Exclude empty fields from struct iteration

Question

I have a struct that will get its value from user input. Now I want to extract only field names that have associated values. Fields with a nil value should not be returned. How can I do that?

Here’s my code:

package main


import "fmt"
import "reflect"

type Users struct {
    Name string
    Password string
}


func main(){
    u := Users{"Robert", ""}

    val := reflect.ValueOf(u)


    for i := 0; i < val.NumField(); i++ {

        fmt.Println(val.Type().Field(i).Name)

    }


}

Current Result:

Name
Password

Expected result:

Name

user4122236 · Accepted Answer · 2014-12-01 06:09:11Z

5

You need to write a function to check for empty:

func empty(v reflect.Value) bool {
    switch v.Kind() {
    case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
        return v.Int() == 0
    case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
        return v.Uint() == 0
    case reflect.String:
        return v.String() == ""
    case reflect.Ptr, reflect.Slice, reflect.Map, reflect.Interface, reflect.Chan:
        return v.IsNil()
    case reflect.Bool:
        return !v.Bool()
    }
    return false
}

playground example.

answered Dec 1, 2014 at 6:09

user4122236

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3 Comments

twotwotwo Over a year ago

default: return reflect.DeepEqual(v.Interface(), reflect.Zero(v.Type()).Interface()) would also (inefficiently) handle fields that are themselves structs.

under5hell Over a year ago

Thanks. But, I simplify it lil bit below. :)

KorbenDallas Over a year ago

Please, use isZero() method from reflect package now. details: stackoverflow.com/a/57613511

under5hell · Accepted Answer · 2014-12-01 08:24:43Z

0

I think I found a solution. Case closed. :)

import "fmt"
import "reflect"

type Users struct {
    Name string
    Password string
}


func main(){
    u := Users{"robert", ""}

    val := reflect.ValueOf(u)

    var fFields []string

    for i := 0; i < val.NumField(); i++ {
    f := val.Field(i)

    if f.Interface() != "" {
        fFields = append(fFields, val.Type().Field(i).Name)
    }

    }

   fmt.Println(fFields)
}

http://play.golang.org/p/QVIJaNXGQB

answered Dec 1, 2014 at 8:24

under5hell

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1 Comment

twotwotwo Over a year ago

Great--though I'd be inclined to accept 4of4's answer because the question's phrasing covers non-string fields too, and the more general answer could be useful for folks researching this problem later. (It also doesn't "cost" you anything to accept it, so to speak.)

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