1. Documentation
2. Effective Go

Effective Go

Introduction

Go is an open-source programming language that focuses on simplicity, reliability, and efficiency, specifically designed to make it easy to build software at scale. Although it borrows ideas from existing languages, it has unusual properties that make effective Go programs different in character from programs written in its relatives. A straightforward translation of a C++ or Java program into Go is unlikely to produce a satisfactory result—Java programs are written in Java, not Go. On the other hand, thinking about the problem from a Go perspective could produce a successful but quite different program. In other words, to write Go well, it's important to understand its properties and idioms. It's also important to know the established conventions for programming in Go, such as naming, formatting, program construction, and so on, so that programs you write will be easy for other Go programmers to understand.

This document gives tips for writing clear, idiomatic Go code. It augments the language specification, the Tour of Go, and How to Write Go Code, all of which you should read first.

Examples

The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. Moreover, many of the packages contain working, self-contained executable examples you can run directly from the go.dev web site, such as this one (if necessary, click on the word "Example" to open it up). If you have a question about how to approach a problem or how something might be implemented, the documentation, code and examples in the library can provide answers, ideas and background.

Formatting

Formatting issues are the most contentious but the least consequential. People can adapt to different formatting styles but it's better if they don't have to, and less time is devoted to the topic if everyone adheres to the same style. The problem is how to approach this Utopia without a long prescriptive style guide.

With Go we take an unusual approach and let the machine take care of most formatting issues. The gofmt program (also available as go fmt, which operates at the package level rather than source file level) reads a Go program and emits the source in a standard style of indentation and vertical alignment, retaining and if necessary reformatting comments. If you want to know how to handle some new layout situation, run gofmt; if the answer doesn't seem right, rearrange your program (or file a bug about gofmt), don't work around it.

As an example, there's no need to spend time lining up the comments on the fields of a structure. Gofmt will do that for you. Given the declaration

type T struct {
    name string // name of the object
    value int // its value
}

gofmt will line up the columns:

type T struct {
    name    string // name of the object
    value   int    // its value
}

All Go code in the standard packages has been formatted with gofmt.

Some formatting details remain. Very briefly:

Indentation

We use tabs for indentation and gofmt emits them by default. Use spaces only if you must.

Line length

Go has no line length limit. Don't worry about overflowing a punched card. If a line feels too long, wrap it and indent with an extra tab.

Parentheses

Go needs fewer parentheses than C and Java: control structures (if, for, switch) do not have parentheses in their syntax. Also, the operator precedence hierarchy is shorter and clearer, so

x<<8 + y<<16

means what the spacing implies, unlike in the other languages.

Commentary

Go provides C-style /* */ block comments and C++-style // line comments. Line comments are the norm; block comments appear mostly as package comments, but are useful within an expression or to disable large swaths of code.

Comments that appear before top-level declarations, with no intervening newlines, are considered to document the declaration itself. These “doc comments” are the primary documentation for a given Go package or command. For more about doc comments, see “Go Doc Comments”.

Names

Names are as important in Go as in any other language. They even have semantic effect: the visibility of a name outside a package is determined by whether its first character is upper case. It's therefore worth spending a little time talking about naming conventions in Go programs.

Package names

When a package is imported, the package name becomes an accessor for the contents. After

import "bytes"

the importing package can talk about bytes.Buffer. It's helpful if everyone using the package can use the same name to refer to its contents, which implies that the package name should be good: short, concise, evocative. By convention, packages are given lower case, single-word names; there should be no need for underscores or mixedCaps. Err on the side of brevity, since everyone using your package will be typing that name. And don't worry about collisions a priori. The package name is only the default name for imports; it need not be unique across all source code, and in the rare case of a collision the importing package can choose a different name to use locally. In any case, confusion is rare because the file name in the import determines just which package is being used.

Another convention is that the package name is the base name of its source directory; the package in src/encoding/base64 is imported as "encoding/base64" but has name base64, not encoding_base64 and not encodingBase64.

