Dear Alan, Thank you for the clear explanation about z. I tought that z is a special variable different from other pairs. Now, I know that it is an equation. Thank you again. Best regards, Dalyoung

2018. 6. 1. 오후 11:24, Alan Braslau 작성:

On Fri, 01 Jun 2018 20:29:41 +0900 Jeong Dal wrote:

...

After inserting a line “save x, y;”, it works.

This is also defined as "clearxy ;"

For those learning MetaPost, simplifying, z is defined as "(x,y)" so one can write z = (a,b) ; This is an EQUATION in MetaPost (x,y) = (a,b) ; not an assignment (as in most other languages), actually a pair of equations: x = a ; y = b ; When x and y are unknown, this can be "solved". When x and y contain values, it leads to "inconsistent equations".

One *cannot* write z =: (a,b) ; ("=:" is the assignment operator), for z is just a macro, not a "pair" object or variable.

This can be *very* confusing when learning MetaPost.

Alan

P.S. for i=0 upto n: z[i] = (i,0) ; endfor works as each element of the (pseudo)array z is unknown, at least the first time that it is used.