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ICU 77.1: icu::DecimalFormat Class Reference

IMPORTANT: New users are strongly encouraged to see if numberformatter.h fits their use case. More...

IMPORTANT: New users are strongly encouraged to see if numberformatter.h fits their use case.

Although not deprecated, this header is provided for backwards compatibility only.

DecimalFormat is a concrete subclass of NumberFormat that formats decimal numbers. It has a variety of features designed to make it possible to parse and format numbers in any locale, including support for Western, Arabic, or Indic digits. It also supports different flavors of numbers, including integers ("123"), fixed-point numbers ("123.4"), scientific notation ("1.23E4"), percentages ("12%"), and currency amounts ("$123", "USD123", "123 US dollars"). All of these flavors can be easily localized.

To obtain a NumberFormat for a specific locale (including the default locale) call one of NumberFormat's factory methods such as createInstance(). Do not call the DecimalFormat constructors directly, unless you know what you are doing, since the NumberFormat factory methods may return subclasses other than DecimalFormat.

Example Usage

int32_t locCount;

double myNumber = -1234.56;

UnicodeString countryName;

UnicodeString displayName;

UnicodeString str;

UnicodeString pattern;

Formattable fmtable;

for (int32_t j = 0; j < 3; ++j) {

cout << endl << "FORMAT " << j << endl;

for (int32_t i = 0; i < locCount; ++i) {

if (locales[i].getCountry(countryName).size() == 0) {

continue;

}

switch (j) {

case 0:

case 1:

default:

}

if (form) {

str.remove();

cout << locales[i].getDisplayName(displayName) << ": " << pattern;

cout << " -> " << form->format(myNumber,str) << endl;

form->parse(form->format(myNumber,str), fmtable, success);

delete form;

}

}

}

DecimalFormat(UErrorCode &status)

Create a DecimalFormat using the default pattern and symbols for the default locale.

static NumberFormat * createInstance(UErrorCode &)

Create a default style NumberFormat for the current default locale.

NumberFormat()

Default constructor for subclass use only.

static StringEnumeration * getAvailableLocales()

Return a StringEnumeration over the locales available at the time of the call, including registered l...

static NumberFormat * createPercentInstance(UErrorCode &)

Returns a percentage format for the current default locale.

static NumberFormat * createCurrencyInstance(UErrorCode &)

Returns a currency format for the current default locale.

UErrorCode

Standard ICU4C error code type, a substitute for exceptions.

@ U_ZERO_ERROR

No error, no warning.

Another example use createInstance(style)

Locale* locale = new Locale("en", "US");

double myNumber = 1234.56;

UnicodeString str;

Formattable fmtable;

for (int j=NumberFormat::kNumberStyle;

j<=NumberFormat::kPluralCurrencyStyle;

++j) {

str.remove();

cout << "format result " << form->format(myNumber, str) << endl;

->parse(form->format(myNumber, str), fmtable, success);

delete form;

}

virtual UnicodeString & format(const Formattable &obj, UnicodeString &appendTo, FieldPosition &pos, UErrorCode &status) const override

Format an object to produce a string.

Patterns

A DecimalFormat consists of a pattern and a set of symbols. The pattern may be set directly using applyPattern(), or indirectly using other API methods which manipulate aspects of the pattern, such as the minimum number of integer digits. The symbols are stored in a DecimalFormatSymbols object. When using the NumberFormat factory methods, the pattern and symbols are read from ICU's locale data.

Special Pattern Characters

Many characters in a pattern are taken literally; they are matched during parsing and output unchanged during formatting. Special characters, on the other hand, stand for other characters, strings, or classes of characters. For example, the '#' character is replaced by a localized digit. Often the replacement character is the same as the pattern character; in the U.S. locale, the ',' grouping character is replaced by ','. However, the replacement is still happening, and if the symbols are modified, the grouping character changes. Some special characters affect the behavior of the formatter by their presence; for example, if the percent character is seen, then the value is multiplied by 100 before being displayed.

To insert a special character in a pattern as a literal, that is, without any special meaning, the character must be quoted. There are some exceptions to this which are noted below.

The characters listed here are used in non-localized patterns. Localized patterns use the corresponding characters taken from this formatter's DecimalFormatSymbols object instead, and these characters lose their special status. Two exceptions are the currency sign and quote, which are not localized.