The importer of a package will use the name to refer to its contents, so exported names in the package can use that fact to avoid repetition. (Don't use the import . notation, which can simplify tests that must run outside the package they are testing, but should otherwise be avoided.) For instance, the buffered reader type in the bufio package is called Reader, not BufReader, because users see it as bufio.Reader, which is a clear, concise name. Moreover, because imported entities are always addressed with their package name, bufio.Reader does not conflict with io.Reader. Similarly, the function to make new instances of ring.Ring—which is the definition of a constructor in Go—would normally be called NewRing, but since Ring is the only type exported by the package, and since the package is called ring, it's called just New, which clients of the package see as ring.New. Use the package structure to help you choose good names.

Another short example is once.Do; once.Do(setup) reads well and would not be improved by writing once.DoOrWaitUntilDone(setup). Long names don't automatically make things more readable. A helpful doc comment can often be more valuable than an extra long name.

Getters

Go doesn't provide automatic support for getters and setters. There's nothing wrong with providing getters and setters yourself, and it's often appropriate to do so, but it's neither idiomatic nor necessary to put Get into the getter's name. If you have a field called owner (lower case, unexported), the getter method should be called Owner (upper case, exported), not GetOwner. The use of upper-case names for export provides the hook to discriminate the field from the method. A setter function, if needed, will likely be called SetOwner. Both names read well in practice:

owner := obj.Owner()
if owner != user {
    obj.SetOwner(user)
}

Interface names

By convention, one-method interfaces are named by the method name plus an -er suffix or similar modification to construct an agent noun: Reader, Writer, Formatter, CloseNotifier etc.

There are a number of such names and it's productive to honor them and the function names they capture. Read, Write, Close, Flush, String and so on have canonical signatures and meanings. To avoid confusion, don't give your method one of those names unless it has the same signature and meaning. Conversely, if your type implements a method with the same meaning as a method on a well-known type, give it the same name and signature; call your string-converter method String not ToString.

MixedCaps

Finally, the convention in Go is to use MixedCaps or mixedCaps rather than underscores to write multiword names.

Semicolons

Like C, Go's formal grammar uses semicolons to terminate statements, but unlike in C, those semicolons do not appear in the source. Instead the lexer uses a simple rule to insert semicolons automatically as it scans, so the input text is mostly free of them.

The rule is this. If the last token before a newline is an identifier (which includes words like int and float64), a basic literal such as a number or string constant, or one of the tokens

break continue fallthrough return ++ -- ) }

the lexer always inserts a semicolon after the token. This could be summarized as, “if the newline comes after a token that could end a statement, insert a semicolon”.

A semicolon can also be omitted immediately before a closing brace, so a statement such as

    go func() { for { dst <- <-src } }()

needs no semicolons. Idiomatic Go programs have semicolons only in places such as for loop clauses, to separate the initializer, condition, and continuation elements. They are also necessary to separate multiple statements on a line, should you write code that way.

One consequence of the semicolon insertion rules is that you cannot put the opening brace of a control structure (if, for, switch, or select) on the next line. If you do, a semicolon will be inserted before the brace, which could cause unwanted effects. Write them like this

if i < f() {
    g()
}

not like this

if i < f()  // wrong!
{           // wrong!
    g()
}

Control structures

The control structures of Go are related to those of C but differ in important ways. There is no do or while loop, only a slightly generalized for; switch is more flexible; if and switch accept an optional initialization statement like that of for; break and continue statements take an optional label to identify what to break or continue; and there are new control structures including a type switch and a multiway communications multiplexer, select. The syntax is also slightly different: there are no parentheses and the bodies must always be brace-delimited.

If

In Go a simple if looks like this:

if x > 0 {
    return y
}

Mandatory braces encourage writing simple if statements on multiple lines. It's good style to do so anyway, especially when the body contains a control statement such as a return or break.

