Assign new values later to the underlying object of the interface in go

I am trying to assign new values to the underlying structure of an interface in code below. But it keeps the older values. Below is the example code.

package main

import (
	"fmt"
	"math"
)

type Shape interface {
	Area() float64
}

type Circle struct {
	Radius float64
	Name   string
}

func (c Circle) Area() float64 {
	return math.Pi * c.Radius * c.Radius
}

type Rectangle struct {
	Length float64
	Width  float64
	Name   string
}

func (r Rectangle) Area() float64 {
	return r.Length * r.Width
}

func assignRadius(s Shape, radius float64, name string) {
	switch s := s.(type) {
	case Circle:
		s.Radius = radius
		s.Name = name
	case Rectangle:
		s.Length = radius
		s.Name = name
	}
}

func main() {
	var s Shape
	c := Circle{Radius: 0, Name: "My Circle"}
	s = c
	fmt.Println(s.Area())
	fmt.Println(c.Radius)
	fmt.Println(c.Name)
	assignRadius(s, 10, "My New Circle")
	fmt.Println(c.Radius)
	fmt.Println(c.Name)
}

The type of Shape isn't known beforehand in the assignRadius. I know it has something to do with the pointers. But can't figure it out.

The interface variable s contains a copy of the shape value. To modify it like you attempted to do, it has to contain a pointer to that shape:

var s Shape
c := Circle{Radius: 0, Name: "My Circle"}
s = &c

and in the function modifying them, you have to type-assert the pointer value:

func assignRadius(s Shape, radius float64, name string) {
    switch s := s.(type) {
    case *Circle:
        s.Radius = radius
        s.Name = name
    case *Rectangle:
        s.Length = radius
        s.Name = name
    }




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