Implement String() on a map with interface{} values

Question

How could I write a function to print a map object in Go (Golang)? Right now I have this, but it doesn't compile. It returns cannot convert value (type interface {}) to type reflect.Kind: need type assertion.

package main

type MyDictionary map[string]interface{}

func (d MyDictionary) String() string {
    var stringBuffer bytes.Buffer

    for key, value := range d {
        stringBuffer.WriteString(key)
        stringBuffer.WriteString(": ")

        valueType := reflect.Kind(value)

        switch valueType {
        case reflect.String:
            log.Println("string") // just to check if this block gets executed
            // Add to stringBuffer

        case reflect.Float64:
            log.Println("float64") // just to check if this block gets executed
            // Add to stringBuffer

        default:
            log.Println("Error: type was", valueType)
        }
    }

    return stringBuffer.String()
}

func main() {
    var dict MyDictionary = make(MyDictionary)
    dict["hello"] = "world"
    dict["floating"] = 10.0
    dict["whole"] = 12

    fmt.Println(dict)
}

I want String() to return a string like hello: world\nfloating: 10.0\nwhole: 12\n. That I can then pass to fmt.Println() to print this. In Java, I would use StringBuilder for this.

hello: world
floating: 10.0
whole: 12

I also tried switching on value.(type) with case string: and case float64, but then I didn't know how to write those values to stringBuffer.

ryboe · Accepted Answer · 2017-08-08 19:27:54Z

5

Here's an idiomatic solution.

func (d MyDictionary) String() string {
    var buf bytes.Buffer

    for k, v := range d {
        buf.WriteString(k + ": ")

        // v is an interface{} here
        switch v := v.(type) {
        // The inner v is typed. It shadows the outer interface{} v. That's
        // the idiomatic part.
        case string:
            buf.WriteString(v + "\n") // v is a string
        case int:
            buf.WriteString(fmt.Sprintln(v)) // v is an int
        case float64:
            buf.WriteString(fmt.Sprintln(v)) // v is a float64
        }
    }

    return buf.String()
}

edited Aug 8, 2017 at 19:27

answered Jul 8, 2014 at 0:27

ryboe

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Comments

Simon Whitehead · Accepted Answer · 2014-07-08 00:29:44Z

You can potentially simplify it to this (playground):

func (d MyDictionary) String() string {
    var result string

    for key, value := range d {
        result += fmt.Sprintf("%s: %v\n", key, value)
    }

    return result
}

Which prints:

hello: world
floating: 10
whole: 12

Obviously, the "whole" floating point has the decimals removed (if you set it to 10.5 it will print properly). If that's required, then you'll want to switch on the float and specify precision as well (playground):

func (d MyDictionary) String() string {
    var result string

    for key, value := range d {
        switch value.(type) {
        case float64:
            result += fmt.Sprintf("%s: %.2f\n", key, value)
        default:
            result += fmt.Sprintf("%s: %v\n", key, value)
        }
    }

    return result
}

Which prints:

floating: 10.00
whole: 12
hello: world

fabmilo · Accepted Answer · 2014-07-08 00:05:36Z

-1

You need to get the type of the interface and then switch on the kind of the type.

valueType := reflect.TypeOf(value).Kind()

Working Example: http://play.golang.org/p/a-7SePUzZ-

package main

import (
    "bytes"
    "fmt"
    "log"
    "reflect"
)

type MyDictionary map[string]interface{}

func (d MyDictionary) String() string {
    var stringBuffer bytes.Buffer

    for key, value := range d {
        stringBuffer.WriteString(key)
        stringBuffer.WriteString(": ")

        valueType := reflect.TypeOf(value).Kind()

        switch valueType {
        case reflect.String:
            log.Println("string")
        default:
            log.Println("Type was:", valueType)
        }
    }

    return stringBuffer.String()
}

func main() {
    var dict MyDictionary = make(MyDictionary)
    dict["hello"] = "world"
    dict["floating"] = 10.0
    dict["whole"] = 12

    fmt.Println(dict)
}

Output

2009/11/10 23:00:00 string
2009/11/10 23:00:00 Type was: float64
2009/11/10 23:00:00 Type was: int
hello: floating: whole:

answered Jul 8, 2014 at 0:05

fabmilo

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1 Comment

425nesp Over a year ago

Perhaps I was too vague. I want String() to return a string that concatenates all of the key and values to a single string and then returns that strings.

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