Note: You can use these notes instead, but it's nice to have everything in one page:

• https://github.com/parsiya/Hacking-with-Go
• Gophercises - Lessons Learned

These are my notes when learning go from the Tour of Go and some other sources. A lot of copy/pasted from them and I have used double-quotes to indicate those are not written by me to the best of my ability. This is not something original and is mostly here as a quick lookup reference while learning Go. I will update this as I learn more.

• Packages, variables, and functions
  • Exported names
  • Functions
  • Multiple results
  • Named return values
  • Variables
    • Initialize:
    • Short variable declarations
  • Basic types
  • Casting
  • Constants
• Flow control statements: for, if, else, switch and defer
  • For
  • if
  • else
  • switch
  • defer
• More types: structs, slices, and maps
  • Pointers
  • Structs
  • Arrays
  • Slices
    • Slice length and capacity
    • make slices
    • Range
• Methods and interfaces
  • Methods
    • Pointer Receivers
  • Interfaces
    • Type switch
    • Stringers
  • Solution to the Stringers exercise
  • Errors
  • Solution to the Errors exercise
• Concurrency
  • Channels
    • Buffered channels
    • Closing channels
    • Reading information from channels
    • select
  • sync.Mutex
• Printf from Go by example
  • Switches Verbs
    • Decimals
    • Floats
    • Value
    • Strings
    • Others
• Maps
• Other tips and tricks
  • Errors
  • Hexdump
  • Named imports
  • Importing a package into the current namespace
  • Avoiding the damn unused warnings
  • Unix Timestamp to String
  • Spawn a new thread (goroutine) on the spot
  • Write to a file or io.buffer from goroutines - DON'T
  • Generate godoc HTML for a Single Package
• Stuff learned from Cryptopals
  • Long string on multiple lines
  • Compare two []bytes with bytes.equal
  • Sorting a array/slice of struct by field
  • Append two slices

Packages, variables, and functions

Exported names

In Go, a name is exported if it begins with a capital letter.

When importing a package, you can refer only to its exported names. Any unexported names are not accessible from outside the package.

Functions

Unlike C, type comes after variable name except pointers.

Last int is function return type (obviously).

func add(x int, y int) int {
	return x + y
}

Then use it normally

Multiple results

A function can return any number of results. Gone are the days when we had to use pointers in function parameters as extra return values.

package main

import "fmt"

func addTwo1(x int, y int) (int, int) {
	return x+2, y+2
}

func main() {
	fmt.Println(addTwo1(10,20))
}

Named return values

Go's return values may be named. If so, they are treated as variables defined at the top of the function.

A return statement without arguments returns the named return values. This is known as a "naked" return. Don't use them.

package main

import "fmt"

func addTwo2(x int, y int) (xPlusTwo int, yPlusTwo int) {
	xPlusTwo = x + 2
	yPlusTwo = y + 2

	// Could do naked return too
	return xPlusTwo, yPlusTwo
}

func main() {
	fmt.Println(addTwo2(20,30))
}

Variables

Use var.

var x int

Can be combined:

var x,y int == var x int, y int. Similar to C when we had int x,y;.

Initialize:

var a, b int = 10, 20 == var a int = 10 and var b int = 20.

If initializer is there then the type can be omitted like scripting languages (e.g. Python):

var sampleInt, sampleBoolean, sampleString = 30, true, "Hello"

package main

import "fmt"

func main() {
	var a, b int = 10, 20
	var sampleInt, sampleBoolean, sampleString = 30, true, "Hello"

	fmt.Println(a, b , sampleInt, sampleBoolean, sampleString)
}

If no initial value is assigned to a declared variable, it will get a zero value:

• 0 for numeric types (int, float, etc.)
• false for the boolean type
• "" (the empty string) for strings

Short variable declarations

Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.

Outside a function, every statement begins with a keyword (var, func, and so on) and so the := construct is not available.

package main

import "fmt"

func main() {
	sampleInt, sampleBoolean, sampleString := 30, true, "Hello"

	fmt.Println(sampleInt, sampleBoolean, sampleString)
}

We can also put var statements in different lines (increases readability):

var (
	sampleInt			= 30
	sampleBoolean		= true
	sampleString 		= "Hello"
)

Basic types

bool

string

int  int8  int16  int32  int64	// use int unless you want a specific size
uint uint8 uint16 uint32 uint64 uintptr	// same here, use uint

byte // alias for uint8

rune // alias for int32
     // represents a Unicode code point

float32 float64

complex64 complex128

Casting

Casting needs to be explicit, unlike C where some castings worked out of the box.

