fmt: add a package-level example illustrating basic formats

There is much left out here—the space of possibilities is very
large—but this example shows all that most programmers will need
to know for most printing problems.

Update #27554.

Change-Id: Ib6ae651d5c3720cf7fe1a05ffd0859a5b56a9157
Reviewed-on: https://go-review.googlesource.com/136616Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/example_test.go

@@ -7,8 +7,10 @@ package fmt_test
 import (
 	"fmt"
 	"io"
+	"math"
 	"os"
 	"strings"
+	"time"
 )
 
 // The Errorf function lets us use formatting features
@@ -131,3 +133,150 @@ func ExampleSprint() {
 	// thereare99gophers
 	// 17
 }
+
+// These examples demonstrate the basics of printing using a format string. Printf,
+// Sprintf, and Fprintf all take a format string that specifies how to format the
+// subsequent arguments. For example, %d (we call that a 'verb') says to print the
+// corresponding argument, which must be an integer (or something containing an
+// integer, such as a slice of ints) in decimal. The verb %v ('v' for 'value')
+// always formats the argument in its default form, just how Print or Println would
+// show it. The special verb %T ('T' for 'Type') prints the type of the argument
+// rather than its value. The examples are not exhaustive; see the package comment
+// for all the details.
+func Example_formats() {
+	// A basic set of examples showing that %v is the default format, in this
+	// case decimal for integers, which can be explicitly requested with %d;
+	// the output is just what Println generates.
+	integer := 23
+	// Each of these prints "23" (without the quotes).
+	fmt.Println(integer)
+	fmt.Printf("%v\n", integer)
+	fmt.Printf("%d\n", integer)
+
+	// The special verb %T shows the type of an item rather than its value.
+	fmt.Printf("%T %T\n", integer, &integer)
+	// Result: int *int
+
+	// Println(x) is the same as Printf("%v\n", x) so we will use only Printf
+	// in the following examples. Each one demonstrates how to format values of
+	// a particular type, such as integers or strings. We start each format
+	// string with %v to show the default output and follow that with one or
+	// more custom formats.
+
+	// Booleans print as "true" or "false" with %v or %t.
+	truth := true
+	fmt.Printf("%v %t\n", truth, truth)
+	// Result: true true
+
+	// Integers print as decimals with %v and %d,
+	// or in hex with %x, octal with %o, or binary with %b.
+	answer := 42
+	fmt.Printf("%v %d %x %o %b\n", answer, answer, answer, answer, answer)
+	// Result: 42 42 2a 52 101010
+
+	// Floats have multiple formats: %v and %g print a compact representation,
+	// while %f prints a decimal point and %e uses exponential notation. The
+	// format %6.2f used here shows how to set the width and precision to
+	// control the appearance of a floating-point value. In this instance, 6 is
+	// the total width of the printed text for the value (note the extra spaces
+	// in the output) and 2 is the number of decimal places to show.
+	pi := math.Pi
+	fmt.Printf("%v %g %.2f (%6.2f) %e\n", pi, pi, pi, pi, pi)
+	// Result: 3.141592653589793 3.141592653589793 3.14 (  3.14) 3.141593e+00
+
+	// Complex numbers format as parenthesized pairs of floats, with an 'i'
+	// after the imaginary part.
+	point := 110.7 + 22.5i
+	fmt.Printf("%v %g %.2f %.2e\n", point, point, point, point)
+	// Result: (110.7+22.5i) (110.7+22.5i) (110.70+22.50i) (1.11e+02+2.25e+01i)
+
+	// Runes are integers but when printed with %c show the character with that
+	// Unicode value. The %q verb shows them as quoted characters, %U as a
+	// hex Unicode code point, and %#U as both a code point and a quoted
+	// printable form if the rune is printable.
+	smile := '😀'
+	fmt.Printf("%v %d %c %q %U %#U\n", smile, smile, smile, smile, smile, smile)