Since if and switch accept an initialization statement, it's common to see one used to set up a local variable.

if err := file.Chmod(0664); err != nil {
    log.Print(err)
    return err
}

In the Go libraries, you'll find that when an if statement doesn't flow into the next statement—that is, the body ends in break, continue, goto, or return—the unnecessary else is omitted.

f, err := os.Open(name)
if err != nil {
    return err
}
codeUsing(f)

This is an example of a common situation where code must guard against a sequence of error conditions. The code reads well if the successful flow of control runs down the page, eliminating error cases as they arise. Since error cases tend to end in return statements, the resulting code needs no else statements.

f, err := os.Open(name)
if err != nil {
    return err
}
d, err := f.Stat()
if err != nil {
    f.Close()
    return err
}
codeUsing(f, d)

Redeclaration and reassignment

An aside: The last example in the previous section demonstrates a detail of how the := short declaration form works. The declaration that calls os.Open reads,

f, err := os.Open(name)

This statement declares two variables, f and err. A few lines later, the call to f.Stat reads,

d, err := f.Stat()

which looks as if it declares d and err. Notice, though, that err appears in both statements. This duplication is legal: err is declared by the first statement, but only re-assigned in the second. This means that the call to f.Stat uses the existing err variable declared above, and just gives it a new value.

In a := declaration a variable v may appear even if it has already been declared, provided:

• this declaration is in the same scope as the existing declaration of v (if v is already declared in an outer scope, the declaration will create a new variable §),
• the corresponding value in the initialization is assignable to v, and
• there is at least one other variable that is created by the declaration.

This unusual property is pure pragmatism, making it easy to use a single err value, for example, in a long if-else chain. You'll see it used often.

§ It's worth noting here that in Go the scope of function parameters and return values is the same as the function body, even though they appear lexically outside the braces that enclose the body.

For

The Go for loop is similar to—but not the same as—C's. It unifies for and while and there is no do-while. There are three forms, only one of which has semicolons.

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

// Like a C while
for condition { }

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

Short declarations make it easy to declare the index variable right in the loop.

sum := 0
for i := 0; i < 10; i++ {
    sum += i
}

If you're looping over an array, slice, string, or map, or reading from a channel, a range clause can manage the loop.

for key, value := range oldMap {
    newMap[key] = value
}

If you only need the first item in the range (the key or index), drop the second:

for key := range m {
    if key.expired() {
        delete(m, key)
    }
}

If you only need the second item in the range (the value), use the blank identifier, an underscore, to discard the first:

sum := 0
for _, value := range array {
    sum += value
}

The blank identifier has many uses, as described in a later section.

For strings, the range does more work for you, breaking out individual Unicode code points by parsing the UTF-8. Erroneous encodings consume one byte and produce the replacement rune U+FFFD. (The name (with associated builtin type) rune is Go terminology for a single Unicode code point. See the language specification for details.) The loop

for pos, char := range "日本\x80語" { // \x80 is an illegal UTF-8 encoding
    fmt.Printf("character %#U starts at byte position %d\n", char, pos)
}

prints

character U+65E5 '日' starts at byte position 0
character U+672C '本' starts at byte position 3
character U+FFFD '�' starts at byte position 6
character U+8A9E '語' starts at byte position 7

Finally, Go has no comma operator and ++ and -- are statements not expressions. Thus if you want to run multiple variables in a for you should use parallel assignment (although that precludes ++ and --).

// Reverse a
for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
    a[i], a[j] = a[j], a[i]
}

Switch

Go's switch is more general than C's. The expressions need not be constants or even integers, the cases are evaluated top to bottom until a match is found, and if the switch has no expression it switches on true. It's therefore possible—and idiomatic—to write an if-else-if-else chain as a switch.

func unhex(c byte) byte {
    switch {
    case '0' <= c && c <= '9':
        return c - '0'
    case 'a' <= c && c <= 'f':
        return c - 'a' + 10
    case 'A' <= c && c <= 'F':
        return c - 'A' + 10
    }
    return 0
}

There is no automatic fall through, but cases can be presented in comma-separated lists.

func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&', '=', '#', '+', '%':
        return true
    }
    return false
}

Although they are not nearly as common in Go as some other C-like languages, break statements can be used to terminate a switch early. Sometimes, though, it's necessary to break out of a surrounding loop, not the switch, and in Go that can be accomplished by putting a label on the loop and "breaking" to that label. This example shows both uses.

