02.2.md

#2.2 Go foundation In this section, we are going to teach you how to define constants, variables with elementary types and some skills in Go programming.

##Define variables There are many forms of syntax that can define variables in Go.

Use keyword var is the basic form to define variables, notice that Go puts variable type after variable name.

// define a variable with name “variableName” and type "type"
var variableName type

Define multiple variables.

// define three variables which types are "type"
var vname1, vname2, vname3 type

Define a variable with initial value.

// define a variable with name “variableName”, type "type" and value "value"
var variableName type = value

Define multiple variables with initial values.

/*
Define three variables with type "type", and initialize their values.
vname1 is v1, vname2 is v2, vname3 is v3
*/
var vname1, vname2, vname3 type= v1, v2, v3

Do you think it's too tedious to define variables use the way above? Don't worry because Go team found this problem as well. Therefore if you want to define variables with initial values, we can just omit variable type, so the code will look like this:

/*
Define three variables with type "type", and initialize their values.
vname1 is v1，vname2 is v2，vname3 is v3
*/
var vname1, vname2, vname3 = v1, v2, v3

Well, I know this is still not simple enough for you, so do I. Let's see how we fix it.

/*
Define three variables with type "type", and initialize their values.
vname1 is v1，vname2 is v2，vname3 is v3
*/
vname1, vname2, vname3 := v1, v2, v3

Now it looks much better. Use := to replace var and type, this is called brief statement. But wait, it has one limitation that this form can only be used inside of functions. You will get compile errors if you try to use it outside of function bodies. Therefore, we usually use var to define global variables, and we can use this brief statement in var().

_ (blank) is a special name of variable, any value that is given to it will be ignored. For example, we give 35 to b, and discard 34.( This example just show you how it works. It looks useless here because we often use this symbol when we get function return values. )

_, b := 34, 35

If you don't use any variable that you defined in the program, compiler will give you compile errors. Try to compile following code, see what happens.

package main

func main() {
    var i int
}

##Constants So-called constants are the values that are determined in the compile time, and you cannot change them during runtime. In Go, you can use number, boolean or string as type of constants.

Define constants as follows.

const constantName = value
// you can assign type of constants if it's necessary 
const Pi float32 = 3.1415926

More examples.

const Pi = 3.1415926
const i = 10000
const MaxThread = 10
const prefix = "astaxie_"

##Elementary types ###Boolean In Go, we use bool to define a variable as boolean type, the value can only be true or false, and false will be the default value. ( You cannot convert variables' type between number and boolean! )

// sample code
var isActive bool  // global variable
var enabled, disabled = true, false  // omit type of variables
func test() {
	var available bool  // local variable
	valid := false      // brief statement of variable
	available = true    // assign value to variable
}

###Numerical types Integer types including signed and unsigned integer types. Go has int and uint at the same time, they have same length, but specific length depends on your operating system. They use 32-bit in 32-bit operating systems, and 64-bit in 64-bit operating systems. Go also has types that have specific length including rune, int8, int16, int32, int64, byte, uint8, uint16, uint32, uint64. Note that rune is alias of int32 and byte is alias of uint8.

One important thing you should know that you cannot assign values between these types, this operation will cause compile errors.

var a int8

var b int32

c:=a + b

Although int has longer length than uint8, and has same length as int32, but you cannot assign values between them. ( c will be asserted as type int here )

Float types have float32 and float64, and no type called float, latter one is default type if you use brief statement.

That's all? No! Go has complex number as well. complex128 (with a 64-bit real and 64-bit imaginary part)is default type, if you need smaller type, there is one called complex64 (with a 32-bit real and 32-bit imaginary part). Its form is RE+IMi, where RE is real part and IM is imaginary part, the last i is imaginary number. There is a example of complex number.

var c complex64 = 5+5i
//output: (5+5i)
fmt.Printf("Value is: %v", c)

###String We just talked about that Go uses UTF-8 character set. Strings are represented by double quotes "" or backtracks ``` `.

// sample code
var frenchHello string  // basic form to define string
var emptyString string = ""  // define a string with empty string
func test() {
	no, yes, maybe := "no", "yes", "maybe"  // brief statement
	japaneseHello := "Ohaiou"
	frenchHello = "Bonjour"  // basic form of assign values
}

It's impossible to change string values by index, you will get errors when you compile following code.

var s string = "hello"
s[0] = 'c'

What if I really want to change just one character in a string? Try following code.

s := "hello"
c := []byte(s)  // convert string to []byte type
c[0] = 'c'
s2 := string(c)  // convert back to string type
fmt.Printf("%s\n", s2)

You can use operator + to combine tow strings.

s := "hello,"
m := " world"
a := s + m
fmt.Printf("%s\n", a)

and also.

s := "hello"
s = "c" + s[1:] // you cannot change string values by index, but you can get values instead.
fmt.Printf("%s\n", s)

What if I want to have a multiple-line string?

m := `hello
world`

`` ` will not escape any characters in a string.