For example one of my favorites in C was to make a division but put the result in an int to skip the remainder.

Casting in Go is like this: float32(whatever)

package main

import (
	"fmt"
)

func main() {
	var a, b int = 20, 30
	// Need to convert a and b to float32 before the division
	var div float32 = float32(a)/float32(b)
	var divInt = int(div)
	fmt.Println(div, divInt)
}

%T is the print switch verb to print type of a variable. For example fmt.Printf("v is of type %T\n", v).

Constants

Declared with const keyword. Can be character, string, boolean or numeric. Cannot use :=. Make the first character capital for constants (coding standard?).

package main

import "fmt"

const Whatever = "whatever"

func main() {
	fmt.Println(Whatever)

	const One = 1
	fmt.Println(One)
}

Flow control statements: for, if, else, switch and defer

For

Similar to C with two differences:

• No parenthesis around the three components. Having parenthesis will give you an error (sigh) when you are using the three components separated by semicolons.

• Curly braces { } are always required and the first one needs to be in the same line as for, if, etc.

  sum := 0
  for i :=0; i <20; i++ {
  	sum += i
  }
  

Init and post (first and last) components are optional.

i := 0
for ; i <20; {
	i++
}

Without the semicolons, it will be a while (why not just have a while?).

i := 0
for i <20 { // while (i<20)
	i++
}

Without the condition it will loop forever or while(1).

From Effective Go a better C/Golang comparison:

// Like a C for
for init; condition; post { }

// Like a C while
for condition { }

// Like a C for(;;)
for { }

if

Does not need parenthesis (although you still can use them if you do not have a init component which is separated from the condition with a semicolon) but needs curly braces.

"Like for, the if statement can start with a short statement to execute before the condition.

Variables declared by the statement are only in scope until the end of the if."

package main

import "fmt"

func main() {
	if whatever := 20; whatever > 10 {
		fmt.Println("Inside if:", whatever)
	}
	// Cannot use the variable whatever here
}

else

else is similar to C else.

"Variables declared inside an if short statement are also available inside any of the else blocks."

package main

import "fmt"

func main() {

	if whatever := 20; whatever > 100 {
		fmt.Println("Inside if:", whatever)
	} else {
		// Can use whatever here
		fmt.Println("Inside else:", whatever)
	}
	// Cannot use whatever here
}

switch

Similar to C switch with some differences:

• Doesn't automatically go to the next switch statement unless you have fallthough at the end. "The fallthrough must be the last thing in the case."

• Can have a short statement similar to if (the statement before the case that gets executed).

  // Code taken from the tour
  package main
  
  import (
  	"fmt"
  	"runtime"
  )
  
  func main() {
  	fmt.Print("Go runs on ")
  	switch os := runtime.GOOS; os {
  	case "darwin":
  		fmt.Println("OS X.")
  	case "linux":
  		fmt.Println("Linux.")
  	default:
  		// freebsd, openbsd,
  		// plan9, windows...
  		fmt.Printf("%s.", os)
  	}
  }
  

We can still use break although it breaks at the end of every case.

"switch with no value is switch true and can be used to create long if-then-else chains.""

package main

import (
	"fmt"
	"time"
)

func main() {
	t := time.Now()
	switch {
	case t.Hour() < 12:
		fmt.Println("Good morning!")
	case t.Hour() < 17:
		fmt.Println("Good afternoon.")
	default:
		fmt.Println("Good evening.")
	}
}

defer

"A defer statement defers the execution of a function until the surrounding function returns.

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns."

In each block, every time we get to a defer, it is pushed into a stack. When the function surrounding the block ends, then functions are popped from the stack and executed.

package main

import "fmt"

func main() {
	defer fmt.Println("This runs after main")

	fmt.Println("Main ended")
}

More types: structs, slices, and maps

Pointers

I mean come on!!1! (John Oliver).

Similar to C:

• Point with *: var p *int == int *p;
• Generate pointer (get address of) with &: i := 1 and p = &i

"Unlike C, Go has no pointer arithmetic." Thanks.

