CODEWITHSUNDEEP
c++rounding
mvc
mvc

Reputation: 733

Print double with a given precision rounded in given direction

Note: I know there are some similar questions but I couldn't find the answer to what I'm asking here.

I want to print a double to a std::ostream rounded towards +oo or -oo with n decimal places.

Multiplying by 10^n, floor'ing or ceil'ing, and then multiplying the result by 10^-n is not an option, since the result might not be representable.

What I have so far is the following:

#include <iostream>
#include <cfenv>

int main()
{
    double one_third = 1.0/3.0;
    std::cout.precision(4);
    fesetround(FE_DOWNWARD);
    std::cout << one_third << std::endl;
    fesetround(FE_UPWARD);
    std::cout << one_third << std::endl;
}

The output with gcc-4.7.2 is the following:

0.3333
0.3333

I was expecting:

0.3333
0.3334

So what is wrong with this code?

Thanks!

Upvotes: 4

Views: 193

Answers (1)

lapk
lapk

Reputation: 3918

EDIT: This must be a GCC 4.7.2 bug. The code works fine in GCC 4.8.1, but fails in 4.7.2.

This code (example on ideone.com) works just fine with GCC 4.8.1:

#include <iostream>
#include <cfenv>

int main()
{
    double one_third = 1.0/3.0;
    std::cout.precision(4);

    std::fesetround(FE_DOWNWARD);
    std::cout << one_third << std::endl;

    std::fesetround(FE_UPWARD);
    std::cout << one_third << std::endl;

    return (0);
}

Program output:

0.3333
0.3334

Upvotes: 3

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