###Error types Go has one error type for purpose of dealing with error messages. There is also a package called errors to handle errors.

err := errors.New("emit macho dwarf: elf header corrupted")
if err != nil {
	fmt.Print(err)
}

###Underlying data structure The following picture comes from a article about Go data structure in Russ Cox Blog. As you can see, Go gives blocks in memory to store data.

Figure 2.1 Go underlying data structure

##Some skills ###Define by group If you want to define multiple constants, variables or import packages, you can use group form.

Basic form.

import "fmt"
import "os"

const i = 100
const pi = 3.1415
const prefix = "Go_"

var i int
var pi float32
var prefix string

Group form.

import(
	"fmt"
	"os"
)

const(
	i = 100
	pi = 3.1415
	prefix = "Go_"
)

var(
	i int
	pi float32
	prefix string
)

Unless you assign the value of constant is iota, the first value of constant in the group const() will be 0. If following constants don't assign values explicitly, their values will be the same as the last one. If the value of last constant is iota, the values of following constants which are not assigned are iota also.

###iota enumerate Go has one keyword iota, this keyword is to make enum, it begins with 0, increased by 1.

const(
	x = iota  // x == 0
	y = iota  // y == 1
	z = iota  // z == 2
	w  // If there is no expression after constants name, it uses the last expression, so here is saying w = iota implicitly. Therefore w == 3, and y and x both can omit "= iota" as well.
)

const v = iota // once iota meets keyword `const`, it resets to `0`, so v = 0.

const ( 
  e, f, g = iota, iota, iota // e=0,f=0,g=0 values of iota are same in one line.
)

###Some rules The reason that Go is concise because it has some default behaviors.

• Any variable starts with capital letter means it will be exported, private otherwise.
• Same rule for functions and constants, no public or private keyword exists in Go.

##array, slice, map ###array array is array obviously, we define them as follows.

var arr [n]type

in [n]type, n is the length of array, type is the type of its elements. Like other languages, we use [] to get or set element values in array.

var arr [10]int  // an array of type int
arr[0] = 42      // array is 0-based
arr[1] = 13      // assign value to element
fmt.Printf("The first element is %d\n", arr[0])  // get element value, it returns 42
fmt.Printf("The last element is %d\n", arr[9]) //it returns default value of 10th element in this array, which is 0 in this case.

Because length is a part of array type, [3]int and [4]int are different types, so we cannot change length of arrays. When you use arrays as arguments, functions get their copies instead of references! If you want to use reference, you may want to use slice which we will talk about latter.

It's possible to use := when you define arrays.

a := [3]int{1, 2, 3} // define a int array with 3 elements

b := [10]int{1, 2, 3} // define a int array with 10 elements, and first three are assigned, rest of them use default value 0.

c := [...]int{4, 5, 6} // use `…` replace with number of length, Go will calculate it for you.

You may want to use arrays as arrays' elements, let's see how to do it.

// define a two-dimensional array with 2 elements, and each element has 4 elements.
doubleArray := [2][4]int{[4]int{1, 2, 3, 4}, [4]int{5, 6, 7, 8}}

// You can write about declaration in a shorter way.
easyArray := [2][4]int{{1, 2, 3, 4}, {5, 6, 7, 8}}

Array underlying data structure.

Figure 2.2 Multidimensional array mapping relationship

###slice In many situations, array is not a good choice. For example, we don't know how long the array will be when we define it, so we need "dynamic array". This is called slice in Go.

slice is not really dynamic array, it's a reference type. slice points to an underlying array, its declaration is similar to array, but doesn't need length.

// just like to define array, but no length this time
var fslice []int

Then we define a slice, and initialize its data.

slice := []byte {'a', 'b', 'c', 'd'}

slice can redefine from exists slices or arrays. slice use array[i:j] to slice, where i is start index and j is end index, but notice that array[j] will not be sliced, now the length of slice is j-i.

// define a slice with 10 elements which types are byte
var ar = [10]byte {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'}

// define two slices with type []byte
var a, b []byte

// a points to elements from 3rd to 5th in array ar.
a = ar[2:5]
// now a has elements ar[2]、ar[3]和ar[4]

// b is another slice of array ar
b = ar[3:5]
// now b has elements ar[3]和ar[4]

Notice that differences between slice and array when you define them. We use […] let Go calculates length but use [] to define slice only.

Their underlying data structure.

Figure 2.3 Correspondence between slice and array

slice has some convenient operations.

• slice is 0-based, ar[:n] equals to ar[0:n]
• Second index will be the length of slice if you omit it, ar[n:] equals to ar[n:len(ar)].
• You can use ar[:] to slice whole array, reasons are explained in first two statements.

