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jfelix
jfelix
Associate II
May 10, 2012
Question

float rounding Cosmic compiler

  • May 10, 2012
  • 4 replies
  • 1681 views
Posted on May 10, 2012 at 13:01

Dear all,

I have some strange roundings with float values: 

for example when I write 0.05 in eeprom (calibration parameters) it becomes 0.049999997 when I read It.

0.2 becomes  0,199999988.

I use Cosmic compiler, I tried to change FLT_DIG in float.h but nothing changes.

If someone has a clue please help me.

Many thanks in advance

#floating-point---just-say-no #stm8-stm8l-float-rounding-cosmic
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    4 replies

    jdf25252
    jdf25252
    Associate II
    May 10, 2012
    Posted on May 10, 2012 at 16:36

    Neither 0.2 nor 0.05 can be represented exactly in floating point format.  Google IEE754 and check out the Wikipedia entries for both the 1985 & 2008 versions of the standard.  Those should give you a good idea of what you're seeing.

    jdf

    jdf25252
    jdf25252
    Associate II
    May 10, 2012
    Posted on May 10, 2012 at 16:37

    ooops

    I meant IEEE754 (3 E's)

    jdf

    Andrew Neil
    Andrew Neil
    Super User
    May 12, 2012
    Posted on May 12, 2012 at 19:58

    As already noted, this is an inherent artifact of floating-point.

    Another reason to avoid floating-point unless you really, really have a very specific need for it...
    A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    jfelix
    jfelixAuthor
    Associate II
    June 12, 2012
    Posted on June 12, 2012 at 16:34

    Many thanks to all, I finally decide not to use float numbers. 

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