Structs

Similar to C.

Do the field names need to be uppercase? It seems like. Because "lower case fields (labels in general) are not visible outside the defining package, reflect is outside."

package main

import "fmt"

type Student struct {
	FirstName string
	LastName string
}

func main() {
	fmt.Println(Student{"John", "Smith"})

	// But we usually want to make instances(?) of structs
	studentOne := Student{"Parsia", "H"}

	// Now we can access the fields using documents
	fmt.Println(studentOne.FirstName)

	// We can just assign fields using names, anything not assigned will be
	// initialized with zero as we have seen before
	studentTwo := Student{FirstName: "Ender"}

	// We will print "{Ender }" notice the space after Ender which is supposed
	// to be the delimiter between the fields, LastName is nil because it is not
	// given a value
	fmt.Println(studentTwo)	

	// Can make a pointer to a struct
	p := studentOne

	// Now instead of *p.LastName (doesn't work) we can just use p.LastName
	// fmt.Println((*p).LastName) will not work with error message: invalid indirect of p (type Student)
	fmt.Println(p.LastName)

	// Can just create a pointer out of the blue
	p2 := Student{"Hercule", "Poirot"}
	fmt.Println(p2)
}

Arrays

var a [10]int == int a[10];.

Arrays cannot be resized.

package main

import "fmt"

func main() {
	var a [5]int
	a[0] = 10
	a[4] = 20
	fmt.Println(a)

	// We can initialize arrays during creation
	// str[3] is empty
	str := [3]string{"Ronny", "Johnson"}

	fmt.Println(str)
}

Slices

Slice "is a dynamically-sized, flexible view into the elements of an array."

"The type []T is a slice with elements of type T."

Seems like we can create a slice from members of an array.

Slices don't store anything, they reference the array. If we change something via the slice, the array is modified.

package main

import "fmt"

func main() {
	// Our favorite characters from the ThatHappened subreddit
	// If we do not specify a length, it will create a slice literal and
	// an underlying array as we will see later
	thatHappened := [3]string{"RonnyJohnson", "AlbertEinstein", "MargaretHello"}

	// Last index is non-inclusive - should have guessed
	// allMembers []string := thatHappened[0:3]
	var allMembers []string = thatHappened[0:3]
	fmt.Println(allMembers)

	var lastTwoMembers []string = thatHappened[1:3]
	fmt.Println(lastTwoMembers)

	// Replace Ms.Hello with another popular character
	allMembers[2] = "JoeDisney"

	// Joe Disney added to the array
	fmt.Println(thatHappened)

	// The other slice changes
	fmt.Println(lastTwoMembers)

}

We can create array and slice literals. Meaning we can just initialize them by their members instead of assigning a length and then adding members.

If a slice literal is created, the underlying array is also created. For now I do not know how I can reference this underlying array (instead of using the slice). I guess it's maybe impossible.

package main

import "fmt"

func main() {

	// Slice literal of type struct, the underlying array is created automatically
	sliceStruct := []struct {
		a,b int
	}{
		{1,2},
		{3,4},
		{5,6},	// need this comma in the end otherwise it will not work (why?)
	}

	fmt.Println(sliceStruct)

}

When creating an array, if we do not specify a length we will get a slice literal as seen above.

If we do not want to specify a length we can use [...].

package main

import "fmt"

func main() {

	thatHappened := [...]string{"RonnyJohnson", "AlbertEinstein", "MargaretHello"}

	// Joe Disney added to the array
	fmt.Println(thatHappened)

}

Slice length and capacity

Slices have length and capacity.

• Length is the current number of items in the slice and can be returned via len(slice).

• Capacity is the maximum number of items in the slice returned via cap(slice). Capacity is the number of items in the original arrays from the start of the slice and not the size of array. For example if the slice starts from the second item of the array, its capacity is len(array)-1. This ensures that the slice cannot go past the array.

  package main
  
  import "fmt"
  
  func main() {
  
  	ints := [...]int{0, 1, 2, 3, 4, 5}
  	fmt.Println(ints)
  
  	slice1 := ints[2:6]
  
  	// len=4 and cap=4 (from 3rd item of the array until the end)
  	printSlice(slice1)
  
  	slice1 = ints[2:4]
  
  	// len=2 but cap will remain 4
  	printSlice(slice1)
  }
  
  // Copied from the tour
  func printSlice(s []int) {
  	fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
  }
  

make slices

To create dynamically-sized arrays use make. make creates a zero-ed array and returns a slice to that array.

slice1 := make([]int, 10) creates an int array of length 10.

slice2 := make([]int, 5, 10) creates an int array of length 5 and capacity of 10.

