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: 489 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; } __________________ I Like Ike. Vote for Dwight Eisenhower this November. --This message brought to you by the the Procrastinators Club Of America.

Mar 20th, 2008, 1:32 PM	#3
lectricpharaoh Caffeinated Neural Net Join Date: Jun 2005 Location: Wet west coast of Canada Posts: 889 Rep Power: 4	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). __________________ A man's knowledge is like an expanding sphere, the surface corresponding to the boundary between the known and the unknown. As the sphere grows, so does its surface; the more a man learns, the more he realizes how much he does not know. Hence, the most ignorant man thinks he knows it all. - L. Sprague de Camp

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