Packages, variables, and functions. Learn the basic components of any Go program.
The Go Authors https://golang.org
- Packages
Every Go program is made up of packages.
Programs start running in package main
.
This program is using the packages with import paths "fmt"
and "math/rand"
.
By convention, the package name is the same as the last element of the import path. For instance, the "math/rand"
package comprises files that begin with the statement package
rand`.
#appengine: Note: the environment in which these programs are executed is
#appengine: deterministic, so each time you run the example program
#appengine: rand.Intn
will return the same number.
#appengine:
#appengine: (To see a different number, seed the number generator; see [[https://golang.org/pkg/math/rand/#Seed][rand.Seed
]].
#appengine: Time is constant in the playground, so you will need to use something else as the seed.)
.play basics/packages.go
- Imports
This code groups the imports into a parenthesized, "factored" import statement.
You can also write multiple import statements, like:
import "fmt"
import "math"
But it is good style to use the factored import statement.
.play basics/imports.go
- Exported names
In Go, a name is exported if it begins with a capital letter.
For example, Pizza
is an exported name, as is Pi
, which is exported from
the math
package.
pizza
and pi
do not start with a capital letter, so they are not exported.
When importing a package, you can refer only to its exported names. Any "unexported" names are not accessible from outside the package.
Run the code. Notice the error message.
To fix the error, rename math.pi
to math.Pi
and try it again.
.play basics/exported-names.go
- Functions
A function can take zero or more arguments.
In this example, add
takes two parameters of type int
.
Notice that the type comes after the variable name.
(For more about why types look the way they do, see the [[https://blog.golang.org/gos-declaration-syntax][article on Go's declaration syntax]].)
.play basics/functions.go
- Functions continued
When two or more consecutive named function parameters share a type, you can omit the type from all but the last.
In this example, we shortened
x int, y int
to
x, y int
.play basics/functions-continued.go
- Multiple results
A function can return any number of results.
The swap
function returns two strings.
.play basics/multiple-results.go
- Named return values
Go's return values may be named. If so, they are treated as variables defined at the top of the function.
These names should be used to document the meaning of the return values.
A return
statement without arguments returns the named return values. This is known as a "naked" return.
Naked return statements should be used only in short functions, as with the example shown here. They can harm readability in longer functions.
.play basics/named-results.go
- Variables
The var
statement declares a list of variables; as in function argument lists, the type is last.
A var
statement can be at package or function level. We see both in this example.
.play basics/variables.go
- Variables with initializers
A var declaration can include initializers, one per variable.
If an initializer is present, the type can be omitted; the variable will take the type of the initializer.
.play basics/variables-with-initializers.go
- 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.
.play basics/short-variable-declarations.go
- Basic types
Go's basic types are
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
The example shows variables of several types, and also that variable declarations may be "factored" into blocks, as with import statements.
The int
, uint
, and uintptr
types are usually 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems.
When you need an integer value you should use int
unless you have a specific reason to use a sized or unsigned integer type.
.play basics/basic-types.go
- Zero values
Variables declared without an explicit initial value are given their zero_value.
The zero value is:
0
for numeric types,false
for the boolean type, and""
(the empty string) for strings.
.play basics/zero.go
- Type conversions
The expression T(v)
converts the value v
to the type T
.
Some numeric conversions:
var i int = 42
var f float64 = float64(i)
var u uint = uint(f)
Or, put more simply:
i := 42
f := float64(i)
u := uint(f)
Unlike in C, in Go assignment between items of different type requires an
explicit conversion.
Try removing the float64
or uint
conversions in the example and see what happens.
.play basics/type-conversions.go
- Type inference
When declaring a variable without specifying an explicit type (either by using the :=
syntax or var
=` expression syntax), the variable's type is inferred from the value on the right hand side.
When the right hand side of the declaration is typed, the new variable is of that same type:
var i int
j := i // j is an int
But when the right hand side contains an untyped numeric constant, the new variable may be an int
, float64
, or complex128
depending on the precision of the constant:
i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128
Try changing the initial value of v
in the example code and observe how its type is affected.
.play basics/type-inference.go
- Constants
Constants are declared like variables, but with the const
keyword.
Constants can be character, string, boolean, or numeric values.
Constants cannot be declared using the :=
syntax.
.play basics/constants.go
- Numeric Constants
Numeric constants are high-precision values.
An untyped constant takes the type needed by its context.
Try printing needInt(Big)
too.
(An int
can store at maximum a 64-bit integer, and sometimes less.)
.play basics/numeric-constants.go
- Congratulations!
You finished this lesson!
You can go back to the list of [[/list][modules]] to find what to learn next, or continue with the [[javascript:click('.next-page')][next lesson]].