We can append stuff to slices and it grows as needed. slice1 = append(slice1, 1). We can append multiple elements slice1 = append(slice1, 1, 2, 3).

In order to append one slice to the other (obviously they should be of the same type), we have to use ... like this: slice1 = append(slice1, slice2...).

package main

import "fmt"

func main() {

	slice1 := make([]int, 10)
	printSlice(slice1)
	// len=10 cap=10 [0 0 0 0 0 0 0 0 0 0]

	slice2 := make([]int, 5, 10)
	printSlice(slice2)
	// len=5 cap=10 [0 0 0 0 0]

	slice3 := slice2[0:0]
	printSlice(slice3)
	// len=0 cap=10 []

	slice1 = append(slice1, 1, 2)
	printSlice(slice1)
	// len=12 cap=20 [0 0 0 0 0 0 0 0 0 0 1 2]

	slice1 = append(slice1, slice2...)
	printSlice(slice1)
	// len=17 cap=20 [0 0 0 0 0 0 0 0 0 0 1 2 0 0 0 0 0]
}

// Copied from the tour
func printSlice(x []int) {
	fmt.Printf("len=%d cap=%d %v\n", len(x), cap(x), x)
}

Range

range iterates over slices. It returns an index and a copy of the item stored at that index. for index, value := range slice.

value is optional but index is not. To not use index use _.

package main

import "fmt"

func main() {
	thatHappened := [3]string{"Ronny Johnson", "Albert Einstein", "Margaret Hello"}
	for index, value := range thatHappened {
		// No newline in the end of Printf but Println does not support formatting
		fmt.Printf("thatHappened[%d]: %s\n", index, value) 
	}

	fmt.Println("-----------")

	// Only using index
	for index := range thatHappened {
		fmt.Printf("thatHappened[%d]: %s\n", index, thatHappened[index])
	}

	fmt.Println("-----------")
	// Dropping index
	for _, value :=range thatHappened {
		fmt.Printf("%s\n", value)
	}
}

Methods and interfaces

Methods

Go doesn't have classes. Methods can be defined for types (e.g. structs). what is this? C?

A method is a function with a special receiver, receiver is the type that this method is defined for.

How to do methods for slices: For example we can re-write the printSlice function and define it for slices of type int.

Problem is []int is an unnamed type and cannot be a receiver. We have to create a type with that specific slice type (in this case int) like: type IntSlice []int.

With a value receiver like what we have, operations are done on a copy of the object.

package main

import "fmt"

type IntSlice []int

func (x IntSlice) PrintSlice() {
	fmt.Printf("len=%d cap=%d %v\n",
		len(x), cap(x), x)
}

func main() {

	var slice1 IntSlice = make([]int, 10)
	slice1.PrintSlice()	// len=10 cap=10 [0 0 0 0 0 0 0 0 0 0]

	var slice2 IntSlice = make([]int, 5, 10)
	slice2.PrintSlice()	// len=5 cap=10 [0 0 0 0 0]
}

Pointer Receivers

Pointer receivers get a pointer instead of a value but can modify the object that the pointer points to. Pointer receivers can be a pointer to a pointer (e.g. **int).

By value receivers do not modify the object while by pointer receivers do.

In the following code, Tuple's fields will be modified by ModifyTuplePointer() but not by ModifyTupleValue().

However, this is not the case for slices (e.g. IntSlice in the code). Both value and pointer receivers modify the slice. I do not know the reason but I assume it's because the slice is already a reference to an array somewhere.

Pointer receivers are more efficient because they do not copy the original object. Probably only matters when passing large structs.

