Asymptote: Basics
Syntax
In Asymptote, every command you give it should be followed by a semicolon (;). This tells Asymptote how to separate one command from the next. For example, the command draw(A--B) draws a line segment from point to
given as coordinate pairs. Thus, if you wanted to draw two line segments, say one of them from
to
(in units of PostScript points - see below for the explanation of units and size) and the other from
to
, you can create the following document:
draw((0,0)--(50,50)); draw((0,50)--(50,0));
The two commands do not need to be on separate lines; it is the semicolon that separates them. However, putting the commands on separate lines does not change the output, so it is often useful to separate you commands in this way in order to organize your code. Whitespaces before and after commands are also not read by Asymptote, so any line can be indented as far as desired for clarity's sake.
Variables and Data Types
Asymptote parses your code into substrings which have a certain data type, for example a real (like) or a pair (like
). Each new variable that you declare must be declared as a data type that Asymptote recognizes, by the command [datatype] [variable];. For example, if you wanted to declare the variable
to have type integer, you can use the command int n;
After it is declared, you can store a specific value in a variable using the symbol, as in
n=3;
These two commands can be abbreviated by the single command int n=3;, and several integers can also be declared at once (int m,n,d;).
As another example of variable declaration, consider the picture of the X from the Syntax section above. The same picture can be made as follows:
pair A,B,C,D; A=(0,0); B=(50,50); C=(0,50); D=(50,0); draw(A--B); draw(C--D);
In this particular example, variable declarations made the code longer, but as you will see, declaring variables will significantly clean up your code in messier diagrams.
The most commonly used data types in Asymptote are given in the following table:
Size and Unitsize
Asymptote is a primarily coordinate-based graphics language. Each point is a pair where
is the
-coordinate and
is the
-coordinate.
However, there are many ways to choose a Cartesian coordinate system for the plane; one must pick the placement of origin and the scale on each of the - and
-axes. Asymptote will place your image in the center of your output page after it is drawn, so placement of origin is actually irrelevant. By default, the unit length in both the
and
directions is the PostScript bigpoint, which has length
inches. Thus, if you do not change the scaling on the picture, the points
and
are exactly one inch apart when drawn in Asymptote. However, drawing in bigpoints is inconvenient if you wish to draw a figure that is exactly 3cm wide.
The function unitsize can be used to specify the unit length for your picture. This function takes up to 3 arguments: the picture you want to scale the axes for (if this isn't specified, it defaults to currentpicture, the picture you are drawing on), the unit length in the x direction, and the unit length in the y direction. If only one real argument is given, both the x and y unit sizes are set to this number. Thus the command
unitsize(72);
will tell Asymptote that from now on, your unit length is inch. Be careful when you are redefining your unit length - now that unitsize is set to
, the points
and
are actually
inches apart!
Asymptote has the built-in constants pt (1/72.27 inches), inch, cm, and mm for convenience when defining lengths, so the above command can also be stated
unitsize(1inch);
The other useful function is size, which specifies the exact width and height that your picture (if unspecified as a first argument, this will again default to currentpicture) will be drawn to. If only one number is given, that will be the width of the picture and the height will be left free to scale as necessary to keep the x- and y- unit sizes the same. For example, the command
size(5cm,5cm);
will fit the diagram to a cm
cm box regardless of the specified unitsizes.
As an example, make an Asymptote document containing the following two lines:
unitsize(2inch); draw(unitsquare);
and see what happens as you change the inch size to several other values.