How to do nested iteration

php editor Youzi will introduce you how to perform nested iteration. Nested iteration is a technique of using another loop within a loop, which helps us deal with complex data structures or multi-dimensional arrays. When doing nested iterations, we need to pay attention to the order and conditions of the loops to ensure that each element is accessed and processed correctly. This article will explain the principles and usage of nested iteration in detail, and provide some practical examples for reference. Whether you are a beginner or an experienced developer, this article can help you better understand and apply the techniques of nested iteration. Let’s explore together!

Question content

I'm trying to develop an extension to another software that sends requests to an application written in go. In a go program (which I will now call a "program"), one purpose is to convert a json file into an iterable format. Here is an example of the json format I'm using:

{
  "name": "game-name",
  "tree": {
    "$classname": "datamodel",

    "replicatedstorage": {
      "$path": "src/replicatedstorage"
    },

    "serverscriptservice": {
      "$path": "src/serverscriptservice"
    },

    "replicatedfirst": {
      "$path": "src/replicatedfirst"
    },

    "serverstorage": {
      "$path": "src/serverstorage"
    }
  }
}

The idea is:

• Iteration can get the "name"
• Iteration can get "$classname"
• For all instances with "$path" as the index, a folder will be created under the parent src folder containing the index of the parent map. For example, replicatedstorage is the name of the folder with the path src/replicatedstorage

Here is a handler function to do this:

func process(in interface{}) {
v := reflect.ValueOf(in)

    if v.Kind() == reflect.Map {
        for _, key := range v.MapKeys() {
            strct := v.MapIndex(key)
    
            index := key.Interface()
            value := reflect.ValueOf(strct.Interface())
    
            if index == "tree" {
                for _, treeKey := range value.MapKeys() {
                    treeIndex := treeKey.Interface()
    
                    fmt.Println("KEY")
                    fmt.Println(treeIndex)
    
                    if treeIndex != "$className" {
                        fmt.Println("bug")
                        fmt.Println(treeKey)
    
                        a := key.MapIndex(value) // panic serving ...: reflect: call of reflect.Value.MapIndex on string Value
                        b := reflect.ValueOf(a.Interface())
    
                        for _, key2 := range b.MapKeys() {
                            index2 := key2.Interface()
                            value2 := reflect.ValueOf(key2.Interface())
    
                            fmt.Println(index2)
                            fmt.Println(value2)
                        }
                    }
                }
            }
        }
    }

}

Comments pointed out incorrect location and content. One thing I'd also like to do is not have to stack the for loops, because that's really bad code.

Solution

The usual approach is to unmarshal into a go type that matches the data structure. The problem here is that the tree cannot be easily represented as a go type (it has a field $classname of type string, but is otherwise similar to a map with an object value containing a $path field).

Let's continue unmarshaling to interface{} as you have already done.

Use type assertions instead of reflection packages. Use Map Index to look up the value instead of looping through the keys and looking for a match.

func process(in interface{}) error {
    top, ok := in.(map[string]interface{})
    if !ok {
        return errors.New("expected object at top level")
    }
    tree, ok := top["tree"].(map[string]interface{})
    if !ok {
        return errors.New(".tree not found")
    }
    name, ok := top["name"]
    if !ok {
        return errors.New(".name not found")
    }
    className, ok := tree["$className"].(string)
    if !ok {
        return errors.New(".tree.$className not found")
    }
    for k, v := range tree {
        thing, ok := v.(map[string]interface{})
        if !ok {
            continue
        }
        path, ok := thing["$path"].(string)
        if !ok {
            continue
        }
        fmt.Println(name, className, k, path)
    }
    return nil
}

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