Summary - fmt package

Combine Printf, Sprintf and Errorf to write a reporter.

fmt package summary

Three families, three targets: Print* → stdout, Fprint* → io.Writer, Sprint* → string; plus Errorf → error. Verbs: %v %+v %#v %T general, %d %x %b integers, %s %q text, %f %.2f %e decimals. Width/alignment: %8s, %-8s, %08d. String() for your own types, %w for errors.

Explicit argument indexes

To use the same value several times, the %[n] form refers to an argument by its number:

fmt.Printf("%[1]d onluk = %[1]x onaltılık = %[1]b ikili\n", 10)
// 10 onluk = a onaltılık = 1010 ikili

It is also handy in translation strings where word order changes by language.

Exercise Number bases

Print the number 42 in decimal, hex and binary with a single Printf using argument indexes: "42 2a 101010".

Show example solution

package main

import "fmt"

func main() {
    fmt.Printf("%[1]d %[1]x %[1]b\n", 42)
}

Test yourself

Which shows both a value's field names and its type name?
- %s
- %d
- %#v
- %v
Show answer

%#v — %#v writes the value in Go syntax: main.Kisi{Ad:"Ada", Yas:36}. %+v shows field names but not the type.

Same argument

fmt.Printf("%d = %____x
", 255)

Show answer

fmt.Printf("%d = %[1]x
", 255)

Group summary

Sprintf

Produces a string
Fprintf

Writes to a Writer
Errorf + %w

Wraps an error
String()

Custom display

Next lesson: The encoding/json package

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies