在C/ c++(以及该家族的许多语言)中,根据条件声明和初始化变量的常用习语使用三元条件操作符:

int index = val > 0 ? val : -val

Go没有条件运算符。实现上面同一段代码的最惯用的方法是什么?我想出了下面的解决方案,但它似乎相当啰嗦

var index int

if val > 0 {
    index = val
} else {
    index = -val
}

还有更好的办法吗?


当前回答

我已经编译了一些项目并比较了速度。

/*
go test ternary_op_test.go -v -bench="^BenchmarkTernaryOperator" -run=none -benchmem
*/
package _test

import (
    "testing"
)

func BenchmarkTernaryOperatorIfElse(b *testing.B) {
    for i := 0; i < b.N; i++ {
        if i%2 == 0 {
            _ = i
        } else {
            _ = -i
        }
    }
}

// https://stackoverflow.com/a/45886594/9935654
func Ternary(statement bool, a, b interface{}) interface{} {
    if statement {
        return a
    }
    return b
}

func BenchmarkTernaryOperatorTernaryFunc(b *testing.B) {
    for i := 0; i < b.N; i++ {
        _ = Ternary(i%2 == 0, i, -i).(int)
    }
}

// https://stackoverflow.com/a/34636594/9935654
func BenchmarkTernaryOperatorWithFunc(b *testing.B) {
    for i := 0; i < b.N; i++ {
        _ = func() int {
            if i%2 == 0 {
                return i
            } else {
                return -i
            }
        }
    }
}

// https://stackoverflow.com/a/31483763/9935654
func BenchmarkTernaryOperatorMap(b *testing.B) {
    for i := 0; i < b.N; i++ {
        _ = map[bool]int{true: i, false: -i}[i%2 == 0]
    }
}

输出

goos: windows
goarch: amd64
cpu: Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
BenchmarkTernaryOperatorIfElse
BenchmarkTernaryOperatorIfElse-8                1000000000               0.4460 ns/op          0 B/op          0 allocs/op
BenchmarkTernaryOperatorTernaryFunc
BenchmarkTernaryOperatorTernaryFunc-8           1000000000               0.3602 ns/op          0 B/op          0 allocs/op
BenchmarkTernaryOperatorWithFunc
BenchmarkTernaryOperatorWithFunc-8              659517496                1.642 ns/op           0 B/op          0 allocs/op
BenchmarkTernaryOperatorMap
BenchmarkTernaryOperatorMap-8                   13429532                82.48 ns/op            0 B/op          0 allocs/op
PASS
ok      command-line-arguments  4.365s

其他回答

没有括号,map三元很容易读:

c := map[bool]int{true: 1, false: 0} [5 > 4]

No Go没有三元运算符。使用if/else语法是惯用的方法。

Why does Go not have the ?: operator? There is no ternary testing operation in Go. You may use the following to achieve the same result: if expr { n = trueVal } else { n = falseVal } The reason ?: is absent from Go is that the language's designers had seen the operation used too often to create impenetrably complex expressions. The if-else form, although longer, is unquestionably clearer. A language needs only one conditional control flow construct. — Frequently Asked Questions (FAQ) - The Go Programming Language

埃尔德的回答既有趣又有创意,甚至可以说很聪明。

但是,建议改为:

var index int
if val > 0 {
    index = printPositiveAndReturn(val)
} else {
    index = slowlyReturn(-val)  // or slowlyNegate(val)
}

是的,它们都编译成本质上相同的程序集,但是这段代码比调用匿名函数只返回一个可以首先写入变量的值要清晰得多。

基本上,简单清晰的代码比有创意的代码更好。

此外,任何使用map文字的代码都不是一个好主意,因为在Go中映射根本不是轻量级的。自Go 1.3以来,小型地图的随机迭代顺序得到了保证,为了加强这一点,小型地图的内存效率大大降低。

As a result, making and removing numerous small maps is both space-consuming and time-consuming. I had a piece of code that used a small map (two or three keys, are likely, but common use case was only one entry) But the code was dog slow. We're talking at least 3 orders of magnitude slower than the same code rewritten to use a dual slice key[index]=>data[index] map. And likely was more. As some operations that were previously taking a couple of minutes to run, started completing in milliseconds.\

func Ternary(statement bool, a, b interface{}) interface{} {
    if statement {
        return a
    }
    return b
}

func Abs(n int) int {
    return Ternary(n >= 0, n, -n).(int)
}

这不会比if/else更好,需要施放但有效。仅供参考:

基准基准三元-8 100000000 18.8 ns/op

BenchmarkAbsIfElse-8 2000000000 0.27 ns/op

虽然被创作者所回避,但俏皮话也有自己的一席之地。

这个函数解决了惰性求值的问题,它允许你在必要时可选地传递函数来求值:

func FullTernary(e bool, a, b interface{}) interface{} {
    if e {
        if reflect.TypeOf(a).Kind() == reflect.Func {
            return a.(func() interface{})()
        }
        return a
    }
    if reflect.TypeOf(b).Kind() == reflect.Func {
        return b.(func() interface{})()
    }
    return b
}

func demo() {
    a := "hello"
    b := func() interface{} { return a + " world" }
    c := func() interface{} { return func() string { return "bye" } }
    fmt.Println(FullTernary(true, a, b).(string)) // cast shown, but not required
    fmt.Println(FullTernary(false, a, b))
    fmt.Println(FullTernary(true, b, a))
    fmt.Println(FullTernary(false, b, a))
    fmt.Println(FullTernary(true, c, nil).(func() string)())
}

输出

hello
hello world
hello world
hello
bye

传入的函数必须返回一个接口{}以满足内部强制转换操作。 根据上下文,您可以选择将输出强制转换为特定类型。 如果你想从这个函数中返回一个函数,你需要用c来包装它。


这里的独立解决方案也很好,但对于某些用途可能不太清楚。