# README
Go's basic types
bool
string
int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr
byte // alias for uint8
rune // alias for int32
// represents a Unicode code point
float32 float64
complex64 complex128
Zero values
Variables declared without an explicit initial value are given their zero value.
The zero value is:
0for numeric types,falsefor the boolean type, and""(the empty string) for strings.
Type inference
if a variable declaration is accompanied with initializer the type is infered but for numeric types it depends on the precision
i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128
Forever loop
for {
}
More types
Struct fields are accessed using a dot.
To access the field X of a struct when we have the struct pointer p we could write (*p).X. However, that notation is cumbersome, so the language permits us instead to write just p.X, without the explicit dereference.
Struct literals
A struct literal denotes a newly allocated struct value by listing the values of its fields.
You can list just a subset of fields by using the Name: syntax. (And the order of named fields is irrelevant.)
The special prefix & returns a pointer to the struct value.
Arrays
The type [n]T is an array of n values of type T.
The expression
var a [10]int
declares a variable a as an array of ten integers.
An array's length is part of its type, so arrays cannot be resized. This seems limiting, but don't worry; Go provides a convenient way of working with arrays.
Slices
An array has a fixed size. A slice, on the other hand, is a dynamically-sized, flexible view into the elements of an array. In practice, slices are much more common than arrays.
The type []T is a slice with elements of type T.
A slice is formed by specifying two indices, a low and high bound, separated by a colon:
a[low : high]
This selects a half-open range which includes the first element, but excludes the last one.
The following expression creates a slice which includes elements 1 through 3 of a:
a[1:4]
Slices are like references to arrays
A slice does not store any data, it just describes a section of an underlying array.
Changing the elements of a slice modifies the corresponding elements of its underlying array.
Other slices that share the same underlying array will see those changes.
Slice literals
A slice literal is like an array literal without the length.
This is an array literal:
[3]bool{true, true, false}
And this creates the same array as above, then builds a slice that references it:
[]bool{true, true, false}
Slice defaults
When slicing, you may omit the high or low bounds to use their defaults instead.
The default is zero for the low bound and the length of the slice for the high bound.
For the array
var a [10]int
these slice expressions are equivalent:
a[0:10]
a[:10]
a[0:]
a[:]
Slice length and capacity
A slice has both a length and a capacity.
The length of a slice is the number of elements it contains.
The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.
The length and capacity of a slice s can be obtained using the expressions len(s) and cap(s).
You can extend a slice's length by re-slicing it, provided it has sufficient capacity. Try changing one of the slice operations in the example program to extend it beyond its capacity and see what happens.
Nil slices
The zero value of a slice is nil.
A nil slice has a length and capacity of 0 and has no underlying array.
Creating a slice with make
Slices can be created with the built-in make function; this is how you create dynamically-sized arrays.
The make function allocates a zeroed array and returns a slice that refers to that array:
a := make([]int, 5) // len(a)=5
To specify a capacity, pass a third argument to make:
b := make([]int, 0, 5) // len(b)=0, cap(b)=5
b = b[:cap(b)] // len(b)=5, cap(b)=5
b = b[1:] // len(b)=4, cap(b)=4
Append to slice
func append(s []T, vs ...T) []T
The first parameter s of append is a slice of type T, and the rest are T values to append to the slice.
Range
You can skip the index or value by assigning to _.
for i, _ := range pow
for _, value := range pow
If you only want the index, you can omit the second variable.
for i := range pow
Maps
A map maps keys to values.
The zero value of a map is nil. A nil map has no keys, nor can keys be added.
The make function returns a map of the given type, initialized and ready for use.
Mutating Maps
Insert or update an element in map m:
m[key] = elem
Retrieve an element:
elem = m[key]
Delete an element:
delete(m, key)
Test that a key is present with a two-value assignment:
elem, ok = m[key]
If key is in m, ok is true. If not, ok is false.
