Overview
Package fmt implements formatted I/O with functions analogous to C’s printf and scanf. The format ‘verbs’ are derived from C’s but are simpler.
Printing Functions
There are four families of printing functions:
- Print, Println, Printf - write to standard output
- Sprint, Sprintln, Sprintf - return a string
- Fprint, Fprintln, Fprintf - write to an io.Writer
- Append, Appendln, Appendf - append to a byte slice
Printf
Formats according to a format specifier and writes to standard output.
func Printf(format string, a ...any) (n int, err error)
Format string with verbs like %v, %d, %s, etc.
Arguments to format according to the format string
Any write error encountered
fmt.Printf("Hello, %s! You are %d years old.\n", "Alice", 30)
// Output: Hello, Alice! You are 30 years old.
Sprintf
Formats according to a format specifier and returns the resulting string.
func Sprintf(format string, a ...any) string
s := fmt.Sprintf("Score: %d/%d", 85, 100)
// s = "Score: 85/100"
Fprintf
Formats according to a format specifier and writes to w.
func Fprintf(w io.Writer, format string, a ...any) (n int, err error)
Writer to output formatted text to
Any write error encountered
var buf bytes.Buffer
fmt.Fprintf(&buf, "Value: %v", 42)
Print
Formats using the default formats and writes to standard output.
func Print(a ...any) (n int, err error)
Any write error encountered
fmt.Print("Hello", " ", "World")
// Output: Hello World
Println
Formats using default formats, adds spaces between operands and a newline.
func Println(a ...any) (n int, err error)
fmt.Println("Hello", "World")
// Output: Hello World\n
Scanning Functions
Scanf
Scans text read from standard input, storing values into successive arguments.
func Scanf(format string, a ...any) (n int, err error)
Format string specifying how to parse input
Pointers to variables to store scanned values
Number of items successfully scanned
var name string
var age int
fmt.Scanf("%s %d", &name, &age)
Scan
Scans space-separated values from standard input.
func Scan(a ...any) (n int, err error)
var x, y int
fmt.Scan(&x, &y)
Sscanf
Scans the argument string, storing values into successive arguments.
func Sscanf(str string, format string, a ...any) (n int, err error)
Pointers to store parsed values
var name string
var age int
fmt.Sscanf("Alice 30", "%s %d", &name, &age)
General
%v - value in default format%+v - adds field names for structs%#v - Go-syntax representation%T - type of the value%% - literal percent sign
Boolean
%t - the word true or false
Integer
%b - base 2%d - base 10%o - base 8%x - base 16, lowercase%X - base 16, uppercase%c - the character%q - quoted character
Floating-point
%f - decimal point, no exponent%e - scientific notation%g - %e for large exponents, %f otherwise
String
%s - string or slice%q - double-quoted string%x - base 16, lowercase%X - base 16, uppercase
Pointer
%p - base 16 notation with leading 0x
Interfaces
Stringer
Implemented by any value that has a String method.
type Stringer interface {
String() string
}
type Person struct {
Name string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%s (%d)", p.Name, p.Age)
}
p := Person{"Alice", 30}
fmt.Println(p) // Output: Alice (30)
type Formatter interface {
Format(f State, verb rune)
}
State
Represents the printer state passed to custom formatters.
type State interface {
Write(b []byte) (n int, err error)
Width() (wid int, ok bool)
Precision() (prec int, ok bool)
Flag(c int) bool
}
Errorf
Formats according to a format specifier and returns the string as an error.
func Errorf(format string, a ...any) error
err := fmt.Errorf("user %s not found", username)
return fmt.Errorf("failed to connect: %w", originalErr) // wraps error
Complete Example
package main
import "fmt"
type Point struct {
X, Y int
}
func (p Point) String() string {
return fmt.Sprintf("(%d, %d)", p.X, p.Y)
}
func main() {
// Printing
fmt.Println("Hello, World!")
fmt.Printf("Pi: %.2f\n", 3.14159)
// String formatting
s := fmt.Sprintf("Value: %d", 42)
fmt.Println(s)
// Custom Stringer
p := Point{10, 20}
fmt.Println(p) // Output: (10, 20)
// Scanning
var name string
var age int
fmt.Print("Enter name and age: ")
fmt.Scan(&name, &age)
fmt.Printf("Hello %s, you are %d years old\n", name, age)
}