loops - 如何在 for 循环中基于结构声明变量

Question

这个问题已经有答案了:

Struct in for loop initializer (1 个回答)

已关闭 3 年前。

这是一个在 for 循环之前声明变量的工作程序。

package main

import "fmt"

type brackets struct {
    ch   string
    pos  int
}

type stack []brackets

func main() {
    p := brackets{ch: "a"}
    st := make(stack,0)
    for i :=  0; i < 3; i++ {
        p = brackets{ch: "a", pos: 1}
        st = append(st, p) 
        fmt.Println(p)
        fmt.Println(st)
    }
}

我想声明相同的变量作为 for 循环的一部分。如何？这是一个错误的尝试。

package main

import "fmt"

type brackets struct {
    ch   string
    pos  int
}

type stack []brackets

func main() {
//    p := brackets{ch: "a"}
//    st := make(stack,0)
    for st, p, i :=  make(stack,0), brackets{ch: "a"}, 0; i < 3; i++ {
        p = brackets{ch: "a", pos: 1}
        st = append(st, p) 
        fmt.Println(p)
        fmt.Println(st)
    }
}

我收到以下错误:语法错误:st, p, i := make(stack, 0)，括号用作值

我很困惑，因为声明整数和字符串等标准类型非常容易。为什么不是我的结构？

Answer 1

composite literal section of the specification说:

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

通过在复合文字周围添加括号来修复代码:

for st, p, i :=  make(stack,0), (brackets{ch: "a"}), 0; i < 3; i++ {
    p = brackets{ch: "a", pos: 1}
    st = append(st, p) 
    fmt.Println(p)
    fmt.Println(st)
}

Run it on the playground .