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
正如所指出的(希望这并不令人意外)，使用if+else确实是在Go中执行条件语句的惯用方式。
除了完整的var+if+else代码块之外，这种拼写也经常被使用:
index := val
if val <= 0 {
index = -val
}
如果你有一段重复的代码，比如int value = a <= b ?A: b，你可以创建一个函数来保存它:
func min(a, b int) int {
if a <= b {
return a
}
return b
}
...
value := min(a, b)
编译器将内联这些简单的函数，所以它更快，更清楚，更简短。
其他回答
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
如果你所有的分支都产生了副作用，或者计算成本很高，下面是一个语义保留的重构:
index := func() int {
if val > 0 {
return printPositiveAndReturn(val)
} else {
return slowlyReturn(-val) // or slowlyNegate(val)
}
}(); # exactly one branch will be evaluated
通常没有开销(内联)，最重要的是，不会因为只使用一次的辅助函数而使名称空间混乱(这会影响可读性和维护)。生活的例子
注意，如果你天真地应用Gustavo的方法:
index := printPositiveAndReturn(val);
if val <= 0 {
index = slowlyReturn(-val); // or slowlyNegate(val)
}
你会得到一个有着不同行为的程序;在val <= 0的情况下，程序将输出一个非正值，而它不应该!(类似地，如果反转分支，就会不必要地调用缓慢的函数，从而引入开销。)
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
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.\
关于三元运算符的惯用方法，还有一个建议:
package main
import (
"fmt"
)
func main() {
val := -5
index := func (test bool, n, d int) int {
if test {
return n
}
return d
}(val > 0, val, -val)
fmt.Println(index)
}
去操场
