Appendix B. Formatting Cheat Sheet
This appendix is a compact reference to Go’s Printf-style formatting, showing how to turn values into text with a variety of verbs and flags. It highlights the default and type-aware representations alongside controls for width, precision, alignment, padding, and numerical bases. The emphasis is on practical outcomes—what a given verb prints for a given value—so developers can quickly pick the right specifier.
General verbs include %v for the default view, %#v for Go-syntax output, and %T for the value’s type. For integers, it covers decimal and sign handling (%d, %+d), field width and alignment (%4d, %-4d), zero padding (%04d), and base conversions (%b, %o, %x/%X, %#x). Floating-point formats span scientific (%e), fixed (%f) with adjustable precision (like %.2f) and width/padding (%8.2f, %08.2f), plus the compact %g. Character and text sections show runes and strings as raw or quoted (%c, %q), Unicode code points (%U, %#U), string alignment (%s, %8s, %-8s), and byte-level hex dumps for strings/byte slices (%x, % x). Booleans print with %t, and pointers use %p in 0x-prefixed hexadecimal.
It also catalogs special escape sequences for control characters such as alert, backspace, tab, newline, form feed, carriage return, vertical tab, and backslash, along with the way to print a literal percent using %%. All Unicode code points can be embedded in string literals via backslash escapes in four forms: \xHH (two hex digits), \ooo (three octal digits), \uHHHH (four hex digits), and \UHHHHHHHH (eight hex digits). An example demonstrates inserting a Unicode character in a normal string with a \u escape.
FAQ
What are the default formatting verbs to inspect values?
%v prints the value in a default format; %#v prints a Go-syntax representation; %T prints the value’s type. Example with[]int64{0, 1}: %v → [0 1], %#v → []int64{0, 1}, %T → []int64.How do I print integers in different bases?
Use %d (decimal), %b (binary), %o (octal), %x (hex lowercase), %X (hex uppercase). Add# to %x for a 0x prefix: %#x. Example with 15: %d → 15, %b → 1111, %o → 17, %x → f, %X → F, %#x → 0xf.How can I always show the sign with integers?
Use%+d. Example: fmt.Printf("%+d", 15) prints +15 (negatives keep their minus sign).How do I pad or justify integers?
Use a width with %d.%4d pads with spaces on the left (right-justified). %-4d pads on the right (left-justified). %04d pads with leading zeros. Example with 15: "%4d" → ␣␣15, "%-4d" → 15␣␣, "%04d" → 0015.What’s the difference between %f, %e, and %g for floats?
%f uses fixed-point (default precision 6). %e uses scientific notation. %g picks the more compact of %e or %f and omits unnecessary zeros/exponent. Example with 123.456:%f → 123.456000, %e → 1.234560e+02, %g → 123.456.How do I control float precision, width, and zero-padding?
Use%[width].[prec]f. Examples with 123.456: %.2f → 123.46, %8.2f → ␣␣123.46, %08.2f → 00123.46.How do I format runes/characters and show their Unicode code points?
%c prints the character, %q prints a quoted character, %U prints the Unicode code point (e.g., U+0041), and %#U prints both code point and character. Example with 'A':%c → A, %q → 'A', %U → U+0041, %#U → U+0041 'A'.How do I format strings or byte slices (including hex dumps)?
%s prints the string; %q prints a quoted string. Width and justification work:%8s (right), %-8s (left). For byte values as hex, use %x (no spaces) or % x (space-separated). Example "gophers": %s → gophers, %q → "gophers", %x → 676f7068657273, % x → 67 6f 70 68 65 72 73.How do I print booleans and pointers?
%t prints booleans astrue or false. %p prints a pointer address in hex with a leading 0x. Example: fmt.Printf("%t", true) → true; fmt.Printf("%p", new(int)) → e.g., 0xc0000b2000.What escape sequences and Unicode escapes can I use in strings?
Common escapes:\a (bell), \b (backspace), \\ (backslash), \t (tab), \n (newline), \f (form feed), \r (carriage return), \v (vertical tab). To print a percent sign with formatting, use %%. Unicode escapes: \xHH (2 hex), \ooo (3 octal), \uHHHH (4 hex), \UHHHHHHHH (8 hex). Example: "... endear\u00e8d ...".
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