All methods for one type should either have value receivers or pointer receivers, do not mix and match like the code below :).

package main

import "fmt"

type Tuple struct {
	A, B int
}

// Should not change the value of the object as it works on a copy of it
func (x Tuple) ModifyTupleValue() {
	x.A = 2
	x.B = 2
}

// Should change the value of the object
func (x *Tuple) ModifyTuplePointer() {
	x.A = 3
	x.B = 3
}

type IntSlice []int

func (x IntSlice) PrintSlice() {
	fmt.Printf("len=%d cap=%d %v\n",
		len(x), cap(x), x)
}

// Modifies the IntSlice although it's by value
func (x IntSlice) ModifySliceValue()  {
	x[0] = 1
}

// Modifies the IntSlice
func (x *IntSlice) ModifySlicePointer()  {
	(*x)[0] = 2
}

func main() {

	nem := Tuple{1, 1}

	nem.ModifyTupleValue()
	fmt.Println(nem)	// {1 1}


	nem.ModifyTuplePointer()
	fmt.Println(nem)	// {3 3}

	var slice1 IntSlice = make([]int, 5)
	slice1.PrintSlice()	// len=5 cap=5 [0 0 0 0 0]


	slice1.ModifySliceValue()
	slice1.PrintSlice()	// len=5 cap=5 [1 0 0 0 0]

	slice1.ModifySlicePointer()
	slice1.PrintSlice()	// len=5 cap=5 [2 0 0 0 0]
}

Interfaces

"An interface type is defined as a set of method signatures.

A value of interface type can hold any value that implements those methods."

For example we will define an interface which has the method MyPrint(), now each type that has that method can be assigned to an interface of that type

package main

import "fmt"

type MyPrinter interface {
	MyPrint()
}

type MyInt int

func (i MyInt) MyPrint() {
	fmt.Println(i)
}

type MyFloat float64

func (f MyFloat) MyPrint() {
	fmt.Println(f)
}

func main() {

	var thePrinter MyPrinter

	float1 := MyFloat(1.2345)
	int1 := MyInt(10)

	thePrinter = float1
	thePrinter.MyPrint()
	// 1.2345

	thePrinter = int1
	thePrinter.MyPrint()
	// 10

}

Empty Interface is interface {} and can hold any type. Usually used to handle unknown types.

package main

import "fmt"

var emptyInterface interface{}

type Tuple struct {
	A, B int
}

func main() {

	// Use int
	int1 := 10
	emptyInterface = int1
	fmt.Println(emptyInterface)
	// 10

	// Use float
	float1 := 1.2345
	emptyInterface = float1
	fmt.Println(emptyInterface)
	// 1.2345

	// Use custom struct
	tuple1 := Tuple{5, 5}
	emptyInterface = tuple1
	fmt.Println(emptyInterface)
	// {5 5}

}

We can access the value inside the interface like this myFloat := myInterface(float64). If the interface does not contain a float, this will trigger a panic.

In order to handle it properly we use it like this myFloat, ok := myInterface(float64). This will prevent the panic. If the interface has a float, ok will be true and otherwise false.

package main

import "fmt"

func main() {
	var myInterface interface{} = 1234.5

	myFloat, ok := myInterface.(float64)
	fmt.Println(myFloat, ok)
	// 1234.5 true

	myInt, ok := myInterface.(int)
	fmt.Println(myInt, ok)
	// 0 false -- which means it does not contain an int

	// This will trigger a panic
	// myInt = myInterface.(int)
}

Type switch

Do a switch on interface.(type). Similar to what we did above.

// Code copied from the tutorial
package main

import "fmt"

func do(i interface{}) {
	switch v := i.(type) {
	case int:
		fmt.Printf("Twice %v is %v\n", v, v*2)
	case string:
		fmt.Printf("%q is %v bytes long\n", v, len(v))
	default:
		fmt.Printf("I don't know about type %T!\n", v)
	}
}

func main() {
	do(21)		// Twice 21 is 42
	do("hello")	// "hello" is 5 bytes long
	do(true)	// I don't know about type bool!
}

Stringers

Defined by the fmt package. Can describe itself as string.

type Stringer interface {
    String() string
}

If we implement it for any struct, it will be called when calling Println (and others) on it. Essentially create a method for that struct named String() which returns a string.

