使用一个返回值？

Question

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
",i,sz)
    fmt.Printf("%s", test()[1]) //error
}

Answer 1

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())
}