More examples about slice

// define an array
var array = [10]byte{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'}
// define two slices
var aSlice, bSlice []byte

// some convenient operations
aSlice = array[:3] // equals to aSlice = array[0:3] aSlice has elements a,b,c
aSlice = array[5:] // equals to aSlice = array[5:10] aSlice has elements f,g,h,i,j
aSlice = array[:]  // equals to aSlice = array[0:10] aSlice has all elements

// slice from slice
aSlice = array[3:7]  // aSlice has elements d,e,f,g，len=4，cap=7
bSlice = aSlice[1:3] // bSlice contains aSlice[1], aSlice[2], so it has elements e,f
bSlice = aSlice[:3]  // bSlice contains aSlice[0], aSlice[1], aSlice[2], so it has d,e,f
bSlice = aSlice[0:5] // slcie could be expanded in range of cap, now bSlice contains d,e,f,g,h
bSlice = aSlice[:]   // bSlice has same elements as aSlice does, which are d,e,f,g

slice is reference type, so one of them changes will affect others. For instance, aSlice and bSlice above, if you change value of element in aSlice, bSlice will be changed as well.

slice is like a struct by definition, it contains 3 parts.

• A pointer that points to where slice starts.

• length of slice.

• Capacity, the length from start index to end index of slice.

    Array_a := [10]byte{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'}
    Slice_a := Array_a[2:5]

Underlying data structure of code above as follows.

Figure 2.4 Array information of slice

There are some built-in functions for slice.

• len gets length of slice.
• cap gets maximum length of slice
• append appends one or more elements to slice, and returns slice .
• copy copies elements from one slice to the other, and returns number of elements were copied.

Attention: append will change array that slice points to, and affect other slices that point the same array. Also, if there is not enough length for the slice ((cap-len) == 0), append returns new array for this slice, at this point, other slices point to the old array will not be affected.

###map map is like dictionary in Python, use form map[keyType]valueType to define it.

Let's see some code, the set and get value in map is like slice, use key as agent, but index in slice can only be int type, and map can use much more than that, int, string, whatever you want. Also, they are all able to use == and != to compare values.

// use string as key type, int as value type, and you have to use `make` initialize it.
var numbers map[string] int
// another way to define map
numbers := make(map[string]int)
numbers["one"] = 1  // assign value by key
numbers["ten"] = 10 
numbers["three"] = 3

fmt.Println("The third number is: ", numbers["three"]) // get values
// It prints: The third number is: 3

map is like form in our lives, left side are keys, another side are values.

Some notes when you use map.

• map is disorderly, every time you print map will get different results. It's impossible to get value by index, you have to use key.
• map doesn't have fixed length, it's a reference type as slice does.
• len works for map also, it returns how many keys that map has.
• It's quite easy to change value through map, simply use numbers["one"]=11 to change value of key one to 11.

You can use form key:val to initialize map's values, and map has method inside to check if the key exists.

Use delete to delete element in map.

// Initialize a map
rating := map[string]float32 {"C":5, "Go":4.5, "Python":4.5, "C++":2 }
// map has two return values. For second value, if the key doesn't exist，ok is false，true otherwise.
csharpRating, ok := rating["C#"]
if ok {
	fmt.Println("C# is in the map and its rating is ", csharpRating)
} else {
fmt.Println("We have no rating associated with C# in the map")
}

delete(rating, "C")  // delete element with key "c"

As I said above, map is a reference type, if two maps point to same underlying data, any change will affect both of them.

m := make(map[string]string)
m["Hello"] = "Bonjour"
m1 := m
m1["Hello"] = "Salut"  // now the value of m["hello"] is Salut

###make, new make does memory allocation for built-in models, such as map, slice, and channel), new is for types' memory allocation.

new(T) allocates zero-value to type T's memory, returns its memory address, which is the value of type *T. By Go's term, it returns a pointer, which points to type T's zero-value.

new returns pointers.

Built-in function make(T, args) has different purposes from new(T), make can be used for slice, map, and channel, and returns a type T with initial value. The reason of doing this is because these three types' underlying data must be initialized before they point to them. For example, a slice contains a pointer points to underlying array, length and capacity. Before these data were initialized, slice is nil, so for slice, map, channel, make initializes their underlying data, and assigns some suitable values.

make returns non-zero values.

The following picture shows how new and make be different.

Figure 2.5 Underlying memory allocation of make and new

As for zero-value, it doesn't mean empty value. It's the value that variables are not assigned manually, usually is 0, there is list of some zero-values.

int     0
int8    0
int32   0
int64   0
uint    0x0
rune    0 // the actual type of rune is int32
byte    0x0 // the actual type of byte is uint8
float32 0 // length is 4 byte
float64 0 //length is 8 byte
bool    false
string  ""

##Links

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