Top fmt formatting tips [cheat sheet]

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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`

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

Format Verb Description
[0 1] %v Default format
[]int64{0, 1} %#v Go-syntax format
[]int64 %T The type of the value

Integer (indent, base, sign)

Format Verb Description
15 %d Base 10
+15 %+d Always show sign
␣␣15 %4d Pad with spaces (width 4, right justified)
15␣␣ %-4d Pad with spaces (width 4, left justified)
0015 %04d Pad with zeroes (width 4)
1111 %b Base 2
17 %o Base 8
f %x Base 16, lowercase
F %X Base 16, uppercase
0xf %#x Base 16, with leading 0x

Character (quoted, Unicode)

Format Verb Description
A %c Character
'A' %q Quoted character
U+0041 %U Unicode
U+0041 'A' %#U Unicode with character

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)

Format Verb Description
1.234560e+02 %e Scientific notation
123.456000 %f Decimal point, no exponent
123.46 %.2f Default width, precision 2
␣␣123.46 %8.2f Width 8, precision 2
123.456 %g Exponent as needed, necessary digits only

String or byte slice (quote, indent, hex)

Format Verb Description
café %s Plain string
␣␣café %6s Width 6, right justify
café␣␣ %-6s Width 6, left justify
"café" %q Quoted string
636166c3a9 %x Hex dump of byte values
63 61 66 c3 a9 % x Hex dump with spaces

Special values

Value Description
\a U+0007 alert or bell
\b U+0008 backspace
\\ U+005c backslash
\t U+0009 horizontal tab
\n U+000A line feed or newline
\f U+000C form feed
\r U+000D carriage return
\v U+000b vertical tab

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

There are four different formats:

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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