Answering my own problem here.

I've just searched through the mailing list archives and it seems that 
if I add [instance=doublefun] I do not get the Arithmetic Overflow 
error. Joy!

Thanks

Best Wishes

Keith McKay

On 16/05/2020 15:36, Keith McKay wrote:
>
> Colleagues
>
> I'm having problems with an Arithmetic Overflow error when calculating 
> the x and y values for functions of the form:
>
> y = ax^4 + bx^3 + cx^2 + dx + e
>
> to which I then add a bit of randomness and other embellishments. I 
> have had a look in the Metafun and Metafont manuals but nothing jumps 
> out at me. In the minimum working example below, this Arithmetic 
> Overflow occurs when xord =13.5 but I can get the calculation to work 
> if I separate out the equation and then add together, since it seems 
> to be failing on the pow(xord,4) instruction. Is there a switch in 
> MetaFont that should be set or a better way of getting round this 
> Arithmetic Overflow error?
>
> Thanks
>
> Best Wishes
>
> Keith McKay
>
> %%%%%%%%%%%%%MWE%%%%%%%%%%%
>
> \setuppapersize [A4,landscape]
>
> \starttext
>
> \startMPpage
>
> StartPage;
>
> width := PaperWidth ; height := PaperHeight ; unit := cm ;
>
> path p;
>
> p := unitsquare scaled .1cm ;
>
> path pat;
>
> pat := (0.0*cm,19.935*cm);
>
> for xord = 0 step 0.1 until 25:
>
> show xord;
>
> %%%%%%%%fails at xord = 13.5%%%%%%%%%
>
> yord := ((-0.000268117) * pow(xord,4)) +(0.0136949 * pow(xord, 3)) + 
> ((-0.16608) * sqr(xord)) + ((-0.771743) * xord) + 19.935;
>
> %%%%%comment out above and uncomment below and it works%%%%
>
> % yord1 := (-0.000268117) * sqr(xord);
>
> % yord1a := yord1 * sqr(xord);
>
> % yord2 := (0.0136949 * pow(xord, 3));
>
> % yord3 := ((-0.16608) * sqr(xord));
>
> % yord4 := ((-0.771743) * xord);
>
> % yord := yord1 + yord1a + yord2 + yord3 + yord4 + 19.935;
>
> xrand := (uniformdeviate(1) - 0.5)*2;
>
> yrand := uniformdeviate(1) - 0.5;
>
> xcoord := xord + xrand;
>
> ycoord := yord + yrand;
>
> if odd xord:
>
> pat := pat ... (xcoord*cm, ycoord*cm);
>
> else:
>
> pat := pat --- (xcoord*cm, ycoord*cm);
>
> fill p shifted (xcoord*cm, ycoord*cm) withcolor 
> (uniformdeviate(1),uniformdeviate(1),uniformdeviate(1)) ;
>
> fi;
>
> draw pat withpen pencircle scaled (uniformdeviate(0.75)*mm) withcolor 
> (uniformdeviate(1),uniformdeviate(1),uniformdeviate(1)); 
> %withtransparency (1, .5);
>
> drawdot (xcoord*cm, ycoord*cm) withpen pencircle scaled 
> (uniformdeviate(2.5)*mm) withcolor 
> (uniformdeviate(1),uniformdeviate(1),uniformdeviate(1));
>
> endfor;
>
> StopPage;
>
> \stopMPpage
>
> \stoptext
>