A DecimalFormat pattern contains a positive and negative subpattern, for example, "#,##0.00;(#,##0.00)". Each subpattern has a prefix, a numeric part, and a suffix. If there is no explicit negative subpattern, the negative subpattern is the localized minus sign prefixed to the positive subpattern. That is, "0.00" alone is equivalent to "0.00;-0.00". If there is an explicit negative subpattern, it serves only to specify the negative prefix and suffix; the number of digits, minimal digits, and other characteristics are ignored in the negative subpattern. That means that "#,##0.0#;(#)" has precisely the same result as "#,##0.0#;(#,##0.0#)".

The prefixes, suffixes, and various symbols used for infinity, digits, thousands separators, decimal separators, etc. may be set to arbitrary values, and they will appear properly during formatting. However, care must be taken that the symbols and strings do not conflict, or parsing will be unreliable. For example, either the positive and negative prefixes or the suffixes must be distinct for parse() to be able to distinguish positive from negative values. Another example is that the decimal separator and thousands separator should be distinct characters, or parsing will be impossible.

The grouping separator is a character that separates clusters of integer digits to make large numbers more legible. It commonly used for thousands, but in some locales it separates ten-thousands. The grouping size is the number of digits between the grouping separators, such as 3 for "100,000,000" or 4 for "1 0000 0000". There are actually two different grouping sizes: One used for the least significant integer digits, the primary grouping size, and one used for all others, the secondary grouping size. In most locales these are the same, but sometimes they are different. For example, if the primary grouping interval is 3, and the secondary is 2, then this corresponds to the pattern "#,##,##0", and the number 123456789 is formatted as "12,34,56,789". If a pattern contains multiple grouping separators, the interval between the last one and the end of the integer defines the primary grouping size, and the interval between the last two defines the secondary grouping size. All others are ignored, so "#,##,###,####" == "###,###,####" == "##,#,###,####".

Illegal patterns, such as "#.#.#" or "#.###,###", will cause DecimalFormat to set a failing UErrorCode.

Pattern BNF

pattern    := subpattern (';' subpattern)?
subpattern := prefix? number exponent? suffix?
number     := (integer ('.' fraction)?) | sigDigits
prefix     := '\u0000'..'\uFFFD' - specialCharacters
suffix     := '\u0000'..'\uFFFD' - specialCharacters
integer    := '#'* '0'* '0'
fraction   := '0'* '#'*
sigDigits  := '#'* '@' '@'* '#'*
exponent   := 'E' '+'? '0'* '0'
padSpec    := '*' padChar
padChar    := '\u0000'..'\uFFFD' - quote
 
Notation:
  X*       0 or more instances of X
  X?       0 or 1 instances of X
  X|Y      either X or Y
  C..D     any character from C up to D, inclusive
  S-T      characters in S, except those in T

The first subpattern is for positive numbers. The second (optional) subpattern is for negative numbers.

Not indicated in the BNF syntax above:

• The grouping separator ',' can occur inside the integer and sigDigits elements, between any two pattern characters of that element, as long as the integer or sigDigits element is not followed by the exponent element.

• Two grouping intervals are recognized: That between the decimal point and the first grouping symbol, and that between the first and second grouping symbols. These intervals are identical in most locales, but in some locales they differ. For example, the pattern "#,##,###" formats the number 123456789 as "12,34,56,789".

• The pad specifier padSpec may appear before the prefix, after the prefix, before the suffix, after the suffix, or not at all.

• In place of '0', the digits '1' through '9' may be used to indicate a rounding increment.

Parsing

DecimalFormat parses all Unicode characters that represent decimal digits, as defined by u_charDigitValue(). In addition, DecimalFormat also recognizes as digits the ten consecutive characters starting with the localized zero digit defined in the DecimalFormatSymbols object. During formatting, the DecimalFormatSymbols-based digits are output.

During parsing, grouping separators are ignored if in lenient mode; otherwise, if present, they must be in appropriate positions.

For currency parsing, the formatter is able to parse every currency style formats no matter which style the formatter is constructed with. For example, a formatter instance gotten from NumberFormat.getInstance(ULocale, NumberFormat.CURRENCYSTYLE) can parse formats such as "USD1.00" and "3.00 US dollars".

If parse(UnicodeString&,Formattable&,ParsePosition&) fails to parse a string, it leaves the parse position unchanged. The convenience method parse(UnicodeString&,Formattable&,UErrorCode&) indicates parse failure by setting a failing UErrorCode.