This is also what gets printed when we want to print an instance of that type with the %v format string switch verb.

package main

import "fmt"

type Tuple struct {
	A, B int
}

func (t Tuple) String() string {
	return fmt.Sprintf("A: %d, B: %d", t.A, t.B)
}

func main() {

	tuple1 := Tuple{10, 10}
	tuple2 := Tuple{20, 20}
	fmt.Println(tuple1)	// A: 10, B: 10
	fmt.Println(tuple2)	// A: 20, B: 20
}

Solution to the Stringers exercise

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.
func (ip IPAddr) String() string {
	return fmt.Sprintf("%v.%v.%v.%v", ip[0], ip[1], ip[2], ip[3])
}

func main() {
	hosts := map[string]IPAddr{
		"loopback":  {127, 0, 0, 1},
		"googleDNS": {8, 8, 8, 8},
	}
	for name, ip := range hosts {
		fmt.Printf("%v: %v\n", name, ip)
	}
}

Errors

error type is similar to Stringer().

type error interface {
    Error() string
}

Create a method for the struct type named Error() to return error codes/messages.

func (e MyType) Error() string {
	return fmt.Sprintf("Error message")
}

Most built-in and package methods return an error value if an error occurs, otherwise they will return nil for error which means no error.

Solution to the Errors exercise

package main

import "fmt"

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
}

func Sqrt(x float64) (float64, error) {

	if (x < 0) {
		return 0, ErrNegativeSqrt(x)
	}

	// Don't need else here
	return 0, nil
}

func main() {
	fmt.Println(Sqrt(2))
	fmt.Println(Sqrt(-2))
}

Skipped the rest of the module.

Concurrency

go function(a, b) runs the function in parallel and continues with the rest of the program.

Channels

Typed conduit. Support sending and receiving values using <-.

Channels must be created before use. "By default, sends and receives block until the other side is ready."

// Make a channel of type int in honor of the famous hacker
// Note that we can only send/receive int via this channel
fourChan := make(chan int)

// Send to channel
fourChan <- someInt

// Receive data from channel
newInt := <- fourChan

Buffered channels

If channels are buffered then they will only block when the buffer is full. fiveChan := make (chan int, 100) will create a channel with a buffer size of 100.

Closing channels

To test if a channel is closed do someInt, ok := <- fourChan. If channel is not closed, ok with be true, otherwise false means channel is closed. Sending items to a closed channel will cause a panic.

To close a channel do close(fourChan).

Reading information from channels

Use a range in a for to receive values from the channel in a loop until it closes like for i:=range fourChan. If you want to read something from an open channel and there's nothing there, the program will block(?) and wait until it gets something.

select

select has some cases. It will block until one of the cases is ready and runs it. If multiple are ready, it will choose one at random.

select{
case fourChan <- x:
	// Whatever
case c <- fiveChan:
	// Whatever
default:
	// This is run if no other case is ready
}

sync.Mutex

sync.Mutex has two methods, lock and unlock. We can also defer the unlock if we want to return something and then unlock it like the Value method from the example.

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
	c.mux.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	defer c.mux.Unlock()
	return c.v[key]
}

Printf from Go by example

Taken from Go by Example and Effective Go.

These three need a format string:

• fmt.Sprintf returns a string.
• fmt.Fprintf takes any objects that implements io.Writer for example os.Stdout and os.Stderr.
• fmt.Printf prints to stdout(?).

The following are similar to the above but do not need a format string:

• fmt.Print and fmt.Println.
• fmt.Fprint - fmt.Fprint(os.Stdout, "Ronny", "Johnson", "$100%").
• fmt.Sprint.

Switches Verbs

Better info here: https://golang.org/pkg/fmt/#hdr-Printing

Decimals

%d: digits = numbers.

%nd: n = width of number. Right justified and padded with spaces. To left justify use - like %-nd. If n is less than the number of digits nothing happens.

%b: number in binary.

%c: chr(int), prints the character corresponding to the number.

%x: hex.

Floats

%f: float.

%n.mf: n = decimal width, m = float width. Right justified. To left justify use - like %-n.mf. If n is less than the number of digits nothing happens.

%e and %E: scientific notation (output is a bit different from each other).

Value

%v or value: catch all format. Will print based on value.

%+v: will print struct's field names if we are printing a struct. Has no effect on anything else.

%#v: prints a "Go syntax representation of the value, i.e. the source code snippet that would produce that value." For example for a struct instance it will give code that creates such a struct instance and initializes it with the current values of the struct instance.

Strings

%q: "To double-quote strings as in Go source, use %q."

%s: string.

