使用一个返回值吗?

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英文:

Use one return value?

问题

我想调用我的函数test并使用其中一个返回值。我该如何说“给我第一个或第二个值”?我以为下面的代码会给我“one”,但是[1]是错误的用法,导致编译错误。

package main

import (
	"fmt"
)

func test() (int, string) { return 1, "one"; }

func main() {
	i, sz := test()
	fmt.Printf("%d=%s\n", i, sz)
	fmt.Printf("%s", test()[1]) //error
}
英文:

I want to call my function test and use one of the return values. How do I say give me the first or second value? I thought the below would give me "one" but [1] is incorrect usage causing a compile error

package main

import (
	"fmt"
)

func test() (int, string) { return 1, "one"; }

func main() {
	i,sz:=test()
	fmt.Printf("%d=%s\n",i,sz)
	fmt.Printf("%s", test()[1]) //error
}

答案1

得分: 4

据我所知,您无法对函数返回值进行下标操作。您可以这样做:

_, someString := test();
fmt.Println(someString);
英文:

As far as I know, you can't subscript function return values. You can do:

_, someString := test();
fmt.Println(someString);

答案2

得分: 3

引用Go语言规范

形式为a[x]的主表达式表示由x索引的数组、切片、字符串或映射a的元素。值x分别称为索引或映射键。[...] 否则(如果a不是数组、切片、字符串或映射),a[x]是非法的。

然而,在Go语言中,多个返回值并不是返回的数组,而是一种独立的语言特性。这是必须的,因为数组只能保存单一类型的元素,而返回值可以是不同类型的。

但是由于返回值不是数组(或切片、字符串或映射),根据语言规范,a[x]的语法是错误的。因此,正如@dav已经正确指出的那样,你必须将返回值实际分配给一个变量,以便在其他地方使用它。


在特殊情况下,你可以利用这个小技巧来避免变量赋值:

作为一个特例,如果函数或方法g的返回值与另一个函数或方法f的参数数量相等,并且可以逐个赋值给f的参数,则调用f(g(g的参数))将按顺序将g的返回值绑定到f的参数后调用f。

这使得以下操作成为可能:

func foo() (int, string) {
    return 42, "test";
}

func bar(x int, s string) {
    fmt.Println("Int: ", x);
    fmt.Println("String: ", s);
}

func main() {
    bar(foo())
}
英文:

Citing the Go Language Specification:

> A primary expression of the form a[x]
denotes the element of the array, slice, string or map a indexed by x. The value x is called the index or map key, respectively. [...] Otherwise [if a is not an array, slice string or map] a[x] is illegal.

Multiple return values in Go, however, are not arrays being returned, but a separate language feature. This must be so, because an array can only hold elements of a single type, but return values can be of different types.

But since return values are not arrays (or slices, strings or maps), the a[x] syntax is, per language spec, a syntax error. As a result, as @dav has already correctly stated, you will have to actually assign the return value to a variable in order to use it elsewhere.


In special cases, you may be able to use this bit of trivia to avoid variable assignment:

> As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order.

Which makes the following possible:

func foo() (int, string) {
    return 42, "test";
}

func bar(x int, s string) {
    fmt.Println("Int: ", x);
    fmt.Println("String: ", s);
}

func main() {
    bar(foo())
}

huangapple
  • 本文由 发表于 2013年6月26日 02:41:23
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