CP/M/CPM_CDROM.iso/mbug/mbug172.arc/APL-Z.LBR/APL.NZT/APL.NOT

Text File | 1979-12-31 | 17KB | 507 lines

                              APL/Z
                               by
                          S. Gownowicz


                    Programming Introduction
                               for
                              CP/M




                               by
                         Kevin Smathers




















Document:        APL/Z.1
Initial release: 8/87
Update history:

A short introduction

     This  document is tailored for the typical CP/M  programmer,
who  has never used APL before.   It is written to complement  S.
Gownowicz's public domain programming language, APL/Z.
     The  APL  programming language was originally defined  in  a
book by Kenneth Iverson,  _A_Programming_Language_,  published in
1962.   Iverson's  intention  was to create a  symbolic  language
which would be adequate for describing concisely,  many important
algorithms in mathematics.   As a result, the original APL used a
myriad of special symbols including subscripts,  superscripts and
two-dimensional  program  format  with arrows  to  show  flow  of
control.
     A  modified version of the original APL,  termed APL/360 was
implemented  in  the  late 60's by Iverson and a  group  at  IBM.
Since then APL has been implemented on a variety of machines, all
based largely on the APL/360 design.   APL has two distinguishing
characteristics which set it apart from other languages.

1.    It is designed to be interpreted.  As such it enjoys all of
the  unique  capabilities that interactive programming  languages
have.   APL  includes  a  number  of  special  features  designed
exclusively  for an interactive environment,  inluding  immediate
execution  of expressions entered at the terminal,  and  entering
and editing facilities for subprograms that are associated with a
workspace.  Workspaces in APL/Z can be saved and restored in part
or in whole.

2.    APL directly processes entire data structures.   In APL the
primitive  operations  accept entire  arrays  as  arguments,  and
produce  arrays as results.   Thus in APL the basic unit of  data
tends  to  be an array rather than a single array element  as  in
other  languages.   This  emphasis on array-processing gives  APL
programs   a  unique  appearance  quite  different   from   other
languages.

     The  specialized character set of APL lends it enough  power
that a typical one line program can result in a surprising amount
of computation.  APL is ideally suited for the user who wishes to
avoid  key-board  heavy  languages.   It is well  suited  to  the
interactive  environment,  because only a few lines of input  are
required  to get the results you might typically be looking  for.
On  the other hand,  APL is poorly suited for long  programs,  or
programs  which are meant to be maintained over a long period  of
time.

                              -----
    Programming languages specialist Daniel McCracken [1970]
    notes that it once took him four hours to decode a four-
    line APL program.


Introduction to the APL:ASCII keyboard

     APL  (A Programming Language) is an  unstructured  language,
with  a  specialized  character  set devoted  to  performing  the
functions  of  the programming language.   In APL  there  are  no
lowercase  alphabetic symbols.   Unshifted letters produce  their
uppercase counterparts, while shifted letters produce the special
APL symbols.   To avoid confusion,  we will use lowercase symbols
for unshifted APL characters.
     Since CP/M machines typically don't have APL character sets,
we  will be using some special conventions.   They fall into four
categories.

     1. characters which must be remapped
     2. characters which are common to both APL and ASCII
     3. characters which cannot be remapped
     4. extra characters

1.   The characters listed below have been remapped for ascii:
_________________________________________________________________
 APL   ASCII
_____________
  ..
                    Dieresis (Umlaut) is unmapped (unused in APL)
_____________
  __
        _           High-minus becomes underline
_____________

  <=    {           Less-than-or-equal becomes open-brace
_____________

  >=    }           Greater-than-or-equal becomes close-brace
_____________

  /=    #           Not-equal becomes pound-sign
_____________

  \/    !           Logical-or becomes exclamation-point
_____________

  /\    &           Logical-and becomes ampersand
_____________

  x     *           Times becomes asterisk
_____________
  .
  -     %           Divided-by becomes percent-sign
__.__________

  <-    `           Left-arrow becomes grave-accent
_____________

  ->    @           Right-arrow becomes at-sign
__________________________________________________________________
2.    Certain  keys on the APL keyboard also appear on the  ascii
keyboard.  These are not remapped.  They are:
__________________________________________________________________

[  Open bracket
]  close bracket
(  open paren
)  close paren
/  stroke
\  back-stroke
,  comma
.  period
;  semi-colon
:  colon
+  plus
-  minus
<  less-than
>  greater-than
=  Equals
__________________________________________________________________


