迭代Go地图获取索引 [英] Iterate Go map get index

问题描述

为了使用revel的 even 关键字在模板中，我想使用range进行迭代时获取映射条目的索引.有什么办法吗? 我的地图具有以下结构:

In order to use revel's even keyword in templates I would like to get the index of a map entry when iterating with range. Is there any way to do so? My map has the structure:

map[string][]string

推荐答案

您不能仅使用模板操作来执行此操作，但是您可以注册一个提供必要帮助的函数.

You can't do this only with template actions, but you may register a function which provides the necessary help.

您可以注册一个返回一个函数(闭包)的函数，该函数在每次调用时都会改变其返回值(确切地说，奇数"和偶数"索引是如何交替的):

You may register a function which returns a function (closure), which alternates its return value whenever called (exactly how "odd" and "even" indices alternate):

func isEven() func() bool {
    e := false
    return func() bool {
        e = !e
        return e
    }
}

我将其命名为isEven()，以免与ravel的even()发生冲突.使用它:

I named it isEven() to not collide with ravel's even(). Using it:

func main() {
    t := template.Must(template.New("").Funcs(template.FuncMap{
        "isEven": isEven,
    }).Parse(templ))

    m := map[string]string{
        "a": "A", "b": "B", "c": "C", "d": "D",
    }
    if err := t.Execute(os.Stdout, m); err != nil {
        panic(err)
    }
}

const templ = `{{$e := isEven}}
{{- range $k, $v := . -}}
    [even:{{call $e}}] key={{$k}}; value={{$v}}
{{end}}`

输出(在游乐场上尝试):

[even:true] key=a; value=A
[even:false] key=b; value=B
[even:true] key=c; value=C
[even:false] key=d; value=D

如果您想为奇数和偶数迭代提供不同的输出，则可以在{{if}}动作中调用$e，如下所示:

If you want different output for odd and even iterations, you can call $e in an {{if}} action, like this:

const templ = `{{$e := isEven}}
{{- range $k, $v := . -}}
    [{{if call $e}}even{{else}}odd {{end}}] key={{$k}}; value={{$v}}
{{end}}`

此输出(在游乐场上尝试):

[even] key=a; value=A
[odd ] key=b; value=B
[even] key=c; value=C
[odd ] key=d; value=D

引擎盖下

此模板操作:

Under the hood

This template action:

{{$e := isEven}}

创建一个名为$e的新模板变量，其值将为isEven()函数调用的结果(返回值). isEven()返回一个函数值，该闭包可以访问类型为bool的局部变量e.稍后执行{{call $e}}时，您没有在调用isEven() Go函数，而是返回了该函数(闭包)并存储在$e中.该闭包引用了本地bool变量e，直到可以访问isEvent()返回的函数之前，它不会释放".

Creates a new template variable named $e, and its value will be the result (return value) of the isEven() function call. isEven() returns a function value, a closure that has access to a local variable e of type bool. When later you do {{call $e}}, you're not calling the isEven() Go function, but the function it returned (the closure) and is stored in $e. That closure has a reference to the local bool variable e, it is not "freed" until the function returned by isEvent() is accessible.

因此，每当执行{{call $e}}时，它都会调用闭包，该闭包具有"类型为bool的e变量，该变量的值在两次$e的调用之间保留.

So whenever you do {{call $e}}, it calls the closure, which "has" an e variable of type bool, whose value is retained between calls of this $e.

如果您再次在模板中调用isEvent，则将返回一个新函数(闭包)，包装一个局部变量e的新实例，而与由该包装返回的闭包的第一个包装变量无关.第一次isEvent()呼叫.

If you would call isEvent in the template again, that would return a new function (closure), wrapping a new instance of the local variable e, being independent of the first wrapped variable of the closure returned by the first isEvent() call.

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