grammer/

README

Basic Grammer

• Basic Grammer
  • Variable
  • Control Structures
    • If structure
    • Switch Structure
    • For Structure
  • Data Structure
    • Array
    • Slice
    • Map
  • Struct
  • Iteration
    • transverse a slice
    • transverse a map
    • transverse a struct
  • Function
    • pass by value
    • pass by reference
  • Errors
  • String
    • Format
    • String Handling
    • Strconv
  • JSON
  • Time
  • Processes

Variable

1. := and const can automatically infer the variable type, var is typically used for variables whose type is inconsistent with the initialization expression.

num := 1
const pi = 3.14
var a string

1. Scientific notation is supported. eg, const i = 3e20 / h

Control Structures

If structure

1. bool expression does not need (), but content needs {}
2. Variable can be declared just behind if.

if v := math.Pow(x,y); v < limit {
    return v
}

Switch Structure

1. switch - case is essentially a sequence of if - else statements, that is, case can be used without constants. break is not needed.

fmt.Println("When's Saturday?")
today := time.Now().Weekday()
switch time.Saturday {
case today + 0:
    fmt.Println("Today.")
case today + 1:
    fmt.Println("Tomorrow.")
case today + 2:
    fmt.Println("In two days.")
case today + 3:
    fmt.Println("In three days.")
case today + 4:
    fmt.Println("In four days.")
default:
    fmt.Println("Too far away.")
}

1. Can be used to beautify an if - else sequence. Variable can be declared just behind switch.

switch t := time.Now() {
case t.Hour() < 12:
	fmt.Println("morning")
case t.Hour() < 18:
	fmt.Println("afternoon")
default:
	fmt.Println("evening")
}

For Structure

1. Dead loop

for {	}

1. Can be used as while loop in C/C++

i := 1
for i <= 3 {
	fmt.Println(i)
	i = i + 1
}

Data Structure

Array

1. Declaration

// without initialization
var a [5]int

// with initialization
b := [5]int{1, 2, 3, 4, 5}

Other operations are very similar with those in C/C++.

Slice

1. Slice itself does not store the data. It just like reference of variable-length array in C/C++.Changing a value in a slice will change the data it 'points' the array, so other slices 'point' the array(eg, copy) will also change.
2. Declaration

// part of array
a := [5]int{1, 2, 3, 4, 5}
var b []int = a[2:4]

// use make
s := make([]string, 3)

// [](type) without a number in []
good := []string{"g", "o", "o", "d"}

1. append operation needs to be assigned to the original slice.

s = append(s, "good")

1. Initialization

board := [][]int{
	[]int{1, 0, 1},
	[]int{0, 1, 1},
	[]int{1, 1, 0},
}

Map

1. Declaration

m := make(map[key]value)

1. Initialization

var m = map[string]int{
	"four": 4,
	"five": 5,
}

1. CRUD

// create & update
m[key] = value

// read : ok == true if (key, value) in m else false
value, ok := m[key]

// delete
delete(m, key)

Struct

1. Declaration

type user struct {
	name string
	id   int
}

1. Initialization

var user1 = user{}   // name : empty, id : 0
var user2 = user{name: "lulei"} // name : "lulei", id : 0
var user3 = user{"lulei", 1}   // name : "lulei", id : 1

1. Access Member We can access struct's member by using .

var user4 = user{}
user4.id = 2     // name : empty, id : 2

1. Method

func (u user) checkId(id int) bool {
	return u.id == id
}

It can be written as a function.

func checkIdFunc(u *user, id int) bool {
	return u.id == id
}

And just like function, method can also pass by reference.

Iteration

transverse a slice

nums := []int{2, 3, 4}
for index, value := range nums {
	fmt.Println(index, value)
}

The order of output element traversed is determined by its index.

transverse a map

// iterate hole map
for k, v := range m {
	fmt.Println(k, v)
}

// iterate only keys
for k := range m {
	fmt.Println("key", k)
}

// iterate only values
for _, v := range m {
	fmt.Println("value", v)
}

The storage location of the data in the map is random, so a map can NOT be expected to return results in some desired order when traversed.

transverse a struct

// Firstly, use reflect.ValueOf() to get the reflection instance
value := reflect.ValueOf(user3)

// Secondly, traverse through NumField
for i := 0; i < value.NumField(); i++ {
    fmt.Println(i, value.Field(i))  // Thirdly, obtain the field
}

Function

pass by value

Function in GO is pass by value by default

func function_name(variable variable_type) return_type {
    return return_value
}

A function can return multiple values.

func exist(m map[string]int, k string) (v int, err bool) {
    v, err = m[k]
    return v, err
}

pass by reference

Pointers are needed to edit parameters of the function

func increase(a int) {
    a += 1
}

func increase2(a *int) {
    *a += 1
}

num := 1
increase(num)    // pass by value
fmt.Println(num) // 1

increase2(&num)  // pass by reference
fmt.Println(num) // 2

Errors

Usually, we will return a boolean value error alone with return values. nil for no error.
For example, when searching for an element in an array, the return value should contain the found element and error. When the element is found, error should be nil otherwise it should remind the operator of an error.

func search(users []user, name string) (u *user, err error) {
	for _, u := range users {
		if u.name == name {
			return &u, nil
		}
	}
	return nil, errors.New("No such user")
}

When calling a function, we should first check whether the returned error reports an exception.

if err != nil {
	panic(err)
} else {
	fmt.Println(u.id)
}

String

Format

format	meaning
%v	return native value
%+v	expand struct's names and values
%#v	value in syntax format
%b	binary value
%f	float number
%.2f	float number with 2 decimal places

String Handling

method	meaning
Join	Concatenate string arrays (or slices)
Split	Separate a string according to certain rules
Count	Counts the number of non-overlapping occurrences of a substring in a string
Fields	Separate strings with one or more consecutive spaces
HasPrefix/HasSuffix	Check if a string has a certain prefix or suffix
IndexFunc	Find index of a character or substring in a string
ToLower/ToUpper	Case conversion

For more functions like io operations, please refer to https://books.studygolang.com/The-Golang-Standard-Library-by-Example/chapter02/02.1.html

Strconv

Strconv can realize the conversion between string and numeric types

method	meaning
ParseInt	Convert a string to a number, can set base and bitsize
ParseBool	Convert a string to a boolean
ParseFloat	Cconvert a 'float' string to a float number
ParseUnit	Similar to the ParseInt(), but does not accept +/- signs
Atoi	Convert an integer of type string to type int

JSON

Time

Time is a kind of data type.

method	meaning
Now	Current time
Date	Create a Time
Sub	Time difference
Parse	Parse a time string to a Time
Unix	Convert to unix timestamp
Round	Get the Time on the hour/minute/...

For Timer functions, please refer to https://books.studygolang.com/The-Golang-Standard-Library-by-Example/chapter04/04.4.html

Processes