newbie question: floating point addition operation

Can someone explain this:

CG-USER(9): (+ 2.32 0.001)
2.3209999

I have Allegro Common Lisp, 9.0, Windows 7 machine.

What is the proper way to correctly display 2.321 ?

Since floating point numbers are represented internally as numbers by the base of two there isn't a way to represent 0.001 exactly. There will always be rounding errors. This isn't a problem of Lisp but of floating point arithmetic in general. The only way around it is to print it rounded:

(format t "~,5f" (+ 2.32 0.001))
-> 2.32100

Thanks, that suits for me.

... and, BTW

http://docs.oracle.com/cd/E19957-01/806 ... _math.html

Cheers

MA

Maybe it was my high expectations

hajovonta wrote:Maybe it was my high expectations

If you want exact rationals, use exact rationals.

CL-USER> (+ 232/100 1/1000)
2321/1000

ok, but then I want to display the result in floating point format...

You can convert rationals to floats:

(float (+ 232/100 1/1000))
-> 2.321

Konfusius wrote:You can convert rationals to floats:

(float (+ 232/100 1/1000))
-> 2.321

Technically, that (somewhat) reintroduces the problem...

L> (format t "~E" (float (+ 232/100 1/1000)))
2.3210001e+0
NIL
L> (= 2.3210001 (float (+ 232/100 1/1000)))
T

The Right Thing[TM] would be to use something like Wu-Decimal:

L> (ql:quickload "wu-decimal")
; Loading "wu-decimal"
[package wu-decimal]...
("wu-decimal")
L> (wu-decimal:enable-decimal-printing-for-ratios)
NIL
L> (format t "~A" (+ 232/100 1/1000))
2.321
NIL

Of course, I'm nitpicking. If you're just hacking something together to quickly visualize something, using floating point and rounding the results isn't so bad.