Learning Go

Objects in Go

Go does not use the term class.
A struct is a collection of data.
Go uses structs with associated methods.
Go uses a simplified object orientation:
- no inheritance
- no constructors
- no generics

Concurrency

..is the management of multiple tasks that (could theoretically) run at the same time. Concurrent programming enables parallelism:

management of task execution
communication between tasks
synchronization between tasks Gorutines represent concurrent tasks Channels are used to communicate between tasks Select enables task synchronization

Pointer

.. is an address to data in memory
& operator returns the address of some data
- operator returns the data at some address

var x int = 1			//declaration and initialisation of an int
var y int 			//declaration of an int
var ip *int			//declaration of a pointer to an int

ip = &x 				//setting ip to the address of x > ip now points to x
y = *ip				//setting y to the data stored at the address ip > y now is x > y = 1
----------------------------------------------------
var ptr *int = new(int) //new() creates a var and returns pointer to that var, therfore *int
*ptr = 3
fmt.Println(*ptr)		//3
fmt.Println(ptr)		//some address

Variable Scope

A variable is accessible from block Bj if:

the variable is declared in Bi and
Bi >= Bj

Memory: Stack vs Heap

Stack: variables defined within a function - Memory will be deallocated when function call completed (in C for example) Heap: Global variables - Needs to be explicitly deallocated ( in C for example), is fast but error-prone

Type conversion

Use T() to convert, where T stands for the type you want to convert into

var x int32 = 1
var x int16 = 2
x = y //error
x = int32(y)

ASCII

Each char is associated with an 8-bit number. 8 bits —> 0-255, not enough for different alphabets and symbols

Unicode

Each char is associated with an 32-bit number. Unicode is variable in length. From 8-bits to 32-bits. 8-bit UTF codes are the same as ASCII

Strings

— a sequence of bytes, each byte is a “rune” (Go specific, = UTF-8 code point) Good packages: Unicode (for individual runes), String (for strings)

var hello = "hello"
var test = hello[0]
fmt.Println(test) //104
fmt.Println(string(test)) //h

Switch/case statement

If x is 1, enter case 1, if x is 2, enter case 2…….. Compared to C, no break statements are needed. Go will not fall through, it automatically breaks.

switch x {
case 1: 
	fmt.printf("case 1")
case 1: 
	fmt.printf("case 2")
default:
	fmt.printf("default case")

Exported names

When importing a package, you can refer only to its exported names. Any "unexported" names are not accessible from outside the package. A name is exported if it begins with a capital letter.

Variables

var i, j int = 1, 2
var i, j = 1, 2 //int not needed, var will take the type of the initialiser

var c, python, java = true, false, "no!"

Short variable declarations (only within functions)

func main() {
	var i, j int = 1, 2
	k := 3    					//within fucntions we can omit the var keyword and use := 
	c, python, java := true, false, "no!"

	fmt.Println(i, j, k, c, python, java)
}

Functions

Functions are first class - allows functions to be assigned to variables, passed as arguments to other functions and returned from other functions.

func add(x, y int) int {
	return x + y
} //adds x and y, returns integer

Function with multiple results

func swap(x, y string) (string, string) {
	return y, x
} // parameter x and y get swapped, two strings get returned

Named return values

func split(sum int) (x, y int) {
	x = sum * 4 / 9
	y = sum - x
	return
} //a return statement without arguments, returns the named return values, here (x, y int)

Variables as Functions

Declare a variable as func

var funcVar func(int) int 
func incFn(x int) int {
    return x + 1
}
func main() {
    funcVar = incFn // function on RHS without ()
    fmt.Print(funcVar(1))
}

Functions as Arguments

Functions can be passes to another functions as arguments

func applyIt(afunct func (int) int, 
             val int) int {
    return afunct(val)
}
func incFn(x int) int {
    return x + 1
} 
func main() {
    fmt.Print(applyIt(incFn, 2))
}

Anonymous Functions

Don't need to name a function

func applyIt(afunct func (int) int, 
             val int) int {
    return afunct(val)
}

func main() {
    v := applyIt(func (x int) int{return x + 1}, 2)
    fmt.Print(v)
}

