An Introduction to ACTION!#
by Clinton Parker
ANALOG Computing
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PDF: An Introduction to ACTION/introinaction.PDF
This is the first of a 2-part series that will introduce you to the Action! programming language, using a short example program that draws kaleidoscopic patterns on the screen. There's an old saying about fooling people which, unfortunately, holds true for trying to please people as well. The problem in my case is that different readers have different levels of experience. I hope this series will please all of you at least some of the time.
Action! is a true compiled language, whereas Atari BASIC is an interactive interpreter. In both cases, the ultimate goal is to translate programs from a human readable form into something that the computer can understand. The difference is that Action! only performs this translation once, whereas BASIC does it repeatedly. The process is similar to having a speech translated from German to English once and then reading it many times in English (Action!), as opposed to having someone translate the speech to English every time it is read (BASIC). Because Action! statements don't have to be translated each time, they execute much faster.
Action! has three types of numeric variables (BYTEs, CARDinals and INTegers), which are easier for the computer to deal with than the floating-point numbers always used by Atari BASIC. This also contributes to faster program execution, but costs you in terms of flexibility (no fractions or very large numbers) and simplicity (you must declare variables so that the compiler will know what type they are).
BYTE variables can represent numbers from 0 to 255. CARDs can represent numbers from 0 to 65535, and INTs can represent numbers from -32768 to 32767.
Referring to Listing 1, the lines:
CARD period, npts BYTE x0, y0, x1 , y1 , ATRACT=77 BYTE CH=764
are called variable declarations. Note that the BYTE variable ATRACT is defined to reference location 77 in memory, and that variable CH references location 764. More on these later.
In addition to the three basic types described above, Action! allows ARRAYs, POINTERs and user defined TYPEs (records). The following line:
TYPE REC=~[CARD cnt,ax,bx,cx,ay,by,cy]
is a TYPE declaration named REC, and:
REC p, e
is a declaration of two variables (p and e) of type REC. Each of these variables contain all of the variable fields specified in the declaration of REC. Fields of record variables are referenced by first giving the record variable name, then a '.' (period), followed by the field name. The lines:
p.ax = 5221 p.bx = 64449 p.cx = 3 p.ay = 57669 p.by = 64489 p.cy = 3
are examples of assignment statements using record fields.
Second, Action! makes it possible to execute a list of statements if the condition following an IF is false. This is done by placing the keyword ELSE where the FI would normally go, followed by the list of statements for the ELSE, and finally an FI to terminate the structure. ELSE is not used in Listing 1, so don't be concerned if you don't see one.
Action! loops are used to execute a group of statements repeatedly. A simple loop is specified by the keyword DO, followed by a list of statements and ending with the keyword OD (DO spelled backwards). The effect is similar to a group of BASIC statements with a GOTO ( first statement) as the last statement in the group. You can provide control information to specify how many times an Action! loop is to be repeated. One loop control structure — FOR/TO — is very similar to the FOR structure in Atari BASIC. The differences are that, in Action!, the end condition is always tested before the statements within the loop are executed, which means that the loop may never be executed. BASIC always executes a FOR/NEXT loop at least once. Additionally, the STEP increment may only be positive in Action!, whereas BASIC allows both positive and negative STEPs. The other two Action! control structures, WHILE and UNTIL, will be discussed later.
