How to create a Golang struct instance from an array? - go

Question

Let's say I have a bunch of array of strings, for example:
data := [4]string{"a", "3.0", "2.5", "10.7"}
And a struct definition:
type Record struct {
name string
x float64
y float64
mag float64
}
I'd like to create an instance of this struct from each array.
I need to match the first item of the array to the first field of the struct and so on. Is it possible to do this?
Each array corresponds to one line of a file, so I can actually decide how to read these values in case a different approach is better.

Answer 1

An easy way is to use reflection to iterate over the fields of the struct, obtain their address (pointer), and use fmt.Sscan() to scan the string value into the field. fmt.Sscan() will handle the different types of fields for you. This is in no way an efficient solution, it is just a short, easy and flexible solution. If you need an efficient solution, you have to handle all fields explicitly, manually.
This only works if the fields of the struct are exported, e.g.:
type Record struct {
Name string
X float64
Y float64
Mag float64
Age int
}
The function that loads a string slice into a struct value:
func assign(recordPtr interface{}, data []string) error {
v := reflect.ValueOf(recordPtr).Elem()
max := v.NumField()
if max > len(data) {
max = len(data)
}
for i := 0; i < max; i++ {
if _, err := fmt.Sscan(data[i], v.Field(i).Addr().Interface()); err != nil {
return err
}
}
return nil
}
Note that this implementation tries to fill as many fields as possible (e.g. it does not return an error if the struct has more or less fields than input data provided). Also note that this assign() function can fill any other structs, not just Record, that's why it's flexible.
Example testing it:
data := []string{"a", "3.0", "2.5", "10.7", "23"}
var r Record
if err := assign(&r, data); err != nil {
fmt.Println("error:", err)
}
fmt.Printf("%+v", r)
Output (try it on the Go Playground):
{Name:a X:3 Y:2.5 Mag:10.7 Age:23}

Answer 2

There is no simple way to do this. You have to assign struct members one by one.
for _, x := range data {
x, err := strconv.ParseFloat(x[1])
y, err := strconv.ParseFloat(x[2])
max, err := strconv.ParseFloat(x[3])
strData = append(strData, Record{name: x[0], x: x, y: y, mag: mag})
}
You also have to deal with parse errors.

Answer 3

Of course it it possible to do that in Go.
The following code example will assign to s the record filled with the fields of data.
package main
import (
"fmt"
"strconv"
)
type Record struct {
name string
x float64
y float64
mag float64
}
func main() {
data := [4]string{"a", "3.0", "2.5", "10.7"}
x, err := strconv.ParseFloat(data[1], 64)
if err != nil {
panic(err)
}
y, err := strconv.ParseFloat(data[2], 64)
if err != nil {
panic(err)
}
mag, err := strconv.ParseFloat(data[3], 64)
if err != nil {
panic(err)
}
s := Record{ name: data[0], x: x, y: y, mag: mag}
fmt.Println(s)
}