Returning Functions and Closures

Go supports anonymous functions, which can form closures

package main

import "fmt"

func intSeq() func() int {
    i := 0
    return func() int { // returns anonymous func
        i++             // it closes over variable i 
        return i		// to form closure
    }
}

func main() {
    nextInt := intSeq() // function value captures its own i value, which will be updated each time we call nextInt

    fmt.Println(nextInt())
    fmt.Println(nextInt())
    fmt.Println(nextInt())

    newInts := intSeq()
    fmt.Println(newInts())
}

Variadic and Deferred Functions

Variadic means function can take any number of arguments

Variable Argument Number

Functions can take a variable no. of arguments
Uses ellipses **... **to specify
Treated as a slice inside function

func getMax(vals, ...int) int {
    maxV := -1
    for _, v := range vals {
        if v > maxV {
            maxV = V
        }
    }
    return maxV
}

Variadic Slice Argument

can pass slice to a variadic function
Need the ... suffix

func main() {
    fmt.Println(getMax(1,3,6,4))
    vslice := []int{1,3,6,4}
    fmt.Println(getMax(vslice...))
}

Deferred Function Calls
- Call can be deferred until the surrounding function completes
- Typically used for clean up activities
- ```
func main() {
    i := 1
    defer fmt.Println(i+1)
    i++ 
    fmt.Println("Hello")
}
```

Arrays

fixed length in Go
Elements are accessed via [ ]
Index starts at 0
Elements initialized to zero value by default

var x [5]int		 //declares array with 5 zeros of int type
x[0] = 2 			
________________________
//Array literals - An array pre-defined with values
var x [5] int = [5]{1,2,3,4,5}
x := [...]{1,2,3,4} 					//... is subsituted with  the length of the literal
___________________________
//Iterating through arry
for i, v range x {
	fmt.Printf("index %d, value %d", i, v) 
//range returns two values, the current index and the value at the index

Slices

A window on an underlying array, contain a pointer to array
The pointer indicates the start of the array
The length is the number of elements in the array
- length of each slice is difference between it's declared indices
The capacity is the max number of elements (from start of slice to end of array)
- capacities are difference between length of underlying array and starting index of slice.

array := [...]int{1,2,3,4,5}
slice1 := array[1:3] 				//2,3
slice2 := array[2:5] 				//3,4,5
fmt.Printf(cap(slice2)) 			// 3, because starting from index 2, the slice can have 3 elements 										at most

Length and Capacity

len() function returns length

cap() function returns capacity

a1 := [3]string{"a","b","c"}
sli1 := a1[0:1]
fmt.Printf(len(sli1),cap(sli1)) // 1 3

changing a slice changes the underlying array
overlapping arrays refer to the same elements, changing it in one slice, also changes it for the other element

Slice literal

creates the underlying array and then the slice that refers to it
that means the slice and the array are the same.
slice points to the start of the array, length equal to capacity

//Sliche literal
sli := []{1,2,3} 			//slices don't need ... in the [ ]

Variable slices

you can also create a slice by using make()
make either takes two to three arguments:
Two: type and length sli = make([]int, 10) // then length and capacity are equal
Three arguments : : type, length and capacity sli = make([]int, 10,15) // underlying array len 15 and slice len 10
Append() lets you append elements. If there is space left in the underlying array, it will use that until it is full. Once full and you keep appending, it copies the content and creates a new, bigger underlying array

 sli = make([]int, 0, 3) // len of sli is 0
 sli = append(sli, 100)
 fmt.Println(len(s), cap(s)) // 1 3

Hash table

contains key value pairs
each value is associated with a unique key
hash function is used to compute the slot for the key in the hash table. Input=key, output=slot to put value

Maps (Go’s implementation of hash tables)

var idMap map[string]int // key type string value type int
idMap = make(map[string]int)
_____________________________
//Map literal
idMap := map[string]int {
	"joe": 23}

_____________________________
//Accessing maps
fmt.Printf(idMap["joe"]) 				//prints 23

_____________________________
//Adding a key value pair
idMap["Jane"] = 34	//no key "Jane" so the key/val pair gets added

______________________________
//Editing a key value pair
idMap["Jane"] = 98					//overrides the old key/val pair

____________________________
//Deleting a key/val pair
delete(idMap, "joe"

_____________________________
//Two value assignment to test whether key is in map
id, b := idMap["joe"]					//id=23, b=true, because joe is a key in the map

_____________________________	
//Number of key/val pairs
len(idMap)

_____________________________
//Iterating through map
for key, val := range idMap {
	fmt.Prinf(key, val) 
}

Struct

Aggregate data type
Groups together objects of arbitrary types

type struct Person {
	name string
	addr string
	phone string
}
var p1 Person
p2 := new(Person) //creates new Person struct with all fields initialized to defaults 0 ir "" 
p1 := Person(name: "joe", addr: "a str", phone: "0123")			//struct literal
p1.name = "joe"

