Go Package fmt

External

• https://golang.org/pkg/fmt

Internal

• Standard Library

Formatting

Conversion Characters

%v

Render the value as string in the default format.

"%v" and fmt.Errorf() is typically used in the error handling idiom that involves error annotation.

%+v

When printing structs, the plus flag %+v adds field names.

%#v

A Go-syntax representation of the value.

%#v can be used to show that an error instance returned by errors.New("something") is a pointer:

&errors.errorString{s:"something"}

%T

A Go-syntax representation of the type of the value.

%%

A literal percent sign. Consumes no value.

Strings

color := "blue"
fmt.Printf("The color is %s\n", color)

Integers

size := 1
fmt.Printf("The size is %d\n", size)

Characters

c := 'x'
fmt.Printf("The character is %c\n", c)

Booleans

b := true
fmt.Printf("The boolean value is %t\n", b)

Floating Point Numbers

f := 1.0
fmt.Printf("The floating point value is %f\n", f)

Pointers

Pointers in Go | Displaying Pointers

Structs

Go Structs | Printing Structs

Leading Zero Padding for Integers

i := 7
s := fmt.Sprintf("%06d", i) // will produce "000007" (five zeroes)

Leading Space Padding for Integer

i := 7
s := fmt.Sprintf("%6d", i) // will produce "     7" (five spaces)

Functions

Sprintf()

Format a string and returns it as a result:

message := fmt.Sprintf("Hi, %v. Welcome!", name)

For more details on the format string, see:

Printf() Format String

Printf(), Println()

Printing to stdout and stderr

Scanf(), Scanln()

Handling stdin in Go

Errorf()

Go Language Error Handling

Interfaces

fmt.Stringer

type Stringer interface {
  String() string
}

Stringer is implemented by any type that has a String() method, which defines the "native" format for that value. The String() method is used to print values passed as an operand to any format that accepts a string or to an unformatted printer such as Print. For a usage example, see:

The Equivalent of Java toString() in Go