Programming Forums > Application Development > C++

Lakrids · Mar 20th, 2008, 7:34 AM

Hello,

I have been programming for quite a while now, but the following behavior is one of the most weird ones I ever encountered. It really is annoying me, and I just can't get to the bottom of it. So, in short, the other day I was writing a very small program which calculates all possible solutions to a given problem. The program didn't work, and after some debugging, I was astonished that the following code just doesn't seem to work for some obscure reason. The problem is that nothing is written to the screen in the loop :

 cplusplus Syntax (Toggle Plain Text) 
#include <iostream>
 
int main() {
 
    for (double C=0.0; C<100.0; C+=0.1)
        if (C==1.0)
            std::cout << C << std::endl;
 
    return 0;
}
#include <iostream>

int main() {

    for (double C=0.0; C<100.0; C+=0.1)
        if (C==1.0)
            std::cout << C << std::endl;

    return 0;
}

The program always evaluates (C==1.0) as false. Why?

Ancient Dragon · Mar 20th, 2008, 8:13 AM

Never use equality == when working with doubles because as you have found out it will frequently not evaluate correctly due to the way floating point values are stored in memory. More than likely what is happening in your case is that there are some very small decimal places that you did not intend to be there.

Here's how to make it work

(Toggle Plain Text)

int main() {
    for (double C=0.0; C<100.0; C+=0.1)
        if( C > 1.0 && C < 1.2)
            std::cout << C << "\n";

    return 0;
}

lectricpharaoh · Mar 20th, 2008, 1:32 PM

To expand on what Ancient Dragon said, you need to remember that it's binary floating point. There are some numbers that cannot be exactly represented, and 0.1 is one of those (much like one-third cannot be exactly represented in decimal).

Lakrids · Mar 20th, 2008, 2:51 PM

Thank you!

Mar 20th, 2008, 7:34 AM	#1
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Extremely weird program behavior Hello, I have been programming for quite a while now, but the following behavior is one of the most weird ones I ever encountered. It really is annoying me, and I just can't get to the bottom of it. So, in short, the other day I was writing a very small program which calculates all possible solutions to a given problem. The program didn't work, and after some debugging, I was astonished that the following code just doesn't seem to work for some obscure reason. The problem is that nothing is written to the screen in the loop : cplusplus Syntax (Toggle Plain Text) #include <iostream> int main() { for (double C=0.0; C<100.0; C+=0.1) if (C==1.0) std::cout << C << std::endl; return 0; } #include <iostream> int main() { for (double C=0.0; C<100.0; C+=0.1) if (C==1.0) std::cout << C << std::endl; return 0; } The program always evaluates (C==1.0) as false. Why?

Mar 20th, 2008, 8:13 AM	#2
Ancient Dragon PFO God In Training Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 550 Rep Power: 4	Re: Extremely weird program behavior Never use equality == when working with doubles because as you have found out it will frequently not evaluate correctly due to the way floating point values are stored in memory. More than likely what is happening in your case is that there are some very small decimal places that you did not intend to be there. Here's how to make it work (Toggle Plain Text) int main() { for (double C=0.0; C<100.0; C+=0.1) if( C > 1.0 && C < 1.2) std::cout << C << "\n"; return 0; } int main() { for (double C=0.0; C<100.0; C+=0.1) if( C > 1.0 && C < 1.2) std::cout << C << "\n"; return 0; } __________________ True Terror is to wake up one morning and discover that your high school class is running the country - Kurt Vonnegut Jr.

Mar 20th, 2008, 1:32 PM	#3
lectricpharaoh Caffeinated Neural Net Join Date: Jun 2005 Location: Dry west coast of Canada Posts: 1,039 Rep Power: 5	Re: Extremely weird program behavior To expand on what Ancient Dragon said, you need to remember that it's binary floating point. There are some numbers that cannot be exactly represented, and 0.1 is one of those (much like one-third cannot be exactly represented in decimal). __________________ And once again, Probability proves itself willing to sneak into a back alley and service Drama as would a copper-piece harlot. - Vaarsuvius, Order of the Stick

Mar 20th, 2008, 2:51 PM	#4
Lakrids Newbie Join Date: Dec 2007 Posts: 28 Rep Power: 0	Re: Extremely weird program behavior Thank you!

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Programming Forums > Application Development > C++

Extremely weird program behavior