If key is not in the map, then elem is the zero value for the map's element type.
Note: If elem or ok have not yet been declared you could use a short declaration form:
elem, ok := m[key]
Methods are functions
Remember: a method is just a function with a receiver argument.
You can declare a method on non-struct types, too.
In this example we see a numeric type MyFloat with an Abs method.
You can only declare a method with a receiver whose type is defined in the same package as the method. You cannot declare a method with a receiver whose type is defined in another package (which includes the built-in types such as int).
Pointer receivers
You can declare methods with pointer receivers.
This means the receiver type has the literal syntax *T for some type T. (Also, T cannot itself be a pointer such as *int.)
For example, the Scale method here is defined on *Vertex.
Methods with pointer receivers can modify the value to which the receiver points (as Scale does here). Since methods often need to modify their receiver, pointer receivers are more common than value receivers.
With a value receiver, the Scale method operates on a copy of the original Vertex value. (This is the same behavior as for any other function argument.) The Scale method must have a pointer receiver to change the Vertex value declared in the main function.
Choosing a value or pointer receiver
There are two reasons to use a pointer receiver.
The first is so that the method can modify the value that its receiver points to.
The second is to avoid copying the value on each method call. This can be more efficient if the receiver is a large struct, for example.
In this example, both Scale and Abs are methods with receiver type *Vertex, even though the Abs method needn't modify its receiver.
In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both. (We'll see why over the next few pages.)
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Printf("Before scaling: %+v, Abs: %v\n", v, v.Abs())
v.Scale(5)
fmt.Printf("After scaling: %+v, Abs: %v\n", v, v.Abs())
}
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.
Interfaces are implemented implicitly
A type implements an interface by implementing its methods. There is no explicit declaration of intent, no "implements" keyword.
Implicit interfaces decouple the definition of an interface from its implementation, which could then appear in any package without prearrangement.
Interface values
Under the hood, interface values can be thought of as a tuple of a value and a concrete type:
(value, type)
An interface value holds a value of a specific underlying concrete type.
Calling a method on an interface value executes the method of the same name on its underlying type.
The empty interface
The interface type that specifies zero methods is known as the empty interface:
interface{}
An empty interface may hold values of any type. (Every type implements at least zero methods.)
Empty interfaces are used by code that handles values of unknown type. For example, fmt.Print takes any number of arguments of type interface{}.
Type assertions
A type assertion provides access to an interface value's underlying concrete value.
t := i.(T)
This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.
If i does not hold a T, the statement will trigger a panic.
To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.
t, ok := i.(T)
If i holds a T, then t will be the underlying value and ok will be true.
If not, ok will be false and t will be the zero value of type T, and no panic occurs.
Note the similarity between this syntax and that of reading from a map.
package main
import "fmt"
func main() {
var i interface{} = "hello"
s := i.(string)
fmt.Println(s)
s, ok := i.(string)
fmt.Println(s, ok)
f, ok := i.(float64)
fmt.Println(f, ok)
f = i.(float64) // panic
fmt.Println(f)
}
Type switches
A type switch is a construct that permits several type assertions in series.
A type switch is like a regular switch statement, but the cases in a type switch specify types (not values), and those values are compared against the type of the value held by the given interface value.
switch v := i.(type) {
case T:
// here v has type T
case S:
// here v has type S
default:
// no match; here v has the same type as i
}
The declaration in a type switch has the same syntax as a type assertion i.(T), but the specific type T is replaced with the keyword type.
This switch statement tests whether the interface value i holds a value of type T or S. In each of the T and S cases, the variable v will be of type T or S respectively and hold the value held by i. In the default case (where there is no match), the variable v is of the same interface type and value as i.
Stringers
One of the most ubiquitous interfaces is Stringer defined by the fmt package.
type Stringer interface {
String() string
}
A Stringer is a type that can describe itself as a string. The fmt package (and many others) look for this interface to print values.