Formatting

Formatting is guided by several parameters, all of which can be specified either using a pattern or using the API. The following description applies to formats that do not use scientific notation or significant digits.

• If the number of actual integer digits exceeds the maximum integer digits, then only the least significant digits are shown. For example, 1997 is formatted as "97" if the maximum integer digits is set to 2.

• If the number of actual integer digits is less than the minimum integer digits, then leading zeros are added. For example, 1997 is formatted as "01997" if the minimum integer digits is set to 5.

• If the number of actual fraction digits exceeds the maximum fraction digits, then rounding is performed to the maximum fraction digits. For example, 0.125 is formatted as "0.12" if the maximum fraction digits is 2. This behavior can be changed by specifying a rounding increment and/or a rounding mode.

• If the number of actual fraction digits is less than the minimum fraction digits, then trailing zeros are added. For example, 0.125 is formatted as "0.1250" if the minimum fraction digits is set to 4.

• Trailing fractional zeros are not displayed if they occur j positions after the decimal, where j is less than the maximum fraction digits. For example, 0.10004 is formatted as "0.1" if the maximum fraction digits is four or less.

Special Values

NaN is represented as a single character, typically \uFFFD. This character is determined by the DecimalFormatSymbols object. This is the only value for which the prefixes and suffixes are not used.

Infinity is represented as a single character, typically \u221E, with the positive or negative prefixes and suffixes applied. The infinity character is determined by the DecimalFormatSymbols object.

Scientific Notation

Numbers in scientific notation are expressed as the product of a mantissa and a power of ten, for example, 1234 can be expressed as 1.234 x 10³. The mantissa is typically in the half-open interval [1.0, 10.0) or sometimes [0.0, 1.0), but it need not be. DecimalFormat supports arbitrary mantissas. DecimalFormat can be instructed to use scientific notation through the API or through the pattern. In a pattern, the exponent character immediately followed by one or more digit characters indicates scientific notation. Example: "0.###E0" formats the number 1234 as "1.234E3".

• The number of digit characters after the exponent character gives the minimum exponent digit count. There is no maximum. Negative exponents are formatted using the localized minus sign, not the prefix and suffix from the pattern. This allows patterns such as "0.###E0 m/s". To prefix positive exponents with a localized plus sign, specify '+' between the exponent and the digits: "0.###E+0" will produce formats "1E+1", "1E+0", "1E-1", etc. (In localized patterns, use the localized plus sign rather than '+'.)

• The minimum number of integer digits is achieved by adjusting the exponent. Example: 0.00123 formatted with "00.###E0" yields "12.3E-4". This only happens if there is no maximum number of integer digits. If there is a maximum, then the minimum number of integer digits is fixed at one.

• The maximum number of integer digits, if present, specifies the exponent grouping. The most common use of this is to generate engineering notation, in which the exponent is a multiple of three, e.g., "##0.###E0". The number 12345 is formatted using "##0.####E0" as "12.345E3".

• When using scientific notation, the formatter controls the digit counts using significant digits logic. The maximum number of significant digits limits the total number of integer and fraction digits that will be shown in the mantissa; it does not affect parsing. For example, 12345 formatted with "##0.##E0" is "12.3E3". See the section on significant digits for more details.

• The number of significant digits shown is determined as follows: If areSignificantDigitsUsed() returns false, then the minimum number of significant digits shown is one, and the maximum number of significant digits shown is the sum of the minimum integer and maximum fraction digits, and is unaffected by the maximum integer digits. If this sum is zero, then all significant digits are shown. If areSignificantDigitsUsed() returns true, then the significant digit counts are specified by getMinimumSignificantDigits() and getMaximumSignificantDigits(). In this case, the number of integer digits is fixed at one, and there is no exponent grouping.

• Exponential patterns may not contain grouping separators.

Significant Digits

DecimalFormat has two ways of controlling how many digits are shows: (a) significant digits counts, or (b) integer and fraction digit counts. Integer and fraction digit counts are described above. When a formatter is using significant digits counts, the number of integer and fraction digits is not specified directly, and the formatter settings for these counts are ignored. Instead, the formatter uses however many integer and fraction digits are required to display the specified number of significant digits. Examples:

• Significant digit counts may be expressed using patterns that specify a minimum and maximum number of significant digits. These are indicated by the '@' and '#' characters. The minimum number of significant digits is the number of '@' characters. The maximum number of significant digits is the number of '@' characters plus the number of '#' characters following on the right. For example, the pattern "@@@" indicates exactly 3 significant digits. The pattern "@##" indicates from 1 to 3 significant digits. Trailing zero digits to the right of the decimal separator are suppressed after the minimum number of significant digits have been shown. For example, the pattern "@##" formats the number 0.1203 as "0.12".