Blank identifier

If an assignment requires multiple values on the left side, but one of the values will not be used by the program, a blank identifier on the left-hand-side of the assignment avoids the need to create a dummy variable and makes it clear that the value is to be discarded.

if _, err := os.Stat(path); os.IsNotExist(err) {
	fmt.Printf("%s does not exist\n", path)
}

JSON

Format to represent structured Data
Consists of Attribute-Value pairs
All Unicode
Types can be combined recursively

//Go struct
type struct Person {
    name string
    addr string
    phone string
}

p1 := Person(name: "joh", age:24, phone:"039203")

//JSON
{"name": "joh", "age":23, "phone":"0320323"}

//Marshal creates JSON byte array from Go struct
barr, err := json.Marshal(p1)

//Unmarshal converts JSON []byte array into a Go struct
var p2 Person
err := json.Unmarshall(barr, &p2)

//the Go struct needs to have the same field names as the JSON object has attributes
//if Unmarshal worked, err is nil

File Access

Basic operations:
- Open - get handle for access
- Read - read bytes into []byte
- Write - write []bytes into file
- Close - release handle
- Seek - move read/write head
Package “io/ioutil”

dat, err := ioutil.ReadFile("test.txt")
// dat is []byte filled with contents of entire file
//no open/close needed when using ReadFile
//don't use ReadFile with large files

dat = "Hello world"
err := ioutul.WriteFile("outfile.txt", dat, 0777)
// writes []byte to file, creates new file, unix style permission

OS Package

os.Open() -returns file descriptor
os.Close() - close a file
os.Read() - reads from file into []byte, you can control how big []byte is
os.Write() - writes []byte into file

f, err := os.Open("test.txt)
barr := make([]byte, 10)
number, err := f.read(barr) 
f.close()
//fills barr with first 10 bytes from file, calling it the next time it will read the next 10 bytes --> head moves
//number = number of bytes read

Methods

There are two types of methods :
1. Value receivers : Methods that just do calculations on value, receive value is all they can do
2. Pointer receivers : To change a value in struct we need a pointer receiver. You may want to use a pointer receiver type to avoid copying on method calls or to allow the method to mutate the receiving struct
  - No need to de-reference pointer with a *

package main

import "fmt"

type rect struct {
    width, height int
}

func (r rect) GetWidth() int {
    return r.width
}

func (r *rect) SetWidth(newWidth int) {
   r.width = newWidth // modifies struct
}

func main() {
    r := rect{width: 10, height: 5}

    fmt.Println("Width: ", r.GetWidth())
    r.SetWidth(110)	
    fmt.Println("Width: ", r.GetWidth())
}

Encapsulation

A package is the smallest unit of private encapsulation in Go.
All identifiers defined within a package are visible throughout that package.
When importing a package you can access only its exported identifiers.

An identifier is exported if it begins with a capital letter.

Example

In this package, the only exported identifiers are StopWatch and Start.

package timer

import "time"

// A StopWatch is a simple clock utility.
// Its zero value is an idle clock with 0 total time.
type StopWatch struct {
    start   time.Time
    total   time.Duration
    running bool
}

// Start turns the clock on.
func (s *StopWatch) Start() {
    if !s.running {
        s.start = time.Now()
        s.running = true
    }
}

The StopWatch and its exported methods can be imported and used in a different package.

package main

import "timer"

func main() {
    clock := new(timer.StopWatch)
    clock.Start()
    if clock.running { // ILLEGAL
        // …
    }
}

source: yourbasic.org/golang/public-private

Interfaces

Go doesn’t have inheritance – instead composition, embedding and interfaces support code reuse and polymorphism
Interfaces is : Set of method signatures
- name parameters, return value
- Implementation is NOT defined
Used to express conceptual similarity between types
interfaces take care of polymorphism and dynamic dispatch.
reference : https://gobyexample.com/interfaces
Type satisfies an interface if type defines all methods specified in the interface.
- same method signatures
Rectangle and Triangle types satisfy the Shape2D interface
- Must have Area() and Perimeter() methods
- Additional methods are OK
- Similar to inheritance and overriding
```
type Shape2D interface {
    Area() float64
    Perimeter() float64
}
type Triangle {...}
func (t Triangle) Area() float64 {...}
func (t Triangle) Perimeter() float64 {...}
```
Triangle type satisfies the Shape2D interface, no need to state it explicitly.

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