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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.

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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

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Printf

Formats according to a format specifier and writes to standard output. 
func Printf(format string, a ...any) (n int, err error)
​
format
string
required
Format string with verbs like %v, %d, %s, etc.
​
a
...any
Arguments to format according to the format string
​
n
int
Number of bytes written
​
err
error
Any write error encountered
Example: 
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
​
format
string
required
Format string with verbs
​
a
...any
Arguments to format
​
result
string
The formatted string
Example: 
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)
​
w
io.Writer
required
Writer to output formatted text to
​
format
string
required
Format string
​
a
...any
Arguments to format
​
n
int
Number of bytes written
​
err
error
Any write error encountered
Example: 
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)
​
a
...any
Values to print
​
n
int
Number of bytes written
​
err
error
Any write error encountered
Example: 
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)
Example: 
fmt.Println("Hello", "World")
// Output: Hello World\n

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Scanning Functions

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Scanf

Scans text read from standard input, storing values into successive arguments. 
func Scanf(format string, a ...any) (n int, err error)
​
format
string
required
Format string specifying how to parse input
​
a
...any
required
Pointers to variables to store scanned values
​
n
int
Number of items successfully scanned
​
err
error
Error if scanning failed
Example: 
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)
Example: 
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)
​
str
string
required
String to parse
​
format
string
required
Format string
​
a
...any
required
Pointers to store parsed values
Example: 
var name string
var age int
fmt.Sscanf("Alice 30", "%s %d", &name, &age)

​
Format Verbs

​
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

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Stringer

Implemented by any value that has a String method. 
type Stringer interface {
    String() string
}
Example: 
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)

​
Formatter

Implemented by any value that has a Format method for custom formatting. 
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
}

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Error Formatting

​
Errorf

Formats according to a format specifier and returns the string as an error. 
func Errorf(format string, a ...any) error
Example: 
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)
}

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