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diff --git a/src/fmt/doc.go b/src/fmt/doc.go new file mode 100644 index 0000000..c584cc9 --- /dev/null +++ b/src/fmt/doc.go @@ -0,0 +1,343 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +/* + Package fmt implements formatted I/O with functions analogous + to C's printf and scanf. The format 'verbs' are derived from C's but + are simpler. + + + Printing + + The verbs: + + General: + %v the value in a default format + when printing structs, the plus flag (%+v) adds field names + %#v a Go-syntax representation of the value + %T a Go-syntax representation of the type of the value + %% a literal percent sign; consumes no value + + Boolean: + %t the word true or false + Integer: + %b base 2 + %c the character represented by the corresponding Unicode code point + %d base 10 + %o base 8 + %O base 8 with 0o prefix + %q a single-quoted character literal safely escaped with Go syntax. + %x base 16, with lower-case letters for a-f + %X base 16, with upper-case letters for A-F + %U Unicode format: U+1234; same as "U+%04X" + Floating-point and complex constituents: + %b decimalless scientific notation with exponent a power of two, + in the manner of strconv.FormatFloat with the 'b' format, + e.g. -123456p-78 + %e scientific notation, e.g. -1.234456e+78 + %E scientific notation, e.g. -1.234456E+78 + %f decimal point but no exponent, e.g. 123.456 + %F synonym for %f + %g %e for large exponents, %f otherwise. Precision is discussed below. + %G %E for large exponents, %F otherwise + %x hexadecimal notation (with decimal power of two exponent), e.g. -0x1.23abcp+20 + %X upper-case hexadecimal notation, e.g. -0X1.23ABCP+20 + String and slice of bytes (treated equivalently with these verbs): + %s the uninterpreted bytes of the string or slice + %q a double-quoted string safely escaped with Go syntax + %x base 16, lower-case, two characters per byte + %X base 16, upper-case, two characters per byte + Slice: + %p address of 0th element in base 16 notation, with leading 0x + Pointer: + %p base 16 notation, with leading 0x + The %b, %d, %o, %x and %X verbs also work with pointers, + formatting the value exactly as if it were an integer. + + The default format for %v is: + bool: %t + int, int8 etc.: %d + uint, uint8 etc.: %d, %#x if printed with %#v + float32, complex64, etc: %g + string: %s + chan: %p + pointer: %p + For compound objects, the elements are printed using these rules, recursively, + laid out like this: + struct: {field0 field1 ...} + array, slice: [elem0 elem1 ...] + maps: map[key1:value1 key2:value2 ...] + pointer to above: &{}, &[], &map[] + + Width is specified by an optional decimal number immediately preceding the verb. + If absent, the width is whatever is necessary to represent the value. + Precision is specified after the (optional) width by a period followed by a + decimal number. If no period is present, a default precision is used. + A period with no following number specifies a precision of zero. + Examples: + %f default width, default precision + %9f width 9, default precision + %.2f default width, precision 2 + %9.2f width 9, precision 2 + %9.f width 9, precision 0 + + Width and precision are measured in units of Unicode code points, + that is, runes. (This differs from C's printf where the + units are always measured in bytes.) Either or both of the flags + may be replaced with the character '*', causing their values to be + obtained from the next operand (preceding the one to format), + which must be of type int. + + For most values, width is the minimum number of runes to output, + padding the formatted form with spaces if necessary. + + For strings, byte slices and byte arrays, however, precision + limits the length of the input to be formatted (not the size of + the output), truncating if necessary. Normally it is measured in + runes, but for these types when formatted with the %x or %X format + it is measured in bytes. + + For floating-point values, width sets the minimum width of the field and + precision sets the number of places after the decimal, if appropriate, + except that for %g/%G precision sets the maximum number of significant + digits (trailing zeros are removed). For example, given 12.345 the format + %6.3f prints 12.345 while %.3g prints 12.3. The default precision for %e, %f + and %#g is 6; for %g it is the smallest number of digits necessary to identify + the value uniquely. + + For complex numbers, the width and precision apply to the two + components independently and the result is parenthesized, so %f applied + to 1.2+3.4i produces (1.200000+3.400000i). + + Other flags: + + always print a sign for numeric values; + guarantee ASCII-only output for %q (%+q) + - pad with spaces on the right rather than the left (left-justify the field) + # alternate format: add leading 0b for binary (%#b), 0 for octal (%#o), + 0x or 0X for hex (%#x or %#X); suppress 0x for %p (%#p); + for %q, print a raw (backquoted) string if strconv.CanBackquote + returns true; + always print a decimal point for %e, %E, %f, %F, %g and %G; + do not remove trailing zeros for %g and %G; + write e.g. U+0078 'x' if the character is printable for %U (%#U). + ' ' (space) leave a space for elided sign in numbers (% d); + put spaces between bytes printing strings or slices in hex (% x, % X) + 0 pad with leading zeros rather than spaces; + for numbers, this moves the padding after the sign + + Flags are ignored by verbs that do not expect them. + For example there is no alternate decimal format, so %#d and %d + behave identically. + + For each Printf-like function, there is also a Print function + that takes no format and is equivalent to saying %v for every + operand. Another variant Println inserts blanks between + operands and appends a newline. + + Regardless of the verb, if an operand is an interface value, + the internal concrete value is used, not the interface itself. + Thus: + var i interface{} = 23 + fmt.Printf("%v\n", i) + will print 23. + + Except when printed using the verbs %T and %p, special + formatting considerations apply for operands that implement + certain interfaces. In order of application: + + 1. If the operand is a reflect.Value, the operand is replaced by the + concrete value that it holds, and printing continues with the next rule. + + 2. If an operand implements the Formatter interface, it will + be invoked. In this case the interpretation of verbs and flags is + controlled by that implementation. + + 3. If the %v verb is used with the # flag (%#v) and the operand + implements the GoStringer interface, that will be invoked. + + If the format (which is implicitly %v for Println etc.) is valid + for a string (%s %q %v %x %X), the following two rules apply: + + 4. If an operand implements the error interface, the Error method + will be invoked to convert the object to a string, which will then + be formatted as required by the verb (if any). + + 5. If an operand implements method String() string, that method + will be invoked to convert the object to a string, which will then + be formatted as required by the verb (if any). + + For compound operands such as slices and structs, the format + applies to the elements of each operand, recursively, not to the + operand as a whole. Thus %q will quote each element of a slice + of strings, and %6.2f will control formatting for each element + of a floating-point array. + + However, when printing a byte slice with a string-like verb + (%s %q %x %X), it is treated identically to a string, as a single item. + + To avoid recursion in cases such as + type X string + func (x X) String() string { return Sprintf("<%s>", x) } + convert the value before recurring: + func (x X) String() string { return Sprintf("<%s>", string(x)) } + Infinite recursion can also be triggered by self-referential data + structures, such as a slice that contains itself as an element, if + that type has a String method. Such pathologies are rare, however, + and the package does not protect against them. + + When printing a struct, fmt cannot and therefore does not invoke + formatting methods such as Error or String on unexported fields. + + Explicit argument indexes + + In Printf, Sprintf, and Fprintf, the default behavior is for each + formatting verb to format successive arguments passed in the call. + However, the notation [n] immediately before the verb indicates that the + nth one-indexed argument is to be formatted instead. The same notation + before a '*' for a width or precision selects the argument index holding + the value. After processing a bracketed expression [n], subsequent verbs + will use arguments n+1, n+2, etc. unless otherwise directed. + + For example, + fmt.Sprintf("%[2]d %[1]d\n", 11, 22) + will yield "22 11", while + fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6) + equivalent to + fmt.Sprintf("%6.2f", 12.0) + will yield " 12.00". Because an explicit index affects subsequent verbs, + this notation can be used to print the same values multiple times + by resetting the index for the first argument to be repeated: + fmt.Sprintf("%d %d %#[1]x %#x", 16, 17) + will yield "16 17 0x10 0x11". + + Format errors + + If an invalid argument is given for a verb, such as providing + a string to %d, the generated string will contain a + description of the problem, as in these examples: + + Wrong type or unknown verb: %!verb(type=value) + Printf("%d", "hi"): %!d(string=hi) + Too many arguments: %!(EXTRA type=value) + Printf("hi", "guys"): hi%!(EXTRA string=guys) + Too few arguments: %!verb(MISSING) + Printf("hi%d"): hi%!d(MISSING) + Non-int for width or precision: %!(BADWIDTH) or %!(BADPREC) + Printf("%*s", 4.5, "hi"): %!(BADWIDTH)hi + Printf("%.*s", 4.5, "hi"): %!(BADPREC)hi + Invalid or invalid use of argument index: %!(BADINDEX) + Printf("%*[2]d", 7): %!d(BADINDEX) + Printf("%.