Loop:
    for n := 0; n < len(src); n += size {
        switch {
        case src[n] < sizeOne:
            if validateOnly {
                break
            }
            size = 1
            update(src[n])

        case src[n] < sizeTwo:
            if n+1 >= len(src) {
                err = errShortInput
                break Loop
            }
            if validateOnly {
                break
            }
            size = 2
            update(src[n] + src[n+1]<<shift)
        }
    }

Of course, the continue statement also accepts an optional label but it applies only to loops.

To close this section, here's a comparison routine for byte slices that uses two switch statements:

// Compare returns an integer comparing the two byte slices,
// lexicographically.
// The result will be 0 if a == b, -1 if a < b, and +1 if a > b
func Compare(a, b []byte) int {
    for i := 0; i < len(a) && i < len(b); i++ {
        switch {
        case a[i] > b[i]:
            return 1
        case a[i] < b[i]:
            return -1
        }
    }
    switch {
    case len(a) > len(b):
        return 1
    case len(a) < len(b):
        return -1
    }
    return 0
}

Type switch

A switch can also be used to discover the dynamic type of an interface variable. Such a type switch uses the syntax of a type assertion with the keyword type inside the parentheses. If the switch declares a variable in the expression, the variable will have the corresponding type in each clause. It's also idiomatic to reuse the name in such cases, in effect declaring a new variable with the same name but a different type in each case.

var t interface{}
t = functionOfSomeType()
switch t := t.(type) {
default:
    fmt.Printf("unexpected type %T\n", t)     // %T prints whatever type t has
case bool:
    fmt.Printf("boolean %t\n", t)             // t has type bool
case int:
    fmt.Printf("integer %d\n", t)             // t has type int
case *bool:
    fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool
case *int:
    fmt.Printf("pointer to integer %d\n", *t) // t has type *int
}

Functions

Multiple return values

One of Go's unusual features is that functions and methods can return multiple values. This form can be used to improve on a couple of clumsy idioms in C programs: in-band error returns such as -1 for EOF and modifying an argument passed by address.

In C, a write error is signaled by a negative count with the error code secreted away in a volatile location. In Go, Write can return a count and an error: “Yes, you wrote some bytes but not all of them because you filled the device”. The signature of the Write method on files from package os is:

func (file *File) Write(b []byte) (n int, err error)

and as the documentation says, it returns the number of bytes written and a non-nil error when n != len(b). This is a common style; see the section on error handling for more examples.

A similar approach obviates the need to pass a pointer to a return value to simulate a reference parameter. Here's a simple-minded function to grab a number from a position in a byte slice, returning the number and the next position.

func nextInt(b []byte, i int) (int, int) {
    for ; i < len(b) && !isDigit(b[i]); i++ {
    }
    x := 0
    for ; i < len(b) && isDigit(b[i]); i++ {
        x = x*10 + int(b[i]) - '0'
    }
    return x, i
}

You could use it to scan the numbers in an input slice b like this:

    for i := 0; i < len(b); {
        x, i = nextInt(b, i)
        fmt.Println(x)
    }

Named result parameters

The return or result "parameters" of a Go function can be given names and used as regular variables, just like the incoming parameters. When named, they are initialized to the zero values for their types when the function begins; if the function executes a return statement with no arguments, the current values of the result parameters are used as the returned values.

The names are not mandatory but they can make code shorter and clearer: they're documentation. If we name the results of nextInt it becomes obvious which returned int is which.

func nextInt(b []byte, pos int) (value, nextPos int) {

Because named results are initialized and tied to an unadorned return, they can simplify as well as clarify. Here's a version of io.ReadFull that uses them well:

func ReadFull(r Reader, buf []byte) (n int, err error) {
    for len(buf) > 0 && err == nil {
        var nr int
        nr, err = r.Read(buf)
        n += nr
        buf = buf[nr:]
    }
    return
}

Defer

Go's defer statement schedules a function call (the deferred function) to be run immediately before the function executing the defer returns. It's an unusual but effective way to deal with situations such as resources that must be released regardless of which path a function takes to return. The canonical examples are unlocking a mutex or closing a file.

