How to use std::setprecision in C++

Question

I'm experimenting with std::setprecision, is good to say how many decimals you want to show but... I have a problem. I realized that also affects other variables and not only the variable that I want the precision.

Float a = 2.15111;
Float b = 3.15222;

Cout << std::fixed << std:: setprecision(2) << a; // 2.15
Cout << b; // 3.15

In the above code, I want to implement the precision only to variable a, but it also affects to variable b... How can I fix that? I want to print b with the default precision.

Answer 1

I found this funny behaviour with std::setprecision, when I pass a negative value (-1) as argument it appears to reset the previous setting for std::setprecision (not the std::fixed probably), I'm not sure if it violates any rules, but even though it works, I think it's not a good practice : )

#include <iostream>
#include <iomanip>

int main() {
float a = 2.15111;
float b = 3.15222;

std::cout << std::fixed << std:: setprecision(2) << a << std::endl; // 2.15
std::cout << std::setprecision(-1) << b; // 3.152220

return 0;
}

Answer 2

The original precision can be obtained prior to setprecision, via std::cout.precision(). You can then use std::defaultfloat to reset to default floating point notation: http://www.cplusplus.com/reference/ios/defaultfloat/

float a = 2.15111, b = 2.15112;
std::streamsize ss = std::cout.precision();

// original io formatting
std::cout << a << " " << b << "\n";

// edited
std::cout << std::fixed << std::setprecision(2) << a << " " << b;

// restored
std::cout << std::defaultfloat << std::setprecision(ss) << a << " " << b;

However, resetting the entire state of std::cout may be an even better idea, but I haven't figured out how to do so for precision with resetiosflags(), since precision does not appear to be a flag, and is by default 6.

http://www.cplusplus.com/reference/iomanip/resetiosflags/

Answer 3

Thanks Hatsy Rei, I guess your example was the way to go, I'd probably wouldn't use the negative value in the end anyways, kinda illogical : )

Answer 4

Ipang Interesting observations, and presumably accurate too. :>

https://stackoverflow.com/questions/1957532/negative-precision-values-in-ostream

Answer 5

I see how many extra steps you need to do to return again the default precision, I think If I only want to set how many decimals I want to show only for one variable, maybe the old printf is not a bad option... Thank you so much to everyone!