[2]d", 7): %!d(BADINDEX) + + All errors begin with the string "%!" followed sometimes + by a single character (the verb) and end with a parenthesized + description. + + If an Error or String method triggers a panic when called by a + print routine, the fmt package reformats the error message + from the panic, decorating it with an indication that it came + through the fmt package. For example, if a String method + calls panic("bad"), the resulting formatted message will look + like + %!s(PANIC=bad) + + The %!s just shows the print verb in use when the failure + occurred. If the panic is caused by a nil receiver to an Error + or String method, however, the output is the undecorated + string, "<nil>". + + Scanning + + An analogous set of functions scans formatted text to yield + values. Scan, Scanf and Scanln read from os.Stdin; Fscan, + Fscanf and Fscanln read from a specified io.Reader; Sscan, + Sscanf and Sscanln read from an argument string. + + Scan, Fscan, Sscan treat newlines in the input as spaces. + + Scanln, Fscanln and Sscanln stop scanning at a newline and + require that the items be followed by a newline or EOF. + + Scanf, Fscanf, and Sscanf parse the arguments according to a + format string, analogous to that of Printf. In the text that + follows, 'space' means any Unicode whitespace character + except newline. + + In the format string, a verb introduced by the % character + consumes and parses input; these verbs are described in more + detail below. A character other than %, space, or newline in + the format consumes exactly that input character, which must + be present. A newline with zero or more spaces before it in + the format string consumes zero or more spaces in the input + followed by a single newline or the end of the input. A space + following a newline in the format string consumes zero or more + spaces in the input. Otherwise, any run of one or more spaces + in the format string consumes as many spaces as possible in + the input. Unless the run of spaces in the format string + appears adjacent to a newline, the run must consume at least + one space from the input or find the end of the input. + + The handling of spaces and newlines differs from that of C's + scanf family: in C, newlines are treated as any other space, + and it is never an error when a run of spaces in the format + string finds no spaces to consume in the input. + + The verbs behave analogously to those of Printf. + For example, %x will scan an integer as a hexadecimal number, + and %v will scan the default representation format for the value. + The Printf verbs %p and %T and the flags # and + are not implemented. + For floating-point and complex values, all valid formatting verbs + (%b %e %E %f %F %g %G %x %X and %v) are equivalent and accept + both decimal and hexadecimal notation (for example: "2.3e+7", "0x4.5p-8") + and digit-separating underscores (for example: "3.14159_26535_89793"). + + Input processed by verbs is implicitly space-delimited: the + implementation of every verb except %c starts by discarding + leading spaces from the remaining input, and the %s verb + (and %v reading into a string) stops consuming input at the first + space or newline character. + + The familiar base-setting prefixes 0b (binary), 0o and 0 (octal), + and 0x (hexadecimal) are accepted when scanning integers + without a format or with the %v verb, as are digit-separating + underscores. + + Width is interpreted in the input text but there is no + syntax for scanning with a precision (no %5.2f, just %5f). + If width is provided, it applies after leading spaces are + trimmed and specifies the maximum number of runes to read + to satisfy the verb. For example, + Sscanf(" 1234567 ", "%5s%d", &s, &i) + will set s to "12345" and i to 67 while + Sscanf(" 12 34 567 ", "%5s%d", &s, &i) + will set s to "12" and i to 34. + + In all the scanning functions, a carriage return followed + immediately by a newline is treated as a plain newline + (\r\n means the same as \n). + + In all the scanning functions, if an operand implements method + Scan (that is, it implements the Scanner interface) that + method will be used to scan the text for that operand. Also, + if the number of arguments scanned is less than the number of + arguments provided, an error is returned. + + All arguments to be scanned must be either pointers to basic + types or implementations of the Scanner interface. + + Like Scanf and Fscanf, Sscanf need not consume its entire input. + There is no way to recover how much of the input string Sscanf used. + + Note: Fscan etc. can read one character (rune) past the input + they return, which means that a loop calling a scan routine + may skip some of the input. This is usually a problem only + when there is no space between input values. If the reader + provided to Fscan implements ReadRune, that method will be used + to read characters. If the reader also implements UnreadRune, + that method will be used to save the character and successive + calls will not lose data. To attach ReadRune and UnreadRune + methods to a reader without that capability, use + bufio.NewReader. +*/ +package fmt |