C++ : Converting String to Double - c++

Question

It's C++ code written in Visual Studio 2015. It's something as below,
LPSTR *endPtr;
string strDouble = "0.03456";
double valDouble = strtod(strDouble.c_str(), &endPtr);
Now the output in valDouble is "0.0345555566", something like that.
I want the value in valDouble to be exactly "0.03456".
Basically the value of "0.0345555566" needs to be rounded to say "0.03456".
Is there a way it can be achieved?
BTW, the value in strDouble changes all the time. So it's not possible to set precision to say 5 or something like that. Below are few examples that goes in to strDouble.
string strDouble = "0.1889";
string strDouble = "0.00883342";
string strDouble = "0.2111907";
string strDouble = "3.0045";
string strDouble = "1.45";

Answer 1

I want the value in valDouble to be exactly "0.03456".
That's not possible, unless you target a system whose double floating point representation can represent that number.
There exists no representation for 0.03456 in the ubiquitous IEEE 754 binary64 standard which your CPU probably uses. The closest representable number is 3.45600000000000004418687638008E-2. That's the number that you should get whether you use strtod, stod or a character stream to convert the string.
Is there a way it can be achieved?
In order to represent 0.03456 exactly on a system whose floating point cannot represent that number, you must use integers to represent the number. You can implement arbitrary precision arithmetic, fixed-point arithmetic or a decimal floating point using integers.
Basically the value ... needs to be rounded to say "0.03456".
You can round the output when you convert the non-exact float into a string:
std::cout << std::setprecision(4) << 0.03456;
BTW, the value in strDouble changes all the time. So it's not possible to set precision to say 5 or something like that.
Then you have to record the number of significant digits in the input string in order to use the same precision in output.
Here's an example function for that purpose:
template<class Range>
auto get_precision(const Range& r)
{
auto is_significant = [](auto c) {
return std::isdigit(c) && c != '0';
};
auto first = std::find_if(std:: begin(r), std:: end(r), is_significant);
auto last = std::find_if(std::rbegin(r), std::rend(r), is_significant).base();
return std::count_if(first, last, [](auto c) {
return std::isdigit(c);
});
}
// demo
std::cout << get_precision("0.03456"); // 4

Answer 2

Assuming that you want the number of digits after decimal point as some percent of the total number of digits after the decimal, you could do something like,
Calculate the number of digits after decimal point. Let it be n
Now convert the string to decimal just like you are doing. Let this be d
Now if you want 50% of the decimal places to be retained, you could do use an old trick,
double d_new = round(d * pow(10.0, 5)) / pow(10.0, 5). Assuming precision till 5 digits.
Note: Unlike the other answers, here you are rounding the original decimal itself. Not just printing the rounded decimal to stdout.
Example:
#include<stdio.h>
#include<cmath>
int main(){
double a = 0.0345555566;
double b = 0.00883342;
double c = 0.2111907;
double a_new = round(a * pow(10.0, 5)) / pow(10.0, 5);
double b_new = round(b * pow(10.0, 4)) / pow(10.0, 4);
double c_new = round(c * pow(10.0, 3)) / pow(10.0, 3);
printf("%.10f\n", a_new);
printf("%.10f\n", b_new);
printf("%.10f\n", c_new);
}
See the 50% precision
Results:
0.0345600000
0.0088000000
0.2110000000

Answer 3

Use string stream instead of strtod:
#include <iostream>
#include <sstream>
double convert(std::string string) {
std::stringstream s(string);
double ret = 0;
s >> ret;
return ret;
}
int main() {
std::cerr << convert("0.03456") << std::endl;
std::cerr << convert("0.1889") << std::endl;
std::cerr << convert("0.00883342") << std::endl;
std::cerr << convert("0.2111907") << std::endl;
std::cerr << convert("3.0045") << std::endl;
std::cerr << convert("1.45") << std::endl;
return 0;
}
On my system, this gives:
0.03456
0.1889
0.00883342
0.211191
3.0045
1.45
As some have pointed out in the comments, not all numbers can be represented with doubles. But most of the ones you listed can be.