How Can I Iterate Over Unknown Data Structures in Go?

Iterating Over Data Structures in Go Without Knowing Exact Types

Problem:

How can we iterate over a data structure (array or map) in Go without having exact knowledge of its type?

Failed Attempt:

The code below attempts to iterate over an interface representing either a map or an array and execute a function on each item, but it fails due to type checking issues.

func DoTheThingToAllTheThings(data_interface interface{}) int {
    var numThings int

    switch data := data_interface.(type) {
    case map[interface{}]interface{}:
        numThings = len(data)
        // ...
    case []interface{}:
        numThings = len(data)
        // ...
    default:
        fmt.Println("uh oh!")
    }

    return numThings
}

Solution:

The fmt.Printf("%vn", data_interface) function provides a way to iterate over the data structure without type-casting.

func PrintData(data_interface interface{}) {
    fmt.Printf("%v\n", data_interface)
}

This works because the %v verb in fmt.Printf uses reflection to determine the type of the argument and print it accordingly.

Reflection in Go:

The fmt.Printf function internally uses the reflect package to inspect the type of the argument and decide how to format it. The reflect.ValueOf(arg) returns a reflect.Value object, which represents the actual value of the argument, and reflect.TypeOf(arg) returns the type of the value.

Example:

The following code reflects a Board struct, then reconstitutes it into a new variable of the same type.

type Board struct {
    Tboard  [9]string
    Player1 Player
    Player2 Player
}

func main() {
    myBoard := makeBoard()

    v := reflect.ValueOf(*myBoard)
    t := v.Type()

    var b2 Board

    b2 = v.Interface().(Board)
    fmt.Printf("v converted back to: %#v\n", b2)
}

Note:

In order to use reflection, the data structure's type must beexported, which means it must start with an uppercase letter.

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