How to sort struct fields in alphabetical order

Question

How could I get an output of struct, sorted by fields?

type T struct {
    B int
    A int
}

t := &T{B: 2, A: 1}

doSomething(t)

fmt.Println(t)  // &{1 2} --> Sorted by fields

Do you assume all your fields are going to be of the same type? — tomasz, Jun 18 '15 at 11:07
Looking at the question again – do you want to sort the output by the field name or the value behind it? — tomasz, Jun 18 '15 at 13:47
@tomasz by the field name. When I write in Ruby it's gonna be `Hash[{b: 1, a: 2}.sort]`. — tomodian, Jun 18 '15 at 14:47
Ok, misunderstood your intention then. Go for icza's answer, it hopefully covers all you need. — tomasz, Jun 18 '15 at 14:48

icza · Accepted Answer · 2016-10-02T10:07:35.047

A struct is an ordered collection of fields. The fmt package uses reflection to get the fields and values of a struct value, and generates output in the order in which they were defined.

So the simplest solution would be to declare your type where you already have your fields arranged in alphabetical order:

type T struct {
    A int
    B int
}

If you can't modify the order of fields (e.g. memory layout is important), you can implement the Stringer interface by specifying a String() method for your struct type:

func (t T) String() string {
    return fmt.Sprintf("{%d %d}", t.A, t.B)
}

The fmt package checks if the passed value implements Stringer, and if it does, calls its String() method to generate the output.

Cons of this solution is that this is not flexible (e.g. if you add a new field, you have to update the String() method too), also you have to do it for every struct type you want it to work (and you can't define methods for types defined in other packages).

The completely flexible solution can use reflection. You can get the names of fields, sort them by name, and then iterate over the sorted names and get the field values (by name).

Pros of this solution is that this works for any struct, and it keeps working without modification even if you add or remove fields from your structs. It also works for fields of any type, not just for int fields.

Here is an example how to do it (try it on the Go Playground):

func printFields(st interface{}) string {
    t := reflect.TypeOf(st)

    names := make([]string, t.NumField())
    for i := range names {
        names[i] = t.Field(i).Name
    }
    sort.Strings(names)

    v := reflect.ValueOf(st)
    buf := &bytes.Buffer{}
    buf.WriteString("{")
    for i, name := range names {
        val := v.FieldByName(name)
        if !val.CanInterface() {
            continue
        }
        if i > 0 {
            buf.WriteString(" ")
        }
        fmt.Fprintf(buf, "%v", val.Interface())
    }
    buf.WriteString("}")

    return buf.String()
}

+1 for a nice complete answer. I somehow jumped into conclusion OP wanted to sort fields by value (not name), but looking at the question again, and per example, it's now totally ambiguous to me. — tomasz, Jun 18 '15 at 13:46
@icza this really helps me to understand the concepts of sorting in Go. The latter one is flexible for sure! — tomodian, Jun 18 '15 at 14:37

score 1 · Answer 2 · answered Jun 18 '15 at 11:09

1

Make T implement the Stringer interface (see package fmt) and do either print A orb B first.

BTW. This is a stupid idea.

answered Jun 18 '15 at 11:09

Volker

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now I understand what you mean by implementing Stringer interface by reading icza's answer. Robust! – tomodian Jun 18 '15 at 14:56
2

You should explain why "this is a stupid idea". Instead of simply putting someone's idea down educate them and lift them higher. – Aaron Greenlee Feb 20 '18 at 03:43

How to sort struct fields in alphabetical order

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