How to convert a string to given precision double?

Question

Updated:

I have been trying all methods I could find in stackoverflow, and still could not find a solution.

My point is, I have a string "23.46" and would like to transfer it to double or float anyway. This is in order to be used in another library.

But no matter how I trancate,floor,*100,add,round, it always gives me 24.4600000001 or something like this.

I know there are some precision issue while transfer. But I do need a way to give me a number that double d = 24.46 precisely.

============================================================================== I have many string values and some of them are double with converted precision as below:

char pt[100];
sprintf(pt, "%.2lf", i);
return string(pt);

Now on the other side of the code, I need to convert the strings back to double, but I tried strtod and atof with precision loss.

My questions are:

1. what is the good way to check if a string could be a double?

2. how to convert string back to double with given precision? I only need it to %.2lf be like:

0.21, 35.45, ...

Thanks so much!

Answer 1

Given that you say that using std::strtod is not giving you a solution to the problem, you can use stringstreams to parse strings as doubles, you can also use its flags to assert if the contents of the string are convertible.

Here is an example with some conversions back and forth, and with checks to see if the whole string (not just some digits in it), is parseable as double:

Live demo

#include <iostream>
#include <sstream>
#include <iomanip>

int main()
{
    std::string str = "23.4k7"; //string with a character in the middle
    std::istringstream ss(str); //put string in a stream
    double num;
    ss >> num; //convert string to double

    if(!ss.fail() && ss.eof()) { //check if the whole string is parseable
        std::cout << "is parseable" << std::endl;
    }
    else {
         std::cout << "is not parseable";
         return EXIT_FAILURE;
    }

    std::stringstream to_num;
    to_num  << std::fixed << std::setprecision(2) << num; //double to string 2 decimal places
    std::cout << to_num.str();

    to_num >> num; //final conversion to double
}

Since the string has an alphabetic character in it, this will output:

is not parseable

But if you use a valid number it will output the converted value:

string str:

234.2345

Output:

is parseable
234.23

Note that you could use

Live demo

if(ss >> num)
    std::cout << "is parseable";

This, however, has a weakness, it will still parse if you have for instance 123.45rt56, 123.45 will be parsed, the rest will be discarded, the way it is in the sample code, if the string has any character, it will return an error. You can choose the more appropriate way for your needs.