Question

格式化数字是不经常用到的，只有在例如丑陋的"0.30000000000000004"出现在你的数轴标签上时，或者你想要使用固定精度将几千的数字组织为更加可读的形式，例如"$1,240.10"，再或者你可能只想展示一个特定的数字的显著位。D3使用标准的数字格式化使得一切变得简单，例如，创建一个用0补齐4位数字的函数，可以这样：

var zero = d3.format("04d");

现在，你就可以调用 zero 来很方便的格式化你的数字了：

zero(2); // "0002"
zero(123); // "0123"

当然，除了数字，D3还支持格式化和解析日期，逗号分隔字串。

Numbers

# d3.format(specifier)

返回给定的字符串（specifier）的格式化函数（等同于适用默认的美国英语语言环境的locale.numberFormat)。唯一的入参是数字，返回代表格式化数字的字符串。这个格式化规范模拟的是Python 3.1内置的格式化规范语言[[format specification mini-language|http://docs.python.org/release/3.1.3/library/string.html#formatspec]]。规范（specifier）的通常格式如下:

 [​[fill]align][sign][symbol][0][width][,][.precision][type]

fill可以是任意字符，除了 “{“ 和 “}”，fill 由紧跟它的align选项标识。

align有三种选项：

• ("<") 在可用的区域左对齐。
• (">") 在可用的区域右对齐（默认）。
• ("^") 在可用的区域居中。

sign可能是：

• plus ("+") - 可以用于正数或负数。
• minus ("-") - 仅仅用于负数（默认）。
• space (" ") - 前面的空格应该用在正数前面，而减号必须用在负数！

symbol可能是：

• currency ("$") - a currency symbol should be prefixed (or suffixed) per the locale.
• base ("#") - for binary, octal, or hexadecimal output, prefix by "0b", "0o", or "0x", respectively.

The "0" option enables zero-padding.

The width defines the minimum field width. If not specified, then the width will be determined by the content.

The comma (",") option enables the use of a comma for a thousands separator.

The precision indicates how many digits should be displayed after the decimal point for a value formatted with types "f" and "%", or before and after the decimal point for a value formatted with types "g", "r" and "p".

The available type values are:

• exponent ("e") - use [[Number.toExponential|http://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number/toExponential]].
• general ("g") - use [[Number.toPrecision|http://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number/toPrecision]].
• fixed ("f") - use [[Number.toFixed|http://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number/toFixed]].
• integer ("d") - use [[Number.toString|http://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Number/toString]], but ignore any non-integer values.
• rounded ("r") - round to precision significant digits, padding with zeroes where necessary in similar fashion to fixed ("f"). If no precision is specified, falls back to general notation.
• percentage ("%") - like fixed, but multiply by 100 and suffix with "%".
• rounded percentage ("p") - like rounded, but multiply by 100 and suffix with "%".
• binary ("b") - outputs the number in base 2.
• octal ("o") - outputs the number in base 8.
• hexadecimal ("x") - outputs the number in base 16, using lower-case letters for the digits above 9.
• hexadecimal ("X") - outputs the number in base 16, using upper-case letters for the digits above 9.
• character ("c") - converts the integer to the corresponding unicode character before printing.
• SI-prefix ("s") - like rounded, but with a unit suffixed such as "9.5M" for mega, or "1.00µ" for micro.

The type "n" is also supported as shorthand for ",g".

# d3.formatPrefix(value[, precision])

Returns the SI prefix for the specified value. If an optional precision is specified, the value is rounded accordingly using d3.round before computing the prefix. The returned prefix object has two properties:

• symbol - the prefix symbol, such as "M" for millions.
• scale - the scale function, for converting numbers to the appropriate prefixed scale.

For example:

var prefix = d3.formatPrefix(1.21e9);
console.log(prefix.symbol); // "G"
console.log(prefix.scale(1.21e9)); // 1.21

This method is used by d3.format for the s format.

# d3.round(x[, n])

Returns the value x rounded to n digits after the decimal point. If n is omitted, it defaults to zero. The result is a number. Values are rounded to the closest multiple of 10 to the power minus n; if two multiples are equally close, the value is rounded up in accordance with the built-in [[round|http://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Math/round]] function. For example:

d3.round(1.23); // 1
d3.round(1.23, 1); // 1.2
d3.round(1.25, 1); // 1.3
d3.round(12.5, 0); // 13
d3.round(12, -1); // 10

Note that the resulting number when converted to a string may be imprecise due to IEEE floating point precision; to format a number to a string with a fixed number of decimal points, use d3.format instead.

Strings

# d3.requote(string)

Returns a quoted (escaped) version of the specified string such that the string may be embedded in a regular expression as a string literal.

d3.requote("[]"); // "\[\]"

Dates

See the [[d3.time|Time-Formatting]] module.