Solved

Strange output..

Posted on 2006-07-20
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Last Modified: 2010-04-15
Consider the following small code.
#include <stdio.h>
float a = 0.7;
main()
{
      if (a<0.7)
            printf("Hello\n");
      else
            printf("World\n");
}

The output of this  is “Hello”…….
Can anybody explain this ?….

If I change 0.7 to 0.8 then output will be “World”…

Why is the reason behind this?

Thanks in advance
0
Comment
Question by:jpsarkar
6 Comments
 
LVL 14

Expert Comment

by:StillUnAware
ID: 17144928
try using a cast:

 if (a < (float)0.7)

I guess the compiler interprets the 0.7 as double, it is more accurate than float, and the values does't match, because of different precision.
0
 
LVL 8

Expert Comment

by:deepu chandran
ID: 17144935
hi,

Try this code segment will get you more about this problem

printf("%20.10f %20.10f\n",0.5, 0.5f);
output is: 0.5000000000 0.4999999881

if you want more details just visit

http://docs.sun.com/source/806-3568/ncg_goldberg.html
cheers

deepu
0
 
LVL 45

Expert Comment

by:sunnycoder
ID: 17144981
Hi jpsarkar,

Floating point comparisons are not exact ... typically you would assume float1 == float2 if fabs(float1-float2)<delta where delta is a very small number. Alternatively, you can scale the numbers e.g. in the above example you could have multiplied a by 10 and then compared with 7 instead of 0.7 ... Ofcourse for this you need to know the precision in advance.

Here are some previous discussions on this topic which should provide all the details you are looking for ... In case you need further clarifications, feel free to post back

http://www.experts-exchange.com/Programming/Programming_Languages/C/Q_20707050.html -> long nice explanation
http://www.experts-exchange.com/Programming/Programming_Languages/C/Q_21385870.html
http://www.experts-exchange.com/Programming/Programming_Languages/C/Q_20690437.html
http://www.experts-exchange.com/Programming/Programming_Languages/C/Q_10350949.html
http://www.experts-exchange.com/Programming/Programming_Languages/C/Q_20392174.html

Cheers!
sunnycoder
0
 

Author Comment

by:jpsarkar
ID: 17154451
OK deepu…
The same logic should be true and same type of output should come for 0.8 or even 0.6, or 0.5 etc…….
But for those the output is not like 0.7
……

What is the reason behind this ??….
0
 
LVL 8

Accepted Solution

by:
manish_regmi earned 50 total points
ID: 17162219
Firstly,
    The floating point operations are only approximations. Because the precision is infinite.

secondly,
   It is Architecture and Compiler Dependent how the floating point numbers are temporarily stored for calculation. So different compilers and different  Arch results different answers.

The correct way to do is pointed out by sunny.

Read this if you need more explanation
http://www.cygnus-software.com/papers/comparingfloats/Comparing%20floating%20point%20numbers.htm

regards
Manish Regmi
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