fmt.Printf

Learn Printf and format verbs.

fmt.Printf

fmt.Printf takes a format string and values; the verbs in the string (placeholders starting with %) are filled with the values in order. Unlike Println it doesn't add a newline by itself.

ad, yas := "Ada", 36
fmt.Printf("%s %d yaşında\n", ad, yas)
// Ada 36 yaşında

If a verb doesn't match the value's type the program doesn't crash; a warning like %!d(string=Ada) is written to the output. go vet catches these mistakes before compiling.

Width and precision

Width and precision can be added before the verb: %8.2f means a number at least 8 characters wide with 2 decimals. - left-aligns and 0 pads with zeros:

fmt.Printf("|%6d|%-6d|%06d|\n", 42, 42, 42)
// |    42|42    |000042|
fmt.Printf("%.2f\n", 3.14159)  // 3.14

These fields are very handy for tabular output.

Exercise Price table

Print two products aligned with Printf: name left-aligned in 8 characters (%-8s), price 7 characters wide with 2 decimals (%7.2f). Products: Kalem 12.5, Defter 7.

Show example solution

package main

import "fmt"

func main() {
    fmt.Printf("%-8s%7.2f\n", "Kalem", 12.5)
    fmt.Printf("%-8s%7.2f\n", "Defter", 7.0)
}

Test yourself

What does <code>fmt.Printf("%d\n", "on")</code> print?
- 10
- on
- The program crashes
- %!d(string=on)
Show answer

%!d(string=on) — fmt marks the wrong verb in the output; there's no run-time panic. go vet warns about this line.

Two decimals

fmt.Printf("____
", 9.876)  // 9.88

Show answer

fmt.Printf("%.2f
", 9.876)  // 9.88

Format and output (42)

%d

42
%5d

" 42"
%-5d|

"42 |"
%05d

00042

Next lesson: Format verbs

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