How to convert float or currency to a localized string?

Question

In Delphi¹, using FloatToStrF or CurrToStrF will automatically use the DecimalSeparator character to represent a decimal mark. Unfortunately DecimalSeparator is declared in SysUtils as Char^1,2:

var 
  DecimalSeparator: Char;

While the LOCALE_SDECIMAL is allowed to be up to three characters:

Character(s) used for the decimal separator, for example, "." in "3.14" or "," in "3,14". The maximum number of characters allowed for this string is four, including a terminating null character.

This causes Delphi to fail to read the decimal separator correctly; falling back to assume a default decimal separator of ".":

DecimalSeparator := GetLocaleChar(DefaultLCID, LOCALE_SDECIMAL, '.');

On my computer, which is quite a character, this cause floating point and currency values to be incorrectly localized with a U+002E (full stop) decimal mark.

i am willing to call the Windows API functions directly, which are designed to convert floating point, or currency, values into a localized string:

• GetNumberFormat
• GetCurrencyFormat

Except these functions take a string of picture codes, where the only characters allowed are:

• Characters "0" through "9" (U+0030..U+0039)
• One decimal point (.) if the number is a floating-point value (U+002E)
• A minus sign in the first character position if the number is a negative value (U+002D)

What would be a good way¹ to convert a floating point, or currency, value to a string that obeys those rules? e.g.

• 1234567.893332
• -1234567

given that the local user's locale (i.e. my computer):

• might not use a - to indicate negative (e.g. --)
• might not use a . to indicate a decimal point (e.g. ,,)
• might not use the latin alphabet 0123456789 to represent digits (e.g. [removed arabic digits that crash SO javascript parser])

---

A horrible, horrible, hack, which i could use:

function FloatToLocaleIndependantString(const v: Extended): string;
var
   oldDecimalSeparator: Char;
begin
   oldDecimalSeparator := SysUtils.DecimalSeparator;
   SysUtils.DecimalSeparator := '.'; //Windows formatting functions assume single decimal point
   try
      Result := FloatToStrF(Value, ffFixed, 
            18, //Precision: "should be 18 or less for values of type Extended"
            9 //Scale 0..18.   Sure...9 digits before decimal mark, 9 digits after. Why not
      );
   finally
      SysUtils.DecimalSeparator := oldDecimalSeparator;
   end;
end;

---

Additional info on the chain of functions the VCL uses:

• FloatToStrF and CurrToStrF calls:
  • FloatToText calls:
    • FloatToDecimal

Note

• DecimalSeparator: Char, the single character global is deprecated, and replaced with another single character decimal separator

¹ in my version of Delphi
² and in current versions of Delphi

Answer 1

Delphi does provide a procedure called FloatToDecimal that converts floating point (e.g. Extended) and Currency values into a useful structure for further formatting. e.g.:

FloatToDecimal(..., 1234567890.1234, ...);

gives you:

TFloatRec
   Digits: array[0..20] of Char = "12345678901234"
   Exponent: SmallInt =           10
   IsNegative: Boolean =          True

Where Exponent gives the number of digits to the left of decimal point.

There are some special cases to be handled:

• Exponent is zero

     Digits: array[0..20] of Char = "12345678901234"
     Exponent: SmallInt =           0
     IsNegative: Boolean =          True
  

  means there are no digits to the left of the decimal point, e.g. .12345678901234

• Exponent is negative

     Digits: array[0..20] of Char = "12345678901234"
     Exponent: SmallInt =           -3
     IsNegative: Boolean =          True
  

  means you have to place zeros in between the decimal point and the first digit, e.g. .00012345678901234

• Exponent is -32768 (NaN, not a number)

     Digits: array[0..20] of Char = ""
     Exponent: SmallInt =           -32768
     IsNegative: Boolean =          False
  

  means the value is Not a Number, e.g. NAN

• Exponent is 32767 (INF, or -INF)

     Digits: array[0..20] of Char = ""
     Exponent: SmallInt =           32767
     IsNegative: Boolean =          False
  

  means the value is either positive or negative infinity (depending on the IsNegative value), e.g. -INF

---

We can use FloatToDecimal as a starting point to create a locale-independent string of "pictures codes".

