Re: Reading numbers from a file

In article <1149954290.927527.16870
@h76g2000cwa.googlegroups.com>, grodriguezpiriz@gmail.com
says...


[ ... ]


[...and elsethread mentioned wanting to ignore other data
in the file]

You've gotten a number of replies, but I thought I'd add
one more way this could be done. At least as I understand
the situation, you only want to read the numbers from the
file, and ignore everything else.

One way to handle this would be to create a locale to
reflect that for your purposes, the file contains only
numbers (digits) and other "stuff" you're going to ignore
between the digits. From a viewpoint of reading the
numbers, everything is basically the same as whitespace
-- it separates one number from another, but otherwise
has no meaning. Therefore, we start by creating a ctype
facet that says it IS whitespace:

class number_only: std::ctype<char> {
    number_only() : std::ctype<char>(get_table()) {}
    static std::ctype_base::mask const *get_table() {
        static std::ctype_base::mask *rc;

        if ( rc == 0) {

        // create a character classification table
            rc = new std::ctype_base::mask
                [std::ctype<char>::table_size];

        // say that _everything_ is whitespace
            std::fill_n(rc, std::ctype,char>::table_size,
                        std::ctype_base::space);

        // except for [0-9]:
         for (int i='0'; i<='9'; i++)
            rc[i] = std::ctype_base::digit;
        ]
        return rc;
    }
};

From there, we imbue the stream with a locale using that
facet, and we can just treat it as a file of numbers:

int main() {
    std::ifstream x(wherever);

    number_only n;
    x.imbue(std::locale(std::locale(), n);

    // now we can read in numbers, and everything
    // else is ignored automagically.
    std::vector<int> numbers;

    // read in all the numbers:
    std::copy(std::istream_iterator<int>(x),
        std::istream_iterator<int>(),
        std::back_inserter(numbers));

    // just for grins, we'll display them, one per line:
    std::copy(numbers.begin(), numbers.end(),
        std::ostream_iterator<int>(std::cout, "\n"));

    return 0;
}

--
    Later,
    Jerry.

The universe is a figment of its own imagination.