C++ String to Number Custom function problem - c++

Question

I am trying to convert a string to number(long double) in C++. The problem arises when the number of digits after decimal point is greater than 3. It automatically rounds-off the to the nearest third digit after the decimal.
Additional info:
compiler: mingw
os: win10
Here's the code (test.cpp):
#include<iostream>
#include <string>
#include <math.h>
using namespace std;
double long c2n(string n) {
double long num=0;
bool isdec = false, is_ve=false;
// is_ve checks if number is negative
// isdec checks if the decimal point is reached and numbers can be added after decimal
char c;
// to store the the character which needs to be checked
short i = 0, count=1;
if (n.at(0)=='-')
{
i=1;
is_ve=true;
}
for (; i < n.length(); ++i)
{
c=n.at(i);
if (c=='.'){
isdec=true;
continue;
}
if (!isdec)
num=num*10+(c-'0');
else{
num = num + (c-'0')/pow(10,count);
count++;
}
}
if (is_ve)
{
return -num;
}
return num;
}
int main(int argc, char const *argv[])
{
cout << c2n("-912.301956") << endl;
return 0;
}
Here's the output:
D:\--path-->g++ -o test.exe test.cpp
D:\--path-->test.exe
-912.302
What I discovered later:
if in the main function, we pass "-912.3016"
cout<< c2n("-912.3016") <<endl;
then output comes out to be:
D:\--path-->g++ -o test.exe test.cpp
D:\--path-->test.exe
-912.302
but if we pass "-912.3015"
cout << c2n("-912.3015") <<endl;
then the o/p:
D:\--path-->g++ -o test.exe test.cpp
D:\--path-->test.exe
-912.301
Should I take double instead of long double or there is any other problem?

Answer 1

The default precision of std::cout is 6 as set by std::ios_base::init. So
auto val = 1234.56789;
std::cout<<val<<'\n`;
yields 1234.57 i.e. 6 digits (and rounds it accordingly). Set the precision accordingly using setprecision from the iomanip header and you should be able to see the correct value.
std::cout << std::setprecision(12) << c2n("-912.301956") << std::endl;