Metapost error ! An expression can't begin with `]'

Question

I am trying to reproduce the examples in Metapost showed here but I am having this error:

! An expression can't begin with `]'. 0 ] l.12 vector e[] ,u[];

This is the code:

prologues:=3;                % include all fonts
outputtemplate:="%j-%c.mps"; % output file will be file.mps
beginfig(1);
phi:=25;                     % latitude
lambda:=28;                  % longitude
Omega=35;                    % longitude of ascending node
incl=30;                     % inclination
omega=43;                    % longitude of periapsis in orbital plane
vector e[],u[];
e1=(1,0,0);                  % aries - vernal equinox
e2=(0,1,0);                  % e2=e x e1 - not used
e3=(0,0,1);                  % e3=e=ENP
endfig;
end.

Any ideas why is this happening?

Answer 1

There is no vector type in MetaPost, you have to use pair instead, and as its names suggests, there must be two coordinates, not three. MetaPost is a 2D-based language.

See the featpost package for an example of MetaPost use for 3D drawings. See also the Asymptote language, which is inspired by MetaPost and has full 3D abilities.

Edit Reading the linked article more thoroughly, I finally learnt that the author uses a vector type defined by Denis Roegel in this TUGboat article.

let vector=color;
  let Xp=redpart; let Yp=greenpart; let Zp=bluepart;

In fact, it's down to the same thing used in featpost (and the tres module provided by Metafun, see Henri Menke's comment below): using the color type for 3D-coordinates…