// Contents returns the file's contents as a string.
func Contents(filename string) (string, error) {
    f, err := os.Open(filename)
    if err != nil {
        return "", err
    }
    defer f.Close()  // f.Close will run when we're finished.

    var result []byte
    buf := make([]byte, 100)
    for {
        n, err := f.Read(buf[0:])
        result = append(result, buf[0:n]...) // append is discussed later.
        if err != nil {
            if err == io.EOF {
                break
            }
            return "", err  // f will be closed if we return here.
        }
    }
    return string(result), nil // f will be closed if we return here.
}

Deferring a call to a function such as Close has two advantages. First, it guarantees that you will never forget to close the file, a mistake that's easy to make if you later edit the function to add a new return path. Second, it means that the close sits near the open, which is much clearer than placing it at the end of the function.

The arguments to the deferred function (which include the receiver if the function is a method) are evaluated when the defer executes, not when the call executes. Besides avoiding worries about variables changing values as the function executes, this means that a single deferred call site can defer multiple function executions. Here's a silly example.

for i := 0; i < 5; i++ {
    defer fmt.Printf("%d ", i)
}

Deferred functions are executed in LIFO order, so this code will cause 4 3 2 1 0 to be printed when the function returns. A more plausible example is a simple way to trace function execution through the program. We could write a couple of simple tracing routines like this:

func trace(s string)   { fmt.Println("entering:", s) }
func untrace(s string) { fmt.Println("leaving:", s) }

// Use them like this:
func a() {
    trace("a")
    defer untrace("a")
    // do something....
}

We can do better by exploiting the fact that arguments to deferred functions are evaluated when the defer executes. The tracing routine can set up the argument to the untracing routine. This example:

func trace(s string) string {
    fmt.Println("entering:", s)
    return s
}

func un(s string) {
    fmt.Println("leaving:", s)
}

func a() {
    defer un(trace("a"))
    fmt.Println("in a")
}

func b() {
    defer un(trace("b"))
    fmt.Println("in b")
    a()
}

func main() {
    b()
}

prints

entering: b
in b
entering: a
in a
leaving: a
leaving: b

For programmers accustomed to block-level resource management from other languages, defer may seem peculiar, but its most interesting and powerful applications come precisely from the fact that it's not block-based but function-based. In the section on panic and recover we'll see another example of its possibilities.

Data

Allocation with new

Go has two allocation primitives, the built-in functions new and make. They do different things and apply to different types, which can be confusing, but the rules are simple. Let's talk about new first. It's a built-in function that allocates memory, but unlike its namesakes in some other languages it does not initialize the memory, it only zeros it. That is, new(T) allocates zeroed storage for a new variable of type T and returns its address, a value of type *T. In Go terminology, it returns a pointer to a newly allocated zero value of type T.

Starting with Go 1.26, new also accepts an (value) expression as an argument, which specifies the initial value of the variable. For example, new(int64(300)) allocates a new variable of type int64, initialized to 300, and returns its address.

Since the memory returned by new is zeroed, it's helpful to arrange when designing your data structures that the zero value of each type can be used without further initialization. This means a user of the data structure can create one with new and get right to work. For example, the documentation for bytes.Buffer states that "the zero value for Buffer is an empty buffer ready to use." Similarly, sync.Mutex does not have an explicit constructor or Init method. Instead, the zero value for a sync.Mutex is defined to be an unlocked mutex.

The zero-value-is-useful property works transitively. Consider this type declaration.

type SyncedBuffer struct {
    lock    sync.Mutex
    buffer  bytes.Buffer
}

Values of type SyncedBuffer are also ready to use immediately upon allocation or just declaration. In the next snippet, both p and v will work correctly without further arrangement.

p := new(SyncedBuffer)  // type *SyncedBuffer
var v SyncedBuffer      // type  SyncedBuffer

Constructors and composite literals

Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example derived from package os.

