wtf?!

[holtaf]

Hi guys! I have this simple code

   #include <iostream>
   using namespace std;
   int main() {
   
           for( float f = -0.8f; f <= 0.8f; f+=0.04f )
                   cerr << f << '\n';
   
   
   }

When I run this code ( compiled with g++ ) , the output is

-0.8
-0.76
-0.72
-0.68
-0.64
-0.6
-0.56
-0.52
-0.48
-0.44
-0.4
-0.36
-0.32
-0.28
-0.24
-0.2
-0.16
-0.12
-0.0799999
-0.0399999
5.96046e-08
0.0400001
0.0800001
0.12
0.16
0.2
0.24
0.28
0.32
0.36
0.4
0.44
0.48
0.52
0.56
0.6
0.64
0.68
0.72
0.76

How you see in lines 19-23 the output is in other format. Whats the reason ???
Why it's doing that only when it's closer to zero?
The same result is with double.


Best regards

-holtaf

[1yR4jE8b]

It's because floating point numbers cannot be represented exactly in binary:

Essential Read:
http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html

[anonymous23323124]

Yes, and as such, floating point arithmetic is subject to error. float is 32 bit. Changing float to double (64bit) yields the following output, which is much closer to zero:

-0.8
-0.76
-0.72
-0.68
-0.64
-0.6
-0.56
-0.52
-0.48
-0.44
-0.4
-0.36
-0.32
-0.28
-0.24
-0.2
-0.16
-0.12
-0.08
-0.04
1.249e-016
0.04
0.08
0.12
0.16
0.2
0.24
0.28
0.32
0.36
0.4
0.44
0.48
0.52
0.56
0.6
0.64
0.68
0.72
0.76

(Built with MSVC++10)