Hi,


> here a ; is missing after the q

That was a silly mistake while making the MWE. 
When I corrected it, the MWE compiled correctly. However, the original code didn't have that error and wouldn't compile.
So, I tried another MWE and found out what was causing the error.
In the array I defined, there were two elements with empty string as their value. Once I changed those empty strings to some valid string, there was no error and the file compiled as expected. Here is the new MWE with one array element defined as empty string

\starttext
\switchtobodyfont[30pt]
\startMPpage
StartPage ;
unit := 2 * cm ;
%numeric lm, row, col, bm; 
pair  middle;
row := 1; col := 1; 
picture p, q ; p := nullpicture; q := nullpicture;


string l[], lbl;
l[0] := "" ; l[1]:= "2"; 


for i= 0 upto row:
middle := 0.5[((( i * unit) ), ((2 * unit))), ((((i+1) * unit) ), ((2 * unit) ))];
lbl := l[i];
p := thelabel(lbl, middle); 
q := p reflectedabout (0.5[urcorner p, ulcorner p], 0.5[lrcorner p, llcorner p]);
draw q;


endfor
StopPage ;
\stopMPpage
\stoptext


This will throw an error. If l[0] is defined as, say, "hi", there is no error

I suppose, my work around would be to use a non empty string and then fill that cell dark. Is there another way?

Anyway, thanks for your prompt replies and patience.

Thanks a ton for sharing your efforts with the world

ajith