BigDecimal + Float => Float?

Rails
1

I was just playing around in irb and noticed that while BigDecimal +
Integer
=> BigDecimal, BigDecimal + Float => Float. Is there any reason for
this?
It seems inconsistent and means you have to check the class of any
number you
add to a BigDecimal, which looks very un-Ruby-like to me.

Ashley

2

I’m guessing that the result didn’t need a BigDecimal so a Float
was good enough. Try the same thing with values that are too large
for Float and the result will likely be typed as a BigDecimal.

-Rob

Rob B. http://agileconsultingllc.com
[email protected]
+1 513-295-4739

3

On Friday 31 March 2006 15:15, Rob B. wrote:

I’m guessing that the result didn’t need a BigDecimal so a Float
was good enough. Try the same thing with values that are too large
for Float and the result will likely be typed as a BigDecimal.

-Rob

I thought of that- I figured maybe it was an optimisation. But:

irb(main):004:0> (BigDecimal(“1.0000000000000000000000000001”) +
1.0).class
=> Float

So it looks like it always reduces the available precision.

Ashley

4

I’ve also just realised I meant to post this to ruby-talk :slight_smile:

5

On Friday 31 March 2006 15:29, Justin F. wrote:

It seems right to me that when you add something imprecise (Float) to
something precise (BigDecimal) the result must be imprecise.

I hadn’t thought about it like that - makes sense now. It depends on
the
context though - a float might represent an exact value, although
arguably
you should store is as a rational if it is.

Ashley

6

I thought of that- I figured maybe it was an optimisation. But:

irb(main):004:0> (BigDecimal(“1.0000000000000000000000000001”) + 1.0).class
=> Float

So it looks like it always reduces the available precision.

I would write a bug report in the ruby section of www.rubyforge.org for
this.

Best regards,

Karl

7

Ashley M. wrote:

I was just playing around in irb and noticed that while BigDecimal + Integer
=> BigDecimal, BigDecimal + Float => Float. Is there any reason for this?
It seems inconsistent and means you have to check the class of any number you
add to a BigDecimal, which looks very un-Ruby-like to me.

It seems right to me that when you add something imprecise (Float) to
something precise (BigDecimal) the result must be imprecise.

regards

Justin