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := new(File)
    f.fd = fd
    f.name = name
    f.dirinfo = nil
    f.nepipe = 0
    return f
}

There's a lot of boilerplate in there. We can simplify it using a composite literal, which is an expression that creates a new instance each time it is evaluated.

func NewFile(fd int, name string) *File {
    if fd < 0 {
        return nil
    }
    f := File{fd, name, nil, 0}
    return &f
}

Note that, unlike in C, it's perfectly OK to return the address of a local variable; the storage associated with the variable survives after the function returns. In fact, taking the address of a composite literal allocates a fresh instance each time it is evaluated, so we can combine these last two lines.

    return &File{fd, name, nil, 0}

The fields of a composite literal are laid out in order and must all be present. However, by labeling the elements explicitly as field:value pairs, the initializers can appear in any order, with the missing ones left as their respective zero values. Thus we could say

    return &File{fd: fd, name: name}

As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expressions new(File) and &File{} are equivalent.

Composite literals can also be created for arrays, slices, and maps, with the field labels being indices or map keys as appropriate. In these examples, the initializations work regardless of the values of Enone, Eio, and Einval, as long as they are distinct.

a := [...]string   {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
s := []string      {Enone: "no error", Eio: "Eio", Einval: "invalid argument"}
m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"}

Allocation with make

Back to allocation. The built-in function make(T, args) serves a purpose different from new(T). It creates slices, maps, and channels only, and it returns an initialized (not zeroed) value of type T (not *T). The reason for the distinction is that these three types represent, under the covers, references to data structures that must be initialized before use. A slice, for example, is a three-item descriptor containing a pointer to the data (inside an array), the length, and the capacity, and until those items are initialized, the slice is nil. For slices, maps, and channels, make initializes the internal data structure and prepares the value for use. For instance,

make([]int, 10, 100)

allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of the array. (When making a slice, the capacity can be omitted; see the section on slices for more information.) In contrast, new([]int) returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a nil slice value.

These examples illustrate the difference between new and make.

var p *[]int = new([]int)       // allocates slice structure; *p == nil; rarely useful
var v  []int = make([]int, 100) // the slice v now refers to a new array of 100 ints

// Unnecessarily complex:
var p *[]int = new([]int)
*p = make([]int, 100, 100)

// Idiomatic:
v := make([]int, 100)

Remember that make applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with new or take the address of a variable explicitly.

Arrays

Arrays are useful when planning the detailed layout of memory and sometimes can help avoid allocation, but primarily they are a building block for slices, the subject of the next section. To lay the foundation for that topic, here are a few words about arrays.

There are major differences between the ways arrays work in Go and C. In Go,

• Arrays are values. Assigning one array to another copies all the elements.
• In particular, if you pass an array to a function, it will receive a copy of the array, not a pointer to it.
• The size of an array is part of its type. The types [10]int and [20]int are distinct.

The value property can be useful but also expensive; if you want C-like behavior and efficiency, you can pass a pointer to the array.

func Sum(a *[3]float64) (sum float64) {
    for _, v := range *a {
        sum += v
    }
    return
}

array := [...]float64{7.0, 8.5, 9.1}
x := Sum(&array)  // Note the explicit address-of operator

But even this style isn't idiomatic Go. Use slices instead.

Slices

Slices wrap arrays to give a more general, powerful, and convenient interface to sequences of data. Except for items with explicit dimension such as transformation matrices, most array programming in Go is done with slices rather than simple arrays.