%ns: control width of string. Right justified, padded with spaces. To left justify use - like %-ns. If n is less than the length of the string, nothing happens.

Others

%t: boolean.

%T: prints the type of a value. For example int or main.myType.

package main

import "fmt"

type myType struct {
	field1 int
	field2 string
	field3 float64
}

func main() {

	// struct type
	struct1 := myType{10, "Ronny", -10.2}
	fmt.Printf("%v\n", struct1)		// {10 Ronny -10.2}
	fmt.Printf("%+v\n", struct1)	// {field1:10 field2:Ronny field3:-10.2}
	fmt.Printf("%#v\n", struct1)	// main.myType{field1:10, field2:"Ronny", field3:-10.2}
	fmt.Printf("%T\n", struct1)		// main.myType

	// int
	int1 := 123
	fmt.Printf("%v\n", int1)		// 123
	fmt.Printf("%d\n", int1)		// 123
	fmt.Printf("|%6d|\n", int1)		// |   123|
	fmt.Printf("|%-6d|\n", int1)	// |123   |
	fmt.Printf("%T\n", int1)		// int
	fmt.Printf("%x\n", int1)		// 7b
	fmt.Printf("%b\n", int1)		// 1111011
	fmt.Printf("%e\n", int1)		// %!e(int=123)
	fmt.Printf("%c\n", int1)		// {

	// float
	float1 := 1234.56
	fmt.Printf("%f\n", float1)			// 1234.560000
	fmt.Printf("|%3.2f|\n", float1)		// |1234.56|
	fmt.Printf("|%-3.2f|\n", float1)	// |1234.56|
	fmt.Printf("%e\n", float1)			// 1.234560e+03
	fmt.Printf("%E\n", float1)			// 1.234560E+03

	// string
	string1 := "Ronny"
	fmt.Printf("%s\n", string1)			// Ronny
	fmt.Printf("|%10s|\n", string1)		// |     Ronny|
	fmt.Printf("|%-10s|\n", string1)	// |Ronny     |
	fmt.Printf("%T\n", string1)			// string

	// boolean
	boolean1 := true
	fmt.Printf("%t\n", boolean1)	// true
	fmt.Printf("%T\n", boolean1)	// bool

}

Maps

Go map == hash table. Fast lookup/add/delete. Each key is associated with a value (Python dict?).

Declare an initialized map: mapName := make(map[KeyType]ValueType). KeyType needs to be a comparable type. ValueType can be anything.

If a key does not exist, the result is a zero value. For example 0 for int.

To check if a key exists or not (0 might be a valid value in the map) value, ok := mapName[key]. If ok is true then the key exists (and false if the key is not there).

To test for a key without getting the value drop the value like this _, ok := mapName[key] and then just check ok.

range iterates over the contents of a map as we have seen before for arrays/slices. In this case we get keys instead of indexes. Use it with a for.

for key, value := range mapName.

We can initialize a map using data.

We can also initialize an empty map instead of the make (mapName = map[KeyType]ValueType{}).

Other tips and tricks

Errors

To do a custom error, import the errors package and use it like this.

package main

import "errors"

func randomFunction() (return1 interface{}, err error) {
	// Whatever
	var result interface{}
	return result, errors.New("Custom error string")
}

Hexdump

encoding/hex package is your friend: https://golang.org/pkg/encoding/hex/.

encoding/hex.Dump - func Dump(data []byte) string: Returns a string containing a normal hex dump (e.g. offset - hex - printable characters). Internally it calls the Dumper function - source.

encoding/hex.Dumper - func Dumper(w io.Writer) io.WriteCloser: Returns an io.WriteCloser (I don't know exactly what it is, but it seems like we can call Write)

Seems like there is no way to remove the offset. Either I can modify the source or write my own. There's also this MIT licensed package that looks easier to modify. In both cases, the modification looks pretty straightforward.

Named imports

We can do named imports like Python.

package main

import (
	thisIsFMT "fmt"
)

func main() {
	thisIsFMT.Println("whatever")
}

Importing a package into the current namespace

Using import . "packagename" means we can omit the package name. In the example below we can omit fmt.

