String Formatting

Go offers excellent support for string formatting in the printf tradition. Here are some examples of common string formatting tasks.

package main

import (
    "fmt"
    "os"
)

type point struct {
    x, y int
}

func main() {

    // Go offers several printing “verbs” designed to format general Go values.
    // For example, this prints an instance of our point struct.
    p := point{1, 2}
    fmt.Printf("struct1: %v\n", p)

    // If the value is a struct, the %+v variant will include the struct’s field names.
    fmt.Printf("struct2: %+v\n", p)

    // The %#v variant prints a Go syntax representation of the value
    // i.e. the source code snippet that would produce that value.
    fmt.Printf("struct3: %#v\n", p)

    // To print the type of a value, use %T.
    fmt.Printf("type: %T\n", p)

    // Formatting booleans is straight-forward.
    fmt.Printf("bool: %t\n", true)

    // There are many options for formatting integers.
    // Use %d for standard, base-10 formatting.
    fmt.Printf("int: %d\n", 123)

    // This prints a binary representation.
    fmt.Printf("bin: %b\n", 14)

    // This prints the character corresponding to the given integer.
    fmt.Printf("char: %c\n", 33)

    // %x provides hex encoding.
    fmt.Printf("hex: %x\n", 456)

    // There are also several formatting options for floats.
    // For basic decimal formatting use %f.
    fmt.Printf("float1: %f\n", 78.9)

    // %e and %E format the float in 
    // (slightly different versions of) scientific notation.
    fmt.Printf("float2: %e\n", 123400000.0)
    fmt.Printf("float3: %E\n", 123400000.0)

    // For basic string printing use %s.
    fmt.Printf("str1: %s\n", "\"string\"")

    // To double-quote strings as in Go source, use %q.
    fmt.Printf("str2: %q\n", "\"string\"")

    // As with integers seen earlier, %x renders the string in base-16
    // with two output characters per byte of input.
    fmt.Printf("str3: %x\n", "hex this")

    // To print a representation of a pointer, use %p.
    fmt.Printf("pointer: %p\n", &p)

    // When formatting numbers you will often want 
    // to control the width and precision of the resulting figure.
    // To specify the width of an integer, use a number after the % in the verb.
    // By default the result will be right-justified and padded with spaces.
    fmt.Printf("width1: |%6d|%6d|\n", 12, 345)

    // You can also specify the width of printed floats
    // though usually you’ll also want to restrict the decimal precision
    // at the same time with the width.precision syntax.
    fmt.Printf("width2: |%6.2f|%6.2f|\n", 1.2, 3.45)

    // To left-justify, use the - flag.
    fmt.Printf("width3: |%-6.2f|%-6.2f|\n", 1.2, 3.45)

    // You may also want to control width when formatting strings
    // especially to ensure that they align in table-like output.
    // For basic right-justified width.
    fmt.Printf("width4: |%6s|%6s|\n", "foo", "b")

    // To left-justify use the - flag as with numbers.
    fmt.Printf("width5: |%-6s|%-6s|\n", "foo", "b")

    // So far we’ve seen Printf
    // which prints the formatted string to os.Stdout.
    // Sprintf formats and returns a string without printing it anywhere.
    s := fmt.Sprintf("sprintf: a %s", "string")
    fmt.Println(s)

    // You can format+print to io.Writers other than os.Stdout using Fprintf.
    fmt.Fprintf(os.Stderr, "io: an %s\n", "error")
}

$ go run string-formatting.go
struct1: {1 2}
struct2: {x:1 y:2}
struct3: main.point{x:1, y:2}
type: main.point
bool: true
int: 123
bin: 1110
char: !
hex: 1c8
float1: 78.900000
float2: 1.234000e+08
float3: 1.234000E+08
str1: "string"
str2: "\"string\""
str3: 6865782074686973
pointer: 0xc0000ba000
width1: |    12|   345|
width2: |  1.20|  3.45|
width3: |1.20  |3.45  |
width4: |   foo|     b|
width5: |foo   |b     |
sprintf: a string
io: an error