A Strategy for Displaying Numeric Data with Significant Figures

Introduction

Problem: Some times numeric data, for example floats and doubles, needs to be displayed with a certain amount of decimal precision and at times with a given amount of significant figures.

For example:

4.5568 displayed with 2 Significant Figures is: 4.6
0.0221 displayed with 3 Significant Figures is: 0.022
5.9 displayed with 1 Significant Figures is: 6

Strategy: Using the log10 function we can calculate the number of digits for the integer part of the number we are manipulating. The decimal part of the number can be displayed properly with a sprintf call or something similar as we can see in our function below. This solution also rounds the final number, which is usually desirable when displaying significant figures.

double CalculateSigFig(double num, int sigfig)
{
    char buff[128];//temp buffer

    double temp = 0; 
    int lognum = 0;
    std::string formatbuff;//format string


    if(sigfig <= 0 || num == 0)//damage control

        return num;

    temp = int(num);
    if(temp * -1 > 0)//check for negative numbers

        temp *= -1;

    lognum = (int)log10(temp);//get significant digits of integer part of num

    ++lognum; //adjust for log10 results

    sigfig -= lognum;//subtract for decimal precision calculation


    if(sigfig < 0)//check for negative results

        sigfig = 0;


    //construct format string

    formatbuff = "%";
    itoa(lognum, buff, 10);//convert lognum to int

    formatbuff += buff;
    formatbuff += ".";
    itoa(sigfig, buff, 10);
    formatbuff += buff;
    formatbuff += "f";
    //end construct format string


    //use format string to get desired results

    sprintf(buff, formatbuff.c_str(), num);

    return atof(buff);//return double converted from string

}