No clue how name collisions are handled (e.g. two packages imported into the namespace having the same function name).

package main

import (
	. "fmt"
)

func main() {
	Println("whatever")
}

Avoiding the damn unused warnings

Yeah it's nice to get "better" code (although that is debatable but I am not a dev so I am biased), but it's a pain when debugging/testing. Send them to _.

package main

import (
    _ "package name" // gone to the dogs
)

func main() {
    unUsedVar := "whatever"
    _ = unUsedVar	// gone to the dogs
}

Unix Timestamp to String

import "strconv"

strconv.FormatInt(time.Now().Unix(), 10)

Spawn a new thread (goroutine) on the spot

func main() {

	// Whatever

	go func() {
		// Whatever happens in this goroutine
	}()

}

Write to a file or io.buffer from goroutines - DON'T

Instead use a buffered channel (will make it async). Make a channel before goroutines, send stuff to the channel from goroutines. Make another goroutine that creates a file, does defer fileHandle.Close() (which makes closes the file after this goroutine ends) and then has an infinite loop where it reads from the channel and writes to a file.

// ...
// Logging channel- buffered so it's async
fourChan := make(chan string, 100)

go func() {
	for {
			// Do something and get a string
			fourChan <- string1
		}
	}
}()

go func() {
	for {

		// Do something else
		fourChan <- string2
		}
	}
}()

go func() {
	// Get a unique filename
	dumpFile, _ := os.Create("whatever.txt")
	defer dumpFile.Close()  // finally a good use for defer
	for {
		dumpFile.WriteString(<- fourChan)
	}
}()
// ...

Generate godoc HTML for a Single Package

If godoc doesn't run locally because not all of your packages can be built (which is normal), create the HTML output for one single package to inspect:

godoc -html cmd/github.com/user/package > package-godoc.html

Stuff learned from Cryptopals

I learned a bunch after I returned to go after a while and tried to do the Cryptopals challenge.

Long string on multiple lines

const Input = "49276d206b696c6c696e6720796f7572" +
              "20627261696e206c696b65206120706f" +
              "69736f6e6f7573206d757368726f6f6d"

Compare two []bytes with bytes.equal

Works with slices too.

Use bytes.equal:

package main

import (
	"bytes"
	"fmt"
)

func main() {

	bytes1 := []byte("Hello")
	bytes2 := []byte("Hello")
	bytes3 := []byte("Bye")

	if eval := bytes.Equal(bytes1, bytes2); eval {
		fmt.Println("1 and 2 are equal")
	} else {
		fmt.Println("1 and 2 are different")
	}

	if eval := bytes.Equal(bytes2, bytes3); eval {
		fmt.Println("2 and 3 are equal")
	} else {
		fmt.Println("2 and 3 are different")
	}
}

Sorting a array/slice of struct by field

Code from https://stackoverflow.com/a/28999886.

Arrays need to be passed through a slice but the underlying array will also be sorted.

type MyObj struct {
    Name string
    Score int
    Code []byte
}

myObjects := make([]MyObj, 10)

// ...

// Sort by Score
// We are passing a slice before sorting but underlying array will be sorted
sort.Slice(myObjects[:], func(i, j int) bool {
    return myObjects[i].Score < myObjects[j].Score
})

Append two slices

With append you can append a slice and a primitive of that slice. For example to append a []byte with a byte you can do:

package main

import "fmt"

func main() {
	bytes := []byte("Some string")
	byte1 := byte(0x31)

	bytes = append(bytes, byte1)
	
	fmt.Println(string(bytes))
}

// Some string1

Now if we want to append two slices (e.g. two []byte) we need to do a bit of magic because the second argument to append needs to be a byte:

package main

import "fmt"

func main() {
	bytes1 := []byte("Some string")
	bytes2 := []byte(" Another string")

	bytes1 = append(bytes1, bytes2)
	
	fmt.Println(string(bytes1))
}

We will get this error:

• cannot use bytes2 (type []byte) as type byte in append

What we can do is pass the second slice with .... I am not quite sure how this works but it seems like we are passing the slice as a collection of all of its members

package main

import "fmt"

func main() {
	bytes1 := []byte("Some string")
	bytes2 := []byte(" Another string")

	bytes1 = append(bytes1, bytes2...)
	
	fmt.Println(string(bytes1))
}

// Some string Another string

According to the documentation "As a special case, it is legal to append a string to a byte slice, like this:""

• slice = append([]byte("hello "), "world"...)"