fmt.Printf formatting tutorial and cheat sheet

• Basics
  • Printf
  • Sprintf (format without printing)
  • Find fmt errors with vet
• Cheat sheet
  • Default formats and type
  • Integer (indent, base, sign)
  • Character (quoted, Unicode)
  • Boolean (true/false)
  • Pointer (hex)
  • Float (indent, precision, scientific notation)
  • String or byte slice (quote, indent, hex)
  • Special values

Basics

With the Go fmt package you can format numbers and strings padded with spaces or zeroes, in different bases, and with optional quotes.

You submit a template string that contains the text you want to format plus some annotation verbs that tell the fmt functions how to format the trailing arguments.

Printf

In this example, fmt.Printf formats and writes to standard output:

fmt.Printf("Binary: %b\\%b", 4, 5) // Prints `Binary: 100\101`

• the template string is "Binary: %b\\%b",
• the annotation verb %b formats a number in binary, and
• the special value \\ is a backslash.

As a special case, the verb %%, which consumes no argument, produces a percent sign:

fmt.Printf("%d %%", 50) // Prints `50 %`

Sprintf (format without printing)

Use fmt.Sprintf to format a string without printing it:

s := fmt.Sprintf("Binary: %b\\%b", 4, 5) // s == `Binary: 100\101`

Find fmt errors with vet

If you try to compile and run this incorrect line of code

fmt.Printf("Binary: %b\\%b", 4) // An argument to Printf is missing.

you’ll find that the program will compile, and then print

Binary: 100\%!b(MISSING)

To catch this type of errors early, you can use the vet command – it can find calls whose arguments do not align with the format string.

$ go vet example.go
example.go:8: missing argument for Printf("%b"): format reads arg 2, have only 1 args

Cheat sheet

Default formats and type

• Value: []int64{0, 1}

Integer (indent, base, sign)

• Value: 15

Character (quoted, Unicode)

• Value: 65   (Unicode letter A)

Boolean (true/false)

Use %t to format a boolean as true or false.

Pointer (hex)

Use %p to format a pointer in base 16 notation with leading 0x.

Float (indent, precision, scientific notation)

• Value: 123.456

String or byte slice (quote, indent, hex)

• Value: "café"

Special values

Arbitrary values can be encoded with backslash escapes and can be used in any "" string literal.

There are four different formats:

• \x followed by exactly two hexadecimal digits,
• \ followed by exactly three octal digits,
• \u followed by exactly four hexadecimal digits,
• \U followed by exactly eight hexadecimal digits.

The escapes \u and \U represent Unicode code points.

fmt.Println("\\caf\u00e9") // Prints \café

Further reading

40+ practical string tips [cheat sheet]

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