Slices hold references to an underlying array, and if you assign one slice to another, both refer to the same array. If a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array. A Read function can therefore accept a slice argument rather than a pointer and a count; the length within the slice sets an upper limit of how much data to read. Here is the signature of the Read method of the File type in package os:

func (f *File) Read(buf []byte) (n int, err error)

The method returns the number of bytes read and an error value, if any. To read into the first 32 bytes of a larger buffer buf, slice (here used as a verb) the buffer.

    n, err := f.Read(buf[0:32])

Such slicing is common and efficient. In fact, leaving efficiency aside for the moment, the following snippet would also read the first 32 bytes of the buffer.

    var n int
    var err error
    for i := 0; i < 32; i++ {
        nbytes, e := f.Read(buf[i:i+1])  // Read one byte.
        n += nbytes
        if nbytes == 0 || e != nil {
            err = e
            break
        }
    }

The length of a slice may be changed as long as it still fits within the limits of the underlying array; just assign it to a slice of itself. The capacity of a slice, accessible by the built-in function cap, reports the maximum length the slice may assume. Here is a function to append data to a slice. If the data exceeds the capacity, the slice is reallocated. The resulting slice is returned. The function uses the fact that len and cap are legal when applied to the nil slice, and return 0.

func Append(slice, data []byte) []byte {
    l := len(slice)
    if l + len(data) > cap(slice) {  // reallocate
        // Allocate double what's needed, for future growth.
        newSlice := make([]byte, (l+len(data))*2)
        // The copy function is predeclared and works for any slice type.
        copy(newSlice, slice)
        slice = newSlice
    }
    slice = slice[0:l+len(data)]
    copy(slice[l:], data)
    return slice
}

We must return the slice afterwards because, although Append can modify the elements of slice, the slice itself (the run-time data structure holding the pointer, length, and capacity) is passed by value.

The idea of appending to a slice is so useful it's captured by the append built-in function. To understand that function's design, though, we need a little more information, so we'll return to it later.

Two-dimensional slices

Go's arrays and slices are one-dimensional. To create the equivalent of a 2D array or slice, it is necessary to define an array-of-arrays or slice-of-slices, like this:

type Transform [3][3]float64  // A 3x3 array, really an array of arrays.
type LinesOfText [][]byte     // A slice of byte slices.

Because slices are variable-length, it is possible to have each inner slice be a different length. That can be a common situation, as in our LinesOfText example: each line has an independent length.

text := LinesOfText{
    []byte("Now is the time"),
    []byte("for all good gophers"),
    []byte("to bring some fun to the party."),
}

Sometimes it's necessary to allocate a 2D slice, a situation that can arise when processing scan lines of pixels, for instance. There are two ways to achieve this. One is to allocate each slice independently; the other is to allocate a single array and point the individual slices into it. Which to use depends on your application. If the slices might grow or shrink, they should be allocated independently to avoid overwriting the next line; if not, it can be more efficient to construct the object with a single allocation. For reference, here are sketches of the two methods. First, a line at a time:

// Allocate the top-level slice.
picture := make([][]uint8, YSize) // One row per unit of y.
// Loop over the rows, allocating the slice for each row.
for i := range picture {
    picture[i] = make([]uint8, XSize)
}

And now as one allocation, sliced into lines:

// Allocate the top-level slice, the same as before.
picture := make([][]uint8, YSize) // One row per unit of y.
// Allocate one large slice to hold all the pixels.
pixels := make([]uint8, XSize*YSize) // Has type []uint8 even though picture is [][]uint8.
// Loop over the rows, slicing each row from the front of the remaining pixels slice.
for i := range picture {
    picture[i], pixels = pixels[:XSize], pixels[XSize:]
}

Maps

Maps are a convenient and powerful built-in data structure that associate values of one type (the key) with values of another type (the element or value). The key can be of any type for which the equality operator is defined, such as integers, floating point and complex numbers, strings, pointers, interfaces (as long as the dynamic type supports equality), structs and arrays. Slices cannot be used as map keys, because equality is not defined on them. Like slices, maps hold references to an underlying data structure. If you pass a map to a function that changes the contents of the map, the changes will be visible in the caller.