• If a pattern uses significant digits, it may not contain a decimal separator, nor the '0' pattern character. Patterns such as "@00" or "@.###" are disallowed.

• Any number of '#' characters may be prepended to the left of the leftmost '@' character. These have no effect on the minimum and maximum significant digits counts, but may be used to position grouping separators. For example, "#,#@#" indicates a minimum of one significant digits, a maximum of two significant digits, and a grouping size of three.

• In order to enable significant digits formatting, use a pattern containing the '@' pattern character. Alternatively, call setSignificantDigitsUsed(true).

• In order to disable significant digits formatting, use a pattern that does not contain the '@' pattern character. Alternatively, call setSignificantDigitsUsed(false).

• The number of significant digits has no effect on parsing.

• Significant digits may be used together with exponential notation. Such patterns are equivalent to a normal exponential pattern with a minimum and maximum integer digit count of one, a minimum fraction digit count of getMinimumSignificantDigits() - 1, and a maximum fraction digit count of getMaximumSignificantDigits() - 1. For example, the pattern "@@###E0" is equivalent to "0.0###E0".

• If significant digits are in use, then the integer and fraction digit counts, as set via the API, are ignored. If significant digits are not in use, then the significant digit counts, as set via the API, are ignored.

Padding

DecimalFormat supports padding the result of format() to a specific width. Padding may be specified either through the API or through the pattern syntax. In a pattern the pad escape character, followed by a single pad character, causes padding to be parsed and formatted. The pad escape character is '*' in unlocalized patterns, and can be localized using DecimalFormatSymbols::setSymbol() with a DecimalFormatSymbols::kPadEscapeSymbol selector. For example, "$*x#,##0.00" formats 123 to "$xx123.00", and 1234 to "$1,234.00".

• When padding is in effect, the width of the positive subpattern, including prefix and suffix, determines the format width. For example, in the pattern "* #0 o''clock", the format width is 10.

• The width is counted in 16-bit code units (char16_ts).

• Some parameters which usually do not matter have meaning when padding is used, because the pattern width is significant with padding. In the pattern "* ##,##,#,##0.##", the format width is 14. The initial characters "##,##," do not affect the grouping size or maximum integer digits, but they do affect the format width.

• Padding may be inserted at one of four locations: before the prefix, after the prefix, before the suffix, or after the suffix. If padding is specified in any other location, applyPattern() sets a failing UErrorCode. If there is no prefix, before the prefix and after the prefix are equivalent, likewise for the suffix.

• When specified in a pattern, the 32-bit code point immediately following the pad escape is the pad character. This may be any character, including a special pattern character. That is, the pad escape escapes the following character. If there is no character after the pad escape, then the pattern is illegal.

Rounding

DecimalFormat supports rounding to a specific increment. For example, 1230 rounded to the nearest 50 is 1250. 1.234 rounded to the nearest 0.65 is 1.3. The rounding increment may be specified through the API or in a pattern. To specify a rounding increment in a pattern, include the increment in the pattern itself. "#,#50" specifies a rounding increment of

1. "#,##0.05" specifies a rounding increment of 0.05.

In the absence of an explicit rounding increment numbers are rounded to their formatted width.

• Rounding only affects the string produced by formatting. It does not affect parsing or change any numerical values.

• A rounding mode determines how values are rounded; see DecimalFormat::ERoundingMode. The default rounding mode is DecimalFormat::kRoundHalfEven. The rounding mode can only be set through the API; it can not be set with a pattern.

• Some locales use rounding in their currency formats to reflect the smallest currency denomination.

• In a pattern, digits '1' through '9' specify rounding, but otherwise behave identically to digit '0'.

Synchronization

DecimalFormat objects are not synchronized. Multiple threads should not access one formatter concurrently.

Subclassing

User subclasses are not supported. While clients may write subclasses, such code will not necessarily work and will not be guaranteed to work stably from release to release.

Definition at line 666 of file decimfmt.h.

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