[Solved] Precision and converting float variables into strings



  • @
    float start = 92.195781;
    QString show = QString::number(start, 'g', 5);
    ui->label->setText(show);
    @
    The above code example has a precision of 5 and outputs 92.196 (3 decimal places).
    Let's say start changes to 101.392093 - the output now becomes 101.39 (2 decimal places).
    And then start changes to 6.928321 - the output is 6.9283 (4 decimal places).
    I want the output to show 2 decimal places regardless of how big the number is. Is there an easy way to do this?



  • Probaly you have problem with the "g" parameter.
    Because: "For the 'g' and 'G' formats, the precision represents the maximum number of significant digits (trailing zeroes are omitted).". I think, you have to use the "e" or "E" or "f" paramater, because: "For the 'e', 'E', and 'f' formats, the precision represents the number of digits after the decimal point.".



  • Calculate the magitude of your number, and adjust the value of precision accordingly.

    The maginitude here is a basically the value of n in 10^n, like you use for scientific notation. In your examples, that would be 1, 2 and 0 repectively: the largest value of n where the result of 10^n is smaller than the number you working with. In your case, it seems you want to precision to be n+2.

    There are several ways to calculate the magnitude. Mathematically, it is simply 10Log(x), but depending on the range you are interested in and the performance you need, a simple loop dividing by 10 or even a big if/else if construct or something like that may be enough for your needs.


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