formatting floating numbers into 7 digits?

Discussion in 'C++' started by Ingo Nolden, May 15, 2005.

  1. Ingo Nolden

    Ingo Nolden Guest


    I want to format floats ( doubles ) into small strings of 7 characters.

    I tried to use std::eek:stream, but I don't get the desired behaviour.
    Especially in scientific notation I am not comfortable with things like
    e+007. This is wasting 2 digits for the two zeros.

    I didn't try boost::format, because it doesn't seem to me it will make
    any difference on this. At least the documentation doesn't point it out.

    Does anyone know a good way?

    Ingo Nolden, May 15, 2005
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  2. Ingo Nolden

    Mike Wahler Guest

    Not all required values of type 'float' or 'double'
    can be represented by only 7 text characters. Do
    you want those values truncated (and from which 'end'),
    or are your values all within seven text characters,
    or what? How many digits on either side of the decimal
    do you want? IOW you don't give enough information.
    Once you've decided the answers to my above questions,
    you can use a 'std::eek:stringstream' object with the manipulators
    'std::fixed' and 'std::setprecision'. The former is declared
    by <ios>, the latter by <iomanip>

    #include <ios>
    #include <iomanip>
    #include <iostream>
    #include <ostream>
    #include <sstream>

    int main()
    double d (3.1416);
    std::eek:stringstream oss;
    oss << std::fixed << std::setprecision(2) << d;
    std::cout << oss.str() << '\n'; /* prints 3.14 */
    return 0;

    Also, if you want to pad an output 'field' to a width
    greater that the actual number of output characters
    generated by <<, see the 'std::setw()' manipulator,
    also declared by <iomanip>.

    std::cout << '*' << std::setw(5) << "abc" << '*' << '\n';

    /* prints *abc * */

    (If the argument to 'setw()' is less than the number of
    output characters generated by <<, the output width is *not*
    truncated to that specified with 'setw()'.)

    std::cout << std::setw(1) << "abc\n";

    /* prints abc, not a */

    Mike Wahler, May 15, 2005
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  3. Ingo Nolden

    Ingo Nolden Guest

    yes, of course and from the end that produces the least error
    Ok, I'll try. I want it to print a value with the highest possible
    accuracy within a maximum of 7 characters.


    1234567890123344.123 -> 1.23e15
    0.000000000012345678 -> 1.2-e11
    1.0 -> 1.0 ( or maybe 1 or 1. )
    0.123 -> 0.123

    yes, it works well if the output is smaller than my 7 digits, but if
    scientific notation is necessary to represent my number as accurate as
    possible, it will use more than 7 digits, and it will also waste digits
    with leading zeros in the exponent.
    I really appreciate your help

    Ingo Nolden, May 15, 2005
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