Challenge 2 — Solution

Task: Using the Shape interface, Circle, and Rectangle from this
chapter, create a slice []Shape containing at least 3 shapes (mixing
circles and rectangles), then loop over it printing each one's area.
Add up all the areas into a single total, printed at the end.

package main

import "fmt"

type Shape interface {
	Area() float64
}

type Circle struct {
	Radius float64
}

func (c Circle) Area() float64 {
	return 3.14159 * c.Radius * c.Radius
}

type Rectangle struct {
	Width, Height float64
}

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

func main() {
	shapes := []Shape{
		Circle{Radius: 3},
		Rectangle{Width: 4, Height: 5},
		Circle{Radius: 1.5},
	}

	total := 0.0
	for _, shape := range shapes {
		area := shape.Area()
		fmt.Printf("%.2f\n", area)
		total += area
	}

	fmt.Printf("Total: %.2f\n", total)
}


Expected output:

28.27
20.00
7.07
Total: 55.34

Notes:
- shapes mixes two different concrete types (Circle and Rectangle)
  in a single []Shape slice — this is only possible because both
  satisfy the same interface; a plain []Circle slice could never
  hold a Rectangle.
- shape.Area() inside the loop calls the correct version automatically
  depending on which concrete type is currently stored in shape — Go
  resolves this at runtime without any if/switch on type needed.
- total accumulates exactly like the running totals from earlier
  Fundamentals chapters — there's nothing interface-specific about
  the summing itself.