Ruby float calculations

ruby-talk
1

Could someone please explain to me why the following happens:

irb(main):001:0> 0.29 - 0.38 + 0.1
=> 0.00999999999999998
irb(main):002:0> 0.29 + (-0.38 + 0.1)
=> 0.00999999999999995
irb(main):003:0> 0.29 + 0.1 - 0.38
=> 0.01

How can I prevent something like this from happening, or at the very
least how can I work around such errors?

Many thanks in advance.

···

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Posted via http://www.ruby-forum.com/.

2

it is a hardware limitation, even a C program will do the same:

     harp:~ > cat a.c
     #include <stdlib.h>
     #include <stdio.h>

     main () {
       printf ("%32.32f\n", 0.29 - 0.38 + 0.1);
       printf ("%32.32f\n", 0.29 + (-0.38 + 0.1));
       printf ("%32.32f\n", 0.29 + 0.1 - 0.38);
     }

     harp:~ > gcc a.c && a.out
     0.00999999999999998112620858137234
     0.00999999999999995337063296574343
     0.01000000000000000888178419700125

with ruby, at least, you can use BigDecimal or the like - but with a serious
speed penalty. in short this is just the way computers work:

   Floating-point arithmetic - Wikipedia

hth.

-a

···

On Fri, 10 Mar 2006, Alexandru Toma wrote:

Could someone please explain to me why the following happens:

irb(main):001:0> 0.29 - 0.38 + 0.1
=> 0.00999999999999998
irb(main):002:0> 0.29 + (-0.38 + 0.1)
=> 0.00999999999999995
irb(main):003:0> 0.29 + 0.1 - 0.38
=> 0.01

How can I prevent something like this from happening, or at the very
least how can I work around such errors?

Many thanks in advance.

--
knowledge is important, but the much more important is the use toward which it
is put. this depends on the heart and mind of the one who uses it.
- h.h. the 14th dali lama

3

Check this out:

d1 = 0.29 - 0.38 + 0.1
d2 = 0.29 + (-0.38 + 0.1)
d3 = 0.29 + 0.1 - 0.38
p (d3 - d2)
p Float::EPSILON
(d3 - d2) < Float::EPSILON #=> true

Computers do not store numbers indefinitely, so there is an "accuracy"
associated with floating point operations. For more information,
check out:
  Floating-point arithmetic - Wikipedia

Cameron

···

On 3/9/06, Alexandru Toma <flash3001@yahoo.com> wrote:

Could someone please explain to me why the following happens:

irb(main):001:0> 0.29 - 0.38 + 0.1
=> 0.00999999999999998
irb(main):002:0> 0.29 + (-0.38 + 0.1)
=> 0.00999999999999995
irb(main):003:0> 0.29 + 0.1 - 0.38
=> 0.01

4

  Floating-point arithmetic - Wikipedia

Thanks for your answers. It's clear now. Expecially this link on the IBM
site made it very clear:
http://www2.hursley.ibm.com/decimal/decifaq1.html#inexact

My question now would be if there is an easy way to implement
floating-point decimal arithmetic in Ruby.

···

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5

My question now would be if there is an easy way to implement
floating-point decimal arithmetic in Ruby.

Forget about this... it has already been answered... I should have been
more careful

with ruby, at least, you can use BigDecimal or the like - but with a
serious
speed penalty. in short this is just the way computers work:

Thanks again

···

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