Functions and Methods

Methods

Methods attach functions to a type.

method receiver A method receiver appears before the method name and tells Go which type the method belongs to.

Methods

width
methods_intro.go
Replay: real traced execution (multi-file project)
package main

import "fmt"

type Rectangle struct {
	Width  int
	Height int
}

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

func main() {
	var width = 4
	shape := Rectangle{Width: width, Height: 3}
	area := shape.Area()

	fmt.Println("width=", shape.Width)
	fmt.Println("height=", shape.Height)
	fmt.Println("area=", area)
}

package main

import "fmt"

type Rectangle struct {
	Width  int
	Height int
}

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

func main() {
	var width = 2
	shape := Rectangle{Width: width, Height: 3}
	area := shape.Area()

	fmt.Println("width=", shape.Width)
	fmt.Println("height=", shape.Height)
	fmt.Println("area=", area)
}

package main

import "fmt"

type Rectangle struct {
	Width  int
	Height int
}

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

func main() {
	var width = 6
	shape := Rectangle{Width: width, Height: 3}
	area := shape.Area()

	fmt.Println("width=", shape.Width)
	fmt.Println("height=", shape.Height)
	fmt.Println("area=", area)
}

  1. width ← 4, shape ← main.Rectangle{Width:4, Height:3}

    14func main() {15  var width→ 4 = 4 //@width=2, 616  shape→ main.Rectangle{Width:4, Height:3} := Rectangle{Width: width4, Height: 3}17  area := shapemain.Rectangle{Width:4, Height:3}.Area()

  2. func (r Rectangle) Area() int

    10func (r Rectangle) Area() int {11  return r.Width4 * r.Height312}

  3. area ← 12

    16  shape := Rectangle{Width: width, Height: 3}17  area→ 12 := shapemain.Rectangle{Width:4, Height:3}.Area()1819  fmt.Println("width=", shape.Width4)20  fmt.Println("height=", shape.Height3)21  fmt.Println("area=", area12)22}
    outputwidth= 4
height= 3
area= 12

  1. width ← 2, shape ← main.Rectangle{Width:2, Height:3}

    14func main() {15  var width→ 2 = 216  shape→ main.Rectangle{Width:2, Height:3} := Rectangle{Width: width2, Height: 3}17  area := shapemain.Rectangle{Width:2, Height:3}.Area()

  2. func (r Rectangle) Area() int

    10func (r Rectangle) Area() int {11  return r.Width2 * r.Height312}

  3. area ← 6

    16  shape := Rectangle{Width: width, Height: 3}17  area→ 6 := shapemain.Rectangle{Width:2, Height:3}.Area()1819  fmt.Println("width=", shape.Width2)20  fmt.Println("height=", shape.Height3)21  fmt.Println("area=", area6)22}
    outputwidth= 2
height= 3
area= 6

  1. width ← 6, shape ← main.Rectangle{Width:6, Height:3}

    14func main() {15  var width→ 6 = 616  shape→ main.Rectangle{Width:6, Height:3} := Rectangle{Width: width6, Height: 3}17  area := shapemain.Rectangle{Width:6, Height:3}.Area()

  2. func (r Rectangle) Area() int

    10func (r Rectangle) Area() int {11  return r.Width6 * r.Height312}

  3. area ← 18

    16  shape := Rectangle{Width: width, Height: 3}17  area→ 18 := shapemain.Rectangle{Width:6, Height:3}.Area()1819  fmt.Println("width=", shape.Width6)20  fmt.Println("height=", shape.Height3)21  fmt.Println("area=", area18)22}
    outputwidth= 6
height= 3
area= 18

Follow the Receiver

  1. width starts as 4.
  2. shape stores Width: 4 and Height: 3.
  3. shape.Area() calls the method with shape as the receiver.
  4. The method returns 4 * 3.
  5. The program prints area= 12. | width | height | area | | --- | --- | --- | | 4 | 3 | 12 | | 2 | 3 | 6 | | 6 | 3 | 18 |

Exercise: methods_intro.go

Reproduce width= 4, height= 3, and area= 12, then use the pinned width variants 2 and 6 to predict area= 6 and area= 18.