Errors
Go programs express error state with error values.
The error type is a built-in interface similar to fmt.Stringer:
type error interface {
Error() string
}
(As with fmt.Stringer, the fmt package looks for the error interface when printing values.)
Functions often return an error value, and calling code should handle errors by testing whether the error equals nil.
i, err := strconv.Atoi("42")
if err != nil {
fmt.Printf("couldn't convert number: %v\n", err)
return
}
fmt.Println("Converted integer:", i)
A nil error denotes success; a non-nil error denotes failure.
Type parameters
Go functions can be written to work on multiple types using type parameters. The type parameters of a function appear between brackets, before the function's arguments.
func Index[T comparable](s []T, x T) int
This declaration means that s is a slice of any type T that fulfills the built-in constraint comparable. x is also a value of the same type.
comparable is a useful constraint that makes it possible to use the == and != operators on values of the type. In this example, we use it to compare a value to all slice elements until a match is found. This Index function works for any type that supports comparison.
package main
import "fmt"
// Index returns the index of x in s, or -1 if not found.
func Index[T comparable](s []T, x T) int {
for i, v := range s {
// v and x are type T, which has the comparable
// constraint, so we can use == here.
if v == x {
return i
}
}
return -1
}
func main() {
// Index works on a slice of ints
si := []int{10, 20, 15, -10}
fmt.Println(Index(si, 15))
// Index also works on a slice of strings
ss := []string{"foo", "bar", "baz"}
fmt.Println(Index(ss, "hello"))
}
GoRoutines
A goroutine is a lightweight thread managed by the Go runtime.
go f(x, y, z)
starts a new goroutine running
f(x, y, z)
The evaluation of f, x, y, and z happens in the current goroutine and the execution of f happens in the new goroutine.
Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won't need them much in Go as there are other primitives.
Channels
Channels are a typed conduit through which you can send and receive values with the channel operator, <-.
ch <- v // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to v.
(The data flows in the direction of the arrow.)
Like maps and slices, channels must be created before use:
ch := make(chan int)
By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.
The example code sums the numbers in a slice, distributing the work between two goroutines. Once both goroutines have completed their computation, it calculates the final result.
Buffered Channels
Channels can be buffered. Provide the buffer length as the second argument to make to initialize a buffered channel:
ch := make(chan int, 100)
Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.
Range and Close
A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after
v, ok := <-ch
ok is false if there are no more values to receive and the channel is closed.
The loop for i := range c receives values from the channel repeatedly until it is closed.
Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.
Another note: Channels aren't like files; you don't usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.
Select
The select statement lets a goroutine wait on multiple communication operations.
A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.
Default Selection
The default case in a select is run if no other case is ready.
Use a default case to try a send or receive without blocking:
select {
case i := <-c:
// use i
default:
// receiving from c would block
}
sync.Mutex
We've seen how channels are great for communication among goroutines.
But what if we don't need communication? What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts?
This concept is called mutual exclusion, and the conventional name for the data structure that provides it is mutex.
Go's standard library provides mutual exclusion with sync.Mutex and its two methods:
Lock
Unlock
We can define a block of code to be executed in mutual exclusion by surrounding it with a call to Lock and Unlock as shown on the Inc method.
We can also use defer to ensure the mutex will be unlocked as in the Value method.
package main
import (
"fmt"
"sync"
"time"
)
// SafeCounter is safe to use concurrently.
type SafeCounter struct {
mu sync.Mutex
v map[string]int
}
// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
c.v[key]++
c.mu.Unlock()
}
// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
c.mu.Lock()
// Lock so only one goroutine at a time can access the map c.v.
defer c.mu.Unlock()
return c.v[key]
}
func main() {
c := SafeCounter{v: make(map[string]int)}
for i := 0; i < 1000; i++ {
go c.Inc("somekey")
}
time.Sleep(time.Second)
fmt.Println(c.Value("somekey"))
}