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This came up during testing where I have to compare values between actual output and expected output.

Code:

float nf = 584227.4649743827f;
printf("Output: \t %.9f \n", nf);

Output:

Output:  584227.437500

I clearly have some gaps in my knowledge in C, so could someone explain to me this situation:

  1. Why is there this deviation (0.027474382659420f) in the print ?
  2. Is this only the limitation of print, or is it the float data type limitation?
  3. which value is actually stored in the variable nf?
  4. How should I work with values like this so I don't lose information like having a deviation of 0.027474382659420f during assignment.

Any other suggestion related to this kind of problem in testing would be also much appriciated.

Jabberwocky
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asif1268
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  • Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) – vgru Feb 05 '19 at 10:23
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    @Groo it's nore really a dupe, but the link you suggest is worth reading anyway. – Jabberwocky Feb 05 '19 at 10:30

1 Answers1

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Why is there this deviation (0.027474382659420f) in the print ?

Because float has an accuracy of about 7 digits, and your deviation starts at the seventh digit.

Is this only the limitation of print, or is it the float data type limitation?

It's a limitation of floating point numbers, so that also includes double (although double has a higher precision). It also has to do with the conversion between binary and decimal numbers, so for instance 0.2 is a repeating decimal (well, binary) in binary representation, so it is suspect to rounding errors, too, and might actually turn into something like 0.200000000000000011.

which value is actually stored in the variable nf?

The one that you see printed. The 584227.4649743827f that you specified most likely won't even exist in the binary of your compiled program and is "translated" to the actually used value during compilation.

How should I work with values like this so I don't lose information like having a deviation of 0.027474382659420f during assignment.

Use double, that has an accuracy of about 15-17 digits. Also, you need to remove the f from the 584227.4649743827f, turning it into a double constant instead. If that is not accurate enough, you may have to use external libraries instead for arbitrary precision numbers, such as GMP.

Your floating point numbers most likely adhere to the IEEE 754 standard, but there's no guarantee.

Blaze
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  • *"It's a limitation of floating point numbers"* That is only half of the problem. The major point missing is that there is decimal -> binary -> decimal conversion, and some numbers cannot exist on either numeric system without very high or infinite precision. – user694733 Feb 05 '19 at 10:36
  • @user694733 Good point, that causes some of the most infamous "is floating point math broken" type errors, so I added a bit of an explanation on that. – Blaze Feb 05 '19 at 10:41