TITLE: formatting numbers

(Newsgroups: comp.lang.c++, 11 Jan 2000)

THOMAS: "Jim Thomas" <jrthomas@wcoil.com>

> I need to know how to put the comma in 10,000. I did it in a 
> rather large function.I would like to find a easier way to 
> display $10,000.00 without having to do modulus math. This 
> would be greatly appreciated.

KUEHL: Dietmar Kuehl <dietmar.kuehl@claas-solutions.de>

I have seen several answers suggesting some kind of post processing.
The "right" answer is to just install a suitable 'std::numpunct<char>'
facet and be done since the numeric formatting then inserts the
thousands separators automatically. More concrete, just do this:

  class my_numpunct: public std::numpunct<char {
    std::string do_grouping() const { return "\3"; };
  };
  int main() {
    std::locale global;
    std::locale loc(global, new my_numpunct);
    std::cout.imbue(loc);
    std::cout << 10000.0 << "\n";
  }

What is happening is this: The numeric formatting functions used by
'std::ostream' actually call methods of the 'std::num_put' facet which
is taken from the current 'std::locale'. A locale is just a collection
of objects which encapsulate certain internationalized options like the
exact formatting of numeric values. Each of the objects is called a
facet. Basically, facets are just objects of classes which conform to
a few basic requirements. The 'std::num_put' functions which transform
a numeric value like an 'int' or a 'float' to sequence of characters
internally use methods from the 'std::numpunct' facet to determine the
exact format. One of these methods is the function 'grouping()' which
returns a string describing the positions at which thousands separators
are to be inserted. The string "\3" says that after groups of three
digits a thousands separator is to be inserted.

Now, the above code creates a new 'std::numpunct<char>' facet with the
suitable grouping. It then goes on to create a 'std::locale' object
which is just the global locale object with only the
'std::numpunct<char>' facets replaced. Finally, the newly created
locale object is installed, or 'imbue()'ed in the stream 'std::cout'.
By default, the global locale at the construction time of the stream is
used. The locale of a stream can be replaced using the function
'imbue()' (defined in 'std::ios_base'). The global locale can be
replaced using the static function 'std::locale::global()'.



_______________________________________________
cpptips mailing list
http://cpptips.hyperformix.com