How to print a double with a comma

Question

In C++ I've got a float/double variable.

When I print this with for example cout the resulting string is period-delimited.

cout << 3.1415 << endl
$> 3.1415

Is there an easy way to force the double to be printed with a comma?

cout << 3.1415 << endl
$> 3,1415

Éric Malenfant · Accepted Answer · 2009-09-14T16:04:49.460

14

imbue() cout with a locale whose numpunct facet's decimal_point() member function returns a comma.

Obtaining such a locale can be done in several ways. You could use a named locale available on your system (std::locale("fr"), perhaps). Alternatively, you could derive your own numpuct, implement the do_decimal_point() member in it.

Example of the second approach:

template<typename CharT>
class DecimalSeparator : public std::numpunct<CharT>
{
public:
    DecimalSeparator(CharT Separator)
    : m_Separator(Separator)
    {}

protected:
    CharT do_decimal_point()const
    {
        return m_Separator;
    }

private:
    CharT m_Separator;
};

Used as:

std::cout.imbue(std::locale(std::cout.getloc(), new DecimalSeparator<char>(',')));

edited Sep 14 '09 at 16:04

answered Sep 14 '09 at 15:19

Éric Malenfant

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using your example results in: std::runtime error locale::facet::_S_create_c_locale name not valid – NomeN Sep 14 '09 at 15:46
This error probably means that, on your platform, the "fr" name does not bind to a locale. – Éric Malenfant Sep 14 '09 at 15:55
I guessed as much, but how could you find out which names *are* bound to a locale. – NomeN Sep 14 '09 at 15:57
Sadly, I don't think there is a portable way. – Éric Malenfant Sep 14 '09 at 16:06
do you know it for *nix perhaps? – NomeN Sep 14 '09 at 16:13
1

On linux: the locale -a command, but I don't know how to do this programmatically. – Éric Malenfant Sep 14 '09 at 16:17
programmatically retrieving the locale was no requirement, this works fine. And I guess anybody needing to do this programmatically could do some search-fu on a system() call. Thx! consider your answer accepted. – NomeN Sep 14 '09 at 16:26

score 4 · Answer 2 · answered Sep 14 '09 at 15:13

This is controlled by your program's locale.

How you set a program's default locale depends on the platform. On POSIX type platforms, it's with the LANG and LC_* environment variables, for instance.

You can force a particular locale -- different from the default -- within a C++ program by calling ios::imbue. Something like this might work:

#include <locale>
cout.imbue(std::locale("German_germany"));

The idea is to force a locale where comma is the decimal separator. You might need to adjust the "German_germany" string to get the behavior you want on your particular platform.

using your example results in: std::runtime error locale::facet::_S_create_c_locale name not valid — NomeN, Sep 14 '09 at 15:28

score 3 · Answer 3 · answered Sep 14 '09 at 15:18

3

You need to impue the stream with a different locale, one whose num_punct (iirc) facet specifies a comma.

If your platform locale formats with commas, then

cout.imbue(locale(""));

should be sufficient.

answered Sep 14 '09 at 15:18

me22

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thx, your answer worked. But I'd like to accept an answer that will be more generic (if you're not so lucky with the correct locale presetup on your platform). Still +1 because it's the easiest method. – NomeN Sep 14 '09 at 15:43

score 1 · Answer 4 · answered Sep 14 '09 at 15:19

1

To be precise, this is controlled by the std::numpunct<charT>::decimal_point() value. You can imbue() another locale with another decimal_point()

answered Sep 14 '09 at 15:19

MSalters

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score 0 · Answer 5 · answered Dec 03 '13 at 10:25

Old thread, but anyway ... One should be aware that using a std::locale makes the string "pretty", complete with correct decimal point, thousands separators and what not, depending on the platform and locale. Most probably, using imbue() will break any parsing of the string after it's formatted. For example:

std::ostringstream s;
std::locale l("fr-fr");
s << "without locale: " << 1234.56L << std::endl;
s.imbue(l);
s << "with fr locale: " << 1234.56L << std::endl;
std::cout << s.str();

Gives the following output:
without locale: 1234.56
with fr locale: 1 234,56

Using strtod() or similar on the second string probably won't work very well ... Also, the space between "1" and "2" in the second output string is a non-breaking one, making the string even prettier :-)

score 0 · Answer 6 · answered May 26 '18 at 15:03

Very old thread, but anyway ... I had the problem to fill a text entry under Gtkmm-3.0 with the outcome of a distance calculation. To make things clearer I add an example, where I have concentrated some wisdoms of several posts I read the last days:

#include <locale>
// next not necessary, added only for clarity
#include <gtkmm-3.0/gtkmm.h>
Gtk::Entry m_Text;
// a distance measured in kilometers
double totalDistance = 35.45678;
std::stringstream str;
// I am using locale of Germany, pay attention to the _
str.imbue(std::locale("de_DE"));
// now we have decimal comma instead of point
str << std::fixed << std::setprecision(4) << std::setw(16) << totalDistance << " km";
// the wished formatting corresponds to "%16.4f km" in printf
m_Text.set_text(str.str());

How to print a double with a comma

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