[Go] Basic grammar ③ Pointer

I've finished watching the "Pointer" edition of the udemy course, so I'll summarize and look back.

This time as well, I learned the basic operation while outputting on the console.

Premise

・ Learn Go by watching [Introduction to Go taught by active Silicon Valley engineers + Development of Bitcoin Systre Fintech app by application]](https://www.udemy.com/course/go-fintech/) on udemy During ~

• Since it is said that "a few lines of code can be introduced on blogs, etc.", the code used in the lecture is quoted.

・ Development environment is VS Code + plug-in (Go version is 1.15) ・ OS is Windows ・ I usually use Java.

Pointer

-If you specify a pointer type variable, you can directly overwrite the contents of the memory. ・ If it is difficult to understand, add "&" and "*" and move it appropriately to understand. -When generating a pointer type, use "new" instead of "make".

python

func one(x int) {
    //Receive only values from the main function
	fmt.Println(&x)
	x = 1
}

func two(x *int) {
    //Receives the address of x in a pointer-type box.
	fmt.Println(&x)
	//Overwrite the contents of the address directly
	*x = 2
}
func main() {
	var n int = 100

    //Memory address is output
	fmt.Println(&n)

	//Integer point type address is entered (same as C language)
	var p *int = &n
	fmt.Println(p)

	//When you want to see the contents of the memory pointed to by the address
	fmt.Println(*p)

	one(n)
	//Since n in the argument of one is a copy, it has no effect on n in the function.
	fmt.Println(n) //100 is output

	two(&n)
	fmt.Println(n)  //2 is output

}

I couldn't understand this "pointer", but the article "Pointers and addresses learned with Go" and the links quoted in the articles. The explanation above was easy to understand. Since it is a concept derived from C language, it seems that there is knowledge that many seniors have struggled to accumulate.

struct

-A structure can be defined with " struct". -To hold a value, it must be defined as a pointer type.

python

type Vertex struct {
	//Variable starts with a capital letter
	//Treated as private when made lowercase
	X int
	Y int
	S string
}

func changeVertex(v Vertex) {
	v.X = 1000
}

func changeVertex2(v *Vertex) {
	v.X = 1000
	(*v).X = 1000 //This is the correct way to write it, but in the case of struct, it automatically refers to the actual pointer.
}

func prg3() {
    //Structure generation
	v := Vertex{X: 1, Y: 2}  //Initial values are entered for variables that are not defined
	changeVertex(v)          //Pass the structure by value (copy)
	fmt.Println(v)           //Output result:{1 2 }
	v.X = 100                //Variables can also be specified
	fmt.Println(v.X, v.Y)    //Output result: 100 2

	v2 := &Vertex{1, 2, "text"}
	changeVertex2(v2)
	fmt.Println(v2)          //Output result:&{1000 2 text}

	var v3 Vertex
	fmt.Println(v3)          //Note that it is not nil. The one with the initial value is constructed
                             //Output result:{0 0 }
	v4 := new(Vertex)
	fmt.Println(v4)          //Pointer type output result:&{0 0 }

	v5 := &Vertex{}
	fmt.Println(v5)          //It has the same contents as the pointer type v6, but it is easier to distinguish it from the pointer type.
}

Impressions

・ It is difficult to understand the concept of pointers. -The struct feels like a java object.

Last post

・ Basic Grammar ① Definition

・ Basic Grammar ② Statement