[llvm-dev] [question] с-double: 35.7 * 100 vs 3570.0

[Matthias Braun via llvm-dev]

Floating point arithmetic is not intuitive. http://www.lsi.upc.edu/~robert/teaching/master/material/p5-goldberg.pdf is a good introduction to the common pitfalls.

TL;DR 35.7 cannot be represented exactly as a floating point number.

- Matthias

> On Mar 21, 2017, at 10:10 AM, Daniil Troshkov via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi all!
> 
> There is a simple c code:
> 
> double a = 3570.0;
> double b = 35.7 * 100;
> int main ()
> {
>   if (b != a) return 1;
>   return 0;
> }
> 
> It returns 1 due to:
> 40abe400 00000000 //a = 3570.0
> 40abe400 00000001 //b = 35.7 * 100;
> 
> gcc do the same thing, so I think it's ok but why?
> 
> For floats:
> 
> float a = 3570.0;
> float b = 35.7 * 100;
> int main ()
> {
>   if (b != a) return 1;
>   return 0;
> }
> 
> It returns 0...
> 455f2000 //a = 3570.0
> 455f2000 //b = 35.7 * 100;
> 
> I will be grateful to get any explanations.
> 
> 
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