3.    Apl also uses additional symbols which we won't be mapping.
These correspond to the shifted keyboard keys.
__________________________________________________________________

Q  Question mark                             see (?) in pt. 4
W  Omega            the greek symbol
E  Element          epsilon
R  Rho              the greek symbol
T  Invert (tilde`)                           see (~) in pt. 4
Y  Up-arrow
U  Down-arrow
I  iota             the greek symbol
O  circle           a large open circle
P  power                                     see (^) in pt. 4
A  alpha            the greek symbol
S  cieling          an upside-down 'L'
D  floor            a right-side-up 'L'
F  underline
G  del              an upside-down delta
H  delta            the greek symbol
J  little-circle    a small open circle
K  single quote                              see (') in pt. 4
L  quad             a large open square
Z  subset           an open semi-circle
X  superset         a close semi-circle
C  intersection     an upside-down 'U'
V  union            a right-side-up 'U'
B  encode           an upside-down 'T'
N  decode           a right-side-up 'T'
M  vertical bar                              see (|) in pt. 4
__________________________________________________________________



4.    That leaves us with several ASCII symbols which haven't yet
been  used,  so  in  order  to simplify  reading  and  typing  in
programs,  the  rest  of the ascii set has been assigned  to  the
characters  on  the  shifed alpha APL keyboard  which  they  most
resemble, or whose function they most closely match.
__________________________________________________________________

|    M
~    T
$    H
^    P
'    K
"    K
?    Q
__________________________________________________________________


Delving into the APL interpreter

     APL lines begin tabbed in 8 spaces.   Here you will type you
command, and the interpreter will respond on the next line.

     2
2
     2+2
4

     Here  are four dyadic (meaning with two operands)  operators
you should recognize.

     2*3                 multiply
6
     4%2                 divide
2
     51                 subract
4
     2+1                 add
3

     An  important  feature to remember about APL is that  dyadic
operators always use whatever they find immediately on their left
side,  and  the result of evaluating everything on  their  right.
This means that '+' has no precedence over '*'.
.___________________.
.                   .
.     2*3+16%3      .
.           +      .
.         +        .
.       +          .
.     +            .
.___________________.

     So that this equation would produce:

     2*3+16%3
4

     This  holds  true for all operators,  not just  mathematical
ones.   However,  this  sequence  can  be  altered  in  APL  when
neccessary,  by grouping the term which are to be evaluated first
within parentheses.

     (2*3)+1(6%3)
5

     Produces the expected result.   These operators can also  be
applied  to  lists.   A  list  is simply a  sequence  of  numbers
separated by spaces.  Try typing

     1+1 2 3
2 3 4

     2*3 2 1
6 4 2

     APL also makes reals available to you,  simply by putting  a
decimal  point within the number.   Exponential notation can also
be used as you would expect.

     3.14159*2
6.28318

     6.22e12%1000
6.022e9

     Remember  that  uppercase  letters  are  reserved  for   APL
functions.   Another  operator you will need to know is the unary
minus.

     _3+5
2

     And the single quote which is used to create arrays of ASCII
data.

     'abcde'
abcde

     abcde is a 5 element list.

     Dyadic  operators can also be used on two arrays.   In  this
case, their shapes must be equal.  (More on shapes later.)

     1 2 3*4 5 6
4 8 10

     Operands  are paired two at a time,  to produce the  result.
Since  arrays are used so frequently in APL,  a simple method  is
provided  for creating arrays which count up.   In  general,  the
monadic  iota  function will produce an array starting at  1  and
counting up to the argument on it's right side.   Iota expects to
find  a scalar,  or a vector of at most one element on it's right
side.

     Monadic Iota:

     I5
1 2 3 4 5

     I5 5
rank error
     I5 5
     ^

     Attempting  to feed iota a higher degree vector will  result
in the error shown.  APL is complaining that the right hand value
is not of the proper rank, also known as dimension.
     Another  dyadic function is the  assignment  function,  left
arrow.   We  will  use  the grave-accent (`)  to  represent  this
character.  Values, such as arrays, character arrays, and scalars
can  be  freely  assigned  to  variables  using  the  left-arrow.
Variables must be alphabetic,  or may additionally start with the
symbol delta ($).

