MetaFun - Pragma ADE

First there is the concept of variables and assignments. There are several data types, some of which
we already have seen.
numeric real number in the range −4096. . . + 4096
boolean a variable that takes one of two states: true or false
pair point or vector in 2--dimensional space
path a piecewise collection of curves and line segments
picture collection of stroked or filled paths
string sequence of characters, like "metapost"
color vector of three (rgb) or four (cmyk) numbers
There are two additional types, transform and pen, but we will not discuss these in depth.
transform transformation vector with six elements
pen pen specification
You can achieve interesting effects by using pens with certain shapes. For the moment you may
consider a pen to be a path itself that is applied to the path that is drawn.
The numeric data type is used so often that it is the default type of any non declared variable. This
means that
n := 10 ;
is the same as
numeric n ; n := 10 ;
When writing collections of macros, it makes sense to use the second method, because you can
never be sure if n isn't already declared as a picture variable, and assigning a numeric to a picture
variable is not permitted.
Because we often deal with collections of objects, such as a series of points, all variables can be
organized in arrays. For instance:
numeric n[] ; n[3] := 10 ; n[5] := 13 ;
An array is a collection of variables of the same type that are assigned and accessed by indexing
the variable name, as in n[3] := 5. Multi--dimensional arrays are also supported. Since you need
a bit of imagination to find an application for 5--dimensional arrays, we restrict ourselves to a two-dimensional
example.
numeric n[][] ; n[2][3] := 10 ;
A nice feature is that the bounds of such an array needs not to be set beforehand. This also means
that each cell that you access is reported as unknown unless you have assigned it a value.
Behind the screens there are not really arrays. It's just a matter of creating hash entries. It might
not be obvious, but the following assignments are all equivalent:
Variables Welcome to MetaPost
29