Accuracy of float types

Question

1.00/5 (6 votes)

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void main()
{
    float a=0.7;
    if(a<0.7)
    printf("A");
    else
    printf("B");
    getch();
}

If i m putting 0.7 and 0.9 in the question answer is coming "A".else for every option as 0.3,0.5,0.6,0.8 it is coming "B".Why is it so??I know double and float concept.please focus on that,that why it is happening with 0.7 and 0.9 only and not on any other.query is this only,if any one can clear in binary form then it will be most helpful.

Posted 11-Oct-10 15:28pm

mstanwar

Updated 14-Oct-10 2:29am

#realJSOP

v4

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Peter_in_2780 11-Oct-10 22:04pm

Cross-post! Ask your question ONCE ONLY! Also, it's not a very interesting question.

Sauro Viti 14-Oct-10 8:18am

Look at the guidelines about posting a question: "3.Keep the subject brief but descriptive."
Your subject doesn't tell us ~~nothing~~anything about your problem, and also it's not an interesting question.

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Emilio Garavaglia · Answer 1 · 2010-10-12T05:17:00

Decimal round numbers (with finite precision) are not necessarily binary round numbers.

0.7, 0.9 and 0.3 are binary rounded to the precision of their binary representation used by the compiler for double-s.

Now, in your case, the problem is that all numeric literals having a decimal point are treated by the compiler as double. And the precision of double is more than the precision of float.

When you assign a double constant to your a variable (of type float), you loose some of the precision.
Then you convert a back to double (adding bogus bits to its mantissa) to compare it with another constant.

Try either to declare a as double, or to postfix an "f" after the constants (like 0.3f) to make them "float"-s.

Mohd Wasif · Answer 2 · 2010-10-11T18:50:00

Solution 1

Program is running correctly I dont know why u are confused
as u have already set the value of a

Posted 11-Oct-10 18:50pm

Mohd Wasif

Aescleal · Answer 3 · 2010-10-12T00:04:00

Not on the compilers I've just tried it (gcc 3.3 and VC++ 2008) on it's not. With warning levels set as high as they go it won't compile (you're assigning a double to a float) as C++ so that may give you some sort of hint as to what's happening on your compiler and system.

To the muppet that gave me a "1" for this answer: Have you compiled the original posters code and managed to reproduce his results? No, didn't think so...

#realJSOP · Answer 4 · 2010-10-14T02:29:00

Solution 4

Because float types are notoriously inaccurate.
Using double would be better until you started doing math with it, and then, all bets are off.

If you want the best possible accuracy, use a decimal type.

Posted 14-Oct-10 2:29am