Problem with mod and the value 2.3 times 100

There seems to be some kind of overflow problem with mod, which occurs on all the Matlab versions I have access to - 2007a, 2010a and 2014b.

> format long
> mod(2.3*100,10) 
> 9.999999999999972

mod(230,10) is fine and gives the right value, zero, but

> qqq=2.3*100
> mod(qqq,10)

Also misbehaves. However

> qqq=2.3*10*10
> mod(qqq,10)

Is fine and does give a result of zero. I'm confused. I was going to submit a bug report, but it seems odd that a bug so easily found could be hanging around for so long. Could somebody offer some insight as to what's going on?

Answer 1

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F