Maps can be constructed using the usual composite literal syntax with colon-separated key-value pairs, so it's easy to build them during initialization.

var timeZone = map[string]int{
    "UTC":  0*60*60,
    "EST": -5*60*60,
    "CST": -6*60*60,
    "MST": -7*60*60,
    "PST": -8*60*60,
}

Assigning and fetching map values looks syntactically just like doing the same for arrays and slices except that the index doesn't need to be an integer.

offset := timeZone["EST"]

An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. A set can be implemented as a map with value type bool. Set the map entry to true to put the value in the set, and then test it by simple indexing.

attended := map[string]bool{
    "Ann": true,
    "Joe": true,
    ...
}

if attended[person] { // will be false if person is not in the map
    fmt.Println(person, "was at the meeting")
}

Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that 0 because it's not in the map at all? You can discriminate with a form of multiple assignment.

var seconds int
var ok bool
seconds, ok = timeZone[tz]

For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report:

func offset(tz string) int {
    if seconds, ok := timeZone[tz]; ok {
        return seconds
    }
    log.Println("unknown time zone:", tz)
    return 0
}

To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value.

_, present := timeZone[tz]

To delete a map entry, use the delete built-in function, whose arguments are the map and the key to be deleted. It's safe to do this even if the key is already absent from the map.

delete(timeZone, "PDT")  // Now on Standard Time

Printing

Formatted printing in Go uses a style similar to C's printf family but is richer and more general. The functions live in the fmt package and have capitalized names: fmt.Printf, fmt.Fprintf, fmt.Sprintf and so on. The string functions (Sprintf etc.) return a string rather than filling in a provided buffer.

You don't need to provide a format string. For each of Printf, Fprintf and Sprintf there is another pair of functions, for instance Print and Println. These functions do not take a format string but instead generate a default format for each argument. The Println versions also insert a blank between arguments and append a newline to the output while the Print versions add blanks only if the operand on neither side is a string. In this example each line produces the same output.

fmt.Printf("Hello %d\n", 23)
fmt.Fprint(os.Stdout, "Hello ", 23, "\n")
fmt.Println("Hello", 23)
fmt.Println(fmt.Sprint("Hello ", 23))

The formatted print functions fmt.Fprint and friends take as a first argument any object that implements the io.Writer interface; the variables os.Stdout and os.Stderr are familiar instances.

Here things start to diverge from C. First, the numeric formats such as %d do not take flags for signedness or size; instead, the printing routines use the type of the argument to decide these properties.

var x uint64 = 1<<64 - 1
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x))

prints

18446744073709551615 ffffffffffffffff; -1 -1

If you just want the default conversion, such as decimal for integers, you can use the catchall format %v (for “value”); the result is exactly what Print and Println would produce. Moreover, that format can print any value, even arrays, slices, structs, and maps. Here is a print statement for the time zone map defined in the previous section.

fmt.Printf("%v\n", timeZone)  // or just fmt.Println(timeZone)

which gives output:

map[CST:-21600 EST:-18000 MST:-25200 PST:-28800 UTC:0]

For maps, Printf and friends sort the output lexicographically by key.

When printing a struct, the modified format %+v annotates the fields of the structure with their names, and for any value the alternate format %#v prints the value in full Go syntax.

type T struct {
    a int
    b float64
    c string
}
t := &T{ 7, -2.35, "abc\tdef" }
fmt.Printf("%v\n", t)
fmt.Printf("%+v\n", t)
fmt.Printf("%#v\n", t)
fmt.Printf("%#v\n", timeZone)

prints

&{7 -2.35 abc   def}
&{a:7 b:-2.35 c:abc     def}
&main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string]int{"CST":-21600, "EST":-18000, "MST":-25200, "PST":-28800, "UTC":0}

(Note the ampersands.) That quoted string format is also available through %q when applied to a value of type string or []byte. The alternate format %#q will use backquotes instead if possible. (The %q format also applies to integers and runes, producing a single-quoted rune constant.) Also, %x works on strings, byte arrays and byte slices as well as on integers, generating a long hexadecimal string, and with a space in the format (% x) it puts spaces between the bytes.

Another handy format is %T, which prints the type of a value.

fmt.Printf("%T\n", timeZone)

prints

map[string]int

If you want to control the default format for a custom type, all that's required is to define a method with the signature String() string on the type. For our simple type T, that might look like this.

func (t *T) String() string {
    return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c)
}
fmt.Printf("%v\n", t)

to print in the format