main

import (
    "fmt"
    "runtime"
)

func main() {
    fmt.Print("Go runs on ")
    switch os := runtime.GOOS; os {
    case "darwin":
        fmt.Println("OS X.")
    case "linux":
        fmt.Println("Linux.")
    default:
        // freebsd, openbsd,
        // plan9, windows...
        fmt.Printf("%s.", os)
    }
}

// Go runs on nacl.

除非使用 fallthrough 語句，否則匹配成功會自動終止

package main

import (
    "fmt"
    "time"
)

func main() {
    fmt.Println("When's Saturday?")
    today := time.Now().Weekday()
    switch time.Saturday {
    case today + 0:
        fmt.Println("Today.")
    case today + 1:
        fmt.Println("Tomorrow.")
    case today + 2:
        fmt.Println("In two days.")
    default:
        fmt.Println("Too far away.")
    }
}

// When's Saturday?
// Too far away.

沒有條件的 switch 語句同 switch true 一樣，用以替代長的 if-then-else 條件式。

package main

import (
    "fmt"
    "time"
)

func main() {
    t := time.Now()
    switch {
    case t.Hour() < 12:
        fmt.Println("Good morning!")
    case t.Hour() < 17:
        fmt.Println("Good afternoon.")
    default:
        fmt.Println("Good evening.")
    }
}

// Good evening.

Original: Big Box "A tour of Go" study notes (b)