This string can then be passed to appropriate Windows GetNumberFormat or GetCurrencyFormat functions to perform the actual correct localization.

i wrote my own CurrToDecimalString and FloatToDecimalString which convert numbers into the required locale independent format:

class function TGlobalization.CurrToDecimalString(const Value: Currency): string;
var
    digits: string;
    s: string;
    floatRec: TFloatRec;
begin
    FloatToDecimal({var}floatRec, Value, fvCurrency, 0{ignored for currency types}, 9999);

    //convert the array of char into an easy to access string
    digits := PChar(Addr(floatRec.Digits[0]));

    if floatRec.Exponent > 0 then
    begin
        //Check for positive or negative infinity (exponent = 32767)
        if floatRec.Exponent = 32767 then //David Heffernan says that currency can never be infinity. Even though i can't test it, i can at least try to handle it
        begin
            if floatRec.Negative = False then
                Result := 'INF'
            else
                Result := '-INF';
            Exit;
        end;

        {
            digits:    1234567 89
              exponent--------^ 7=7 digits on left of decimal mark
        }
        s := Copy(digits, 1, floatRec.Exponent);

        {
            for the value 10000:
                digits:   "1"
                exponent: 5
            Add enough zero's to digits to pad it out to exponent digits
        }
        if Length(s) < floatRec.Exponent then
            s := s+StringOfChar('0', floatRec.Exponent-Length(s));

        if Length(digits) > floatRec.Exponent then
            s := s+'.'+Copy(digits, floatRec.Exponent+1, 20);
    end
    else if floatRec.Exponent < 0 then
    begin
        //check for NaN (Exponent = -32768)
        if floatRec.Exponent = -32768 then  //David Heffernan says that currency can never be NotANumber. Even though i can't test it, i can at least try to handle it
        begin
            Result := 'NAN';
            Exit;
        end;

        {
            digits:   .000123456789
                         ^---------exponent
        }

        //Add zero, or more, "0"'s to the left
        s := '0.'+StringOfChar('0', -floatRec.Exponent)+digits;
    end
    else
    begin
        {
            Exponent is zero.

            digits:     .123456789
                            ^
        }
        if length(digits) > 0 then
            s := '0.'+digits
        else
            s := '0';
    end;

    if floatRec.Negative then
        s := '-'+s;

    Result := s;
end;

Aside from the edge cases of NAN, INF and -INF, i can now pass these strings to Windows:

class function TGlobalization.GetCurrencyFormat(const DecimalString: WideString; const Locale: LCID): WideString;
var
    cch: Integer;
    ValueStr: WideString;
begin
    Locale
        LOCALE_INVARIANT
        LOCALE_USER_DEFAULT     <--- use this one (windows.pas)
        LOCALE_SYSTEM_DEFAULT
        LOCALE_CUSTOM_DEFAULT       (Vista and later)
        LOCALE_CUSTOM_UI_DEFAULT    (Vista and later)
        LOCALE_CUSTOM_UNSPECIFIED   (Vista and later)
}

    cch := Windows.GetCurrencyFormatW(Locale, 0, PWideChar(DecimalString), nil, nil, 0);
    if cch = 0 then
        RaiseLastWin32Error;

    SetLength(ValueStr, cch);
    cch := Windows.GetCurrencyFormatW(Locale, 0, PWideChar(DecimalString), nil, PWideChar(ValueStr), Length(ValueStr));
    if (cch = 0) then
        RaiseLastWin32Error;

    SetLength(ValueStr, cch-1); //they include the null terminator  /facepalm
    Result := ValueStr;
end;

The FloatToDecimalString and GetNumberFormat implementations are left as an exercise for the reader (since i actually haven't written the float one yet, just the currency - i don't know how i'm going to handle exponential notation).

And Bob's yer uncle; properly localized floats and currencies under Delphi.

i already went through the work of properly localizing Integers, Dates, Times, and Datetimes.

Note: Any code is released into the public domain. No attribution required.

Answer 2

Ok, this may not be what you want, but it works with D2007 and up. Thread safe and all.

uses Windows,SysUtils;

var
  myGlobalFormatSettings : TFormatSettings;

// Initialize special format settings record
GetLocaleFormatSettings( 0,myGlobalFormatSettings);
myGlobalFormatSettings.DecimalSeparator := '.';


function FloatToLocaleIndependantString(const value: Extended): string;
begin
  Result := FloatToStrF(Value, ffFixed, 
        18, //Precision: "should be 18 or less for values of type Extended"
        9, //Scale 0..18.   Sure...9 digits before decimal mark, 9 digits after. Why not
        myGlobalFormatSettings
  );
end;