# Ruby float calculations

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?

On Fri, 10 Mar 2006, Alexandru T. wrote:

least how can I work around such errors?

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 3/9/06, Alexandru T. [email protected] 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

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”
check out:
Floating-point arithmetic - Wikipedia

Cameron

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

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

Floating-point arithmetic - Wikipedia