import (
"fmt"
"io"
+ "math"
"os"
"strings"
+ "time"
)
// The Errorf function lets us use formatting features
// 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 := 'π'
+ fmt.Printf("%v %d %c %q %U %#U\n", smile, smile, smile, smile, smile, smile)
+ // Result: 128512 128512 π 'π' U+1F600 U+1F600 'π'
+
+ // 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("aβ")
+ fmt.Printf("%v %d %s %q %x % x\n", cmd, cmd, cmd, cmd, cmd, cmd)
+ // Result: [97 226 140 152] [97 226 140 152] aβ "aβ" 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 π 'π' U+1F600 U+1F600 'π'
+ // 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] aβ "aβ" 61e28c98 61 e2 8c 98
+ // 1973-11-29 21:33:09 +0000 UTC "1973-11-29 21:33:09 +0000 UTC"
+}