+	// Result: 128512 128512 😀 '😀' U+1F600 U+1F600 '😀'
+
+	// Strings are formatted with %v and %s as-is, with %q as quoted strings,
+	// and %#q as backquoted strings.
+	placeholders := `foo "bar"`
+	fmt.Printf("%v %s %q %#q\n", placeholders, placeholders, placeholders, placeholders)
+	// Result: foo "bar" foo "bar" "foo \"bar\"" `foo "bar"`
+
+	// Maps formatted with %v show keys and values in their default formats.
+	// The %#v form (the # is called a "flag" in this context) shows the map in
+	// the Go source format.
+	isLegume := map[string]bool{
+		"peanut": true,
+		// TODO: Include this line when maps are printed in deterministic order.
+		// See Issue #21095
+		// "dachshund": false,
+	}
+	fmt.Printf("%v %#v\n", isLegume, isLegume)
+	// Result: map[peanut:true] map[string]bool{"peanut":true}
+
+	// Structs formatted with %v show field values in their default formats.
+	// The %+v form shows the fields by name, while %#v formats the struct in
+	// Go source format.
+	person := struct {
+		Name string
+		Age  int
+	}{"Kim", 22}
+	fmt.Printf("%v %+v %#v\n", person, person, person)
+	// Result: {Kim 22} {Name:Kim Age:22} struct { Name string; Age int }{Name:"Kim", Age:22}
+
+	// The default format for a pointer shows the underlying value preceded by
+	// an ampersand. The %p verb prints the pointer value in hex. We use a
+	// typed nil for the argument to %p here because the value of any non-nil
+	// pointer would change from run to run; run the commented-out Printf
+	// call yourself to see.
+	pointer := &person
+	fmt.Printf("%v %p\n", pointer, (*int)(nil))
+	// Result: &{Kim 22} 0x0
+	// fmt.Printf("%v %p\n", pointer, pointer)
+	// Result: &{Kim 22} 0x010203 // See comment above.
+
+	// Arrays and slices are formatted by applying the format to each element.
+	greats := [5]string{"Katano", "Kobayashi", "Kurosawa", "Miyazaki", "Ozu"}
+	fmt.Printf("%v %q\n", greats, greats)
+	// Result: [Katano Kobayashi Kurosawa Miyazaki Ozu] ["Katano" "Kobayashi" "Kurosawa" "Miyazaki" "Ozu"]
+
+	kGreats := greats[:3]
+	fmt.Printf("%v %q %#v\n", kGreats, kGreats, kGreats)
+	// Result: [Katano Kobayashi Kurosawa] ["Katano" "Kobayashi" "Kurosawa"] []string{"Katano", "Kobayashi", "Kurosawa"}
+
+	// Byte slices are special. Integer verbs like %d print the elements in
+	// that format. The %s and %q forms treat the slice like a string. The %x
+	// verb has a special form with the space flag that puts a space between
+	// the bytes.
+	cmd := []byte("a⌘")
+	fmt.Printf("%v %d %s %q %x % x\n", cmd, cmd, cmd, cmd, cmd, cmd)
+	// Result: [97 226 140 152] [97 226 140 152] a⌘ "a⌘" 61e28c98 61 e2 8c 98
+
+	// Types that implement Stringer are printed the same as strings. Because
+	// Stringers return a string, we can print them using a string-specific
+	// verb such as %q.
+	now := time.Unix(123456789, 0).UTC() // time.Time implements fmt.Stringer.
+	fmt.Printf("%v %q\n", now, now)
+	// Result: 1973-11-29 21:33:09 +0000 UTC "1973-11-29 21:33:09 +0000 UTC"
+
+	// Output:
+	// 23
+	// 23
+	// 23
+	// int *int
+	// true true
+	// 42 42 2a 52 101010
+	// 3.141592653589793 3.141592653589793 3.14 (  3.14) 3.141593e+00
+	// (110.7+22.5i) (110.7+22.5i) (110.70+22.50i) (1.11e+02+2.25e+01i)
+	// 128512 128512 😀 '😀' U+1F600 U+1F600 '😀'
+	// foo "bar" foo "bar" "foo \"bar\"" `foo "bar"`
+	// map[peanut:true] map[string]bool{"peanut":true}
+	// {Kim 22} {Name:Kim Age:22} struct { Name string; Age int }{Name:"Kim", Age:22}
+	// &{Kim 22} 0x0
+	// [Katano Kobayashi Kurosawa Miyazaki Ozu] ["Katano" "Kobayashi" "Kurosawa" "Miyazaki" "Ozu"]
+	// [Katano Kobayashi Kurosawa] ["Katano" "Kobayashi" "Kurosawa"] []string{"Katano", "Kobayashi", "Kurosawa"}
+	// [97 226 140 152] [97 226 140 152] a⌘ "a⌘" 61e28c98 61 e2 8c 98
+	// 1973-11-29 21:33:09 +0000 UTC "1973-11-29 21:33:09 +0000 UTC"
+}