Haskell question

Mike Gunter [email protected]
28 Dec 2000 16:02:39 -0800


This is fairly subtle.  The fact that the numbers are printed with a
decimal point hints that they are floating point numbers.
Floating-point arithmetic's inexactness makes equality tests less
valuable than for, e.g., Integer arithmetic.  If you change the "**"s
to "^"s you'll get [3,4], etc. (or [3,4,5] if you change "[x,y]" to
"[x,y,z]").

        mike gunter

E.g.:

Main> [[x,y,z] | x<-[1..50],y<-[1..50],z<-[1..50],x^2+y^2==z^2]
[[3,4,5],[4,3,5],[5,12,13],[6,8,10],[7,24,25],  ... ]



> Hello!
>    I am reading Discrete Mathematics Using a computer by Hall and O'Donnell 
> and in the introduction to hugs section, they give this example:
> [[x,y] | x<-[1..50],y<-[1..50],z<-[1..50],x**2+y**2==z**2]
> now I type it in and expect it to return a number of results, but it 
> doesn't, it shows:
> [[8.0,15.0,17.0],[14.0,48.0,50.0],[15.0,8.0,17.0],[15.0,20.0,25.0],[20.0,15.0,25.0],[21.0,28.0,35.0],[27.0,36.0,45.0],[28.0,21.0,35.0],[30.0,40.0,50.0],[36.0,27.0,45.0],[40.0,30.0,50.0],[48.0,14.0,50.0]]
> My question is where is [3.0,4.0,5.0],[5.0,12.0,13.0]?
> I have looked everywhere I could think of to see what I am doing wrong, but 
> I have no idea.  Maybe a rounding error somewhere?
> Thank you,
>   Scott Anderson