A Tour of GO （3）--More Types

1. Pointers
   • *T is type: a pointer to T.
     var x *int
   • Operator & generate a pointer to its operand
     i :=42
     p =&i
   • The * operator denote the pointer's underlying value.
     i = 43
     fmt. Println(i)
2. Struct
   type Vertex struct{
   X int
   Y int
   }

• Fields are accessed using dot.
• Struct fields can be accessed through pointers and dot.
  v := Vertext {1,2}
  p = &v
  p.X = 10
• Struct literals denote the fields of the struct by listing. You can specify only part of them using Name: syntax.
  v1 = Vertex{1, 2} // has type Vertex
  v2 = Vertex{X: 1} // Y:0 is implicit
  v3 = Vertex{} // X:0 and Y:0
  p = &Vertex{1, 2} // has type *Vertex

3. Array
   The type [n]T is an array of n values of type T

• The size of an array is part of its type. So it cannot be resized *

• Slice: a slice points to an array of values and also include a length.

  • []T is a slice of type T

  • len(s) return the length of slice s.

  • Slice can be nested( slice of slices).

  • Slicing slices: Slices can be re-sliced, creating a new slice value that points to the same array.
    s[lo:hi] evaluate slice from lo to hi-1 inclusively.

  • Make slices:
    a := make([]int, 5) // len(a)=5
    b := make([]int, 0, 5) // len(b)=0, cap(b)=5

  • Zero of slice is nil

  • Adding elements to slice.
    func append(s []T, vs ...T) []T

  • Range is a form of for loop. Two values are used. The first one is the index and the second is the value in the array.

     var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
    
     func main() {
        for i, v := range pow {
           fmt.Printf("2**%d = %d\n", i, v)
        }
     }
    

    You can skip the index or value by assigning to _.
    pow := make([]int, 10)
    for i := range pow {
    pow[i] = 1 << uint(i)
    }
    for _, value := range pow {
    fmt.Printf("%d\n", value)
    }

4. Map

• Map must be created with make before used
• var m = map[sting] Vertex key is of type string and value is of type Vertex.
• Mutating maps:
  • Insert
    m[key]=elem
  • Retrieve
    elem= m[key]
  • Delete
    delete (m , key)
  • Test existence
    elem, ok = m[key]
    if the key is in m, ok is true. IF key is not in m, elem is the zero value of that type.

5. Function values
   Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is "bound" to the variables.

func adder() func(int) int {
  sum := 0
  return func(x int) int {
    sum += x  
    return sum
  }
}

func main() {
  pos, neg := adder(), adder()
  for i := 0; i < 10; i++ {
    fmt.Println(
        pos(i),
        neg(-2*i),
    )   
  }
}