     $hello`'Hi there'
     hello
Hi there

     'hello'  is an example of a character vector (its shape  has
one dimension, arrays have more than one dimension in the general
case).  Remember,  variables  cannot be uppercase,  as those  are
reserved for the APL functions.
     The  assignment  statement  introduced  above,   is  another
example of a dyadic function.  This particular function, however,
must use a variable as it's left side.  In all other respects, it
fits  the normal perception of what a dydadic function should be.
For example, you might type a line like this:

     a`1+b`3

     Evaluated right to left, this resolves to ( b`3 ),( a`b+1 ).
In other words, a will have the value 4 and b will have the value
3.   But now that we have these fine variables defined,  we  will
need a way to save and recall them for later use.
     For  this purpose,  there are the so called system commands.
Basically,  system commands are used to make enquiries about  the
status of the environment or work_space.  Here are a few to start
with:

     )VARS               list all defined variables
     )ERASE var_list     deallocate a list of variables
     )SAVE ws_name       save the current workspace as ws_name
     )LOAD ws_name       load a workspace from ws_name
     )FNS                list all defined functions

     Additionally,  it should now be high time to go over some of
the errors that you may encounter within APL

     CHARACTER ERROR
          an unknown character has appeared out of place.
     SYNTAX ERROR
          a badly formed expression was typed
     DOMAIN ERROR
          the function is not defined for the value given.
     LENGTH ERROR
          a binary function was used with unmatched vector shapes
     VALUE ERROR
          the indicated variable has not been assigned a value
     RANK ERROR
          the number of dimensions of the input is not allowed
     WS FULL
          the last function used up all of the unused workspace
     DEFN ERROR
          your definition was illegal,  one of the variable names
          was probably at fault.

     In APL, the standard way of editing is to backspace until
you are over the character in error, press ATTN and type the tail
of  the  string again.   Otherwize,  backspace  will  not  remove
characters from the line.
     Back to functions now, here is another diadic function.  The
function catenation is represented by a comma (,).  For example,

     a`'hi'
     b`' there'
     a,b
hi there

     This  is  an  example  of the catenation  of  two  character
vectors.   Refreshing your memory, a dyadic function is one which
acts  on  two values.   A monadic function works  with  just  one
value.   A  niladic function is one which requires no values  for
input  at all.
     This  all leads to user defined functions.   The great power
of  APL  is  most  closely linked  with  its  ability  to  define
functions  on the fly which can be used in the same way that  all
of the system defined functions are.

     As  with all identifiers,  function names must begin with an
alphabetic (unshifted) letter or delta ($), and may continue with
any other alpha or numeric characters,  but not APL character set
characters.   S$  and  T$ are reserved variables  which  will  be
discussed later.

     A function is defined as follows:

     G function_name
     equations
     G

     The  surrounding  del's  (G)  are  the  function  definition
characters.   They  open  and  close the  function  editor.   The
function  above  would have no explicit  results,  and  no  input
variables.   The  function  would be niladic.   A common use  for
niladic functions without explicit results is to list all of  the
member  functions  of a workspace.   A niladic function would  be
written  which included that documentation in  print  statements,
and would have a name you could remember such as 'menu'.

     G menu
     'This is a file of useless functions:'
     'GRIK: experimental nothing function'
     'GRAK: same as above'
     'YICK: pretty boring huh?'
     G
     menu
This is a file of useless functions:
GRIK: experimental nothing function
GRAK: same as above
YICK: pretty boring huh?

     Non-explicit  functions  always return each of  the  results
generated  on a separate line,  as though they had been typed  at
the  keyboard  in that sequence.   Non-explicit functions do  not
pass values to other functions.   That is the job of the explicit
function.

     G z`pi
     z`3.1415
     G
     pi
3.1415
     2*pi
6.2830

     Before  exiting the function,  the explicit variable in  the
explicit function declaration must be given a value to return  to
the next function.   The next form of the function declaration is
the  monadic  function.   For instance,  the  following  function
would change temperatures from Fahrenheit to Celsius.

     G c ` temp f
     c ` (f-32) * 5 / 9
     G
     temp 32
0

     The full fledged form of the function is a dyadic,  explicit
function.   Listed  below  are all of the  possible  combinations
which can be used to create a function

Argl           Explicit       Non Explicit
 0   Niladic   G z`fn         G fn
 1   Monadic   G z`fn a       G fn a
 2   Dyadic    G z`a fn b     G a fn b

     I'd  like  to take just a moment to digress.   When  talking
about  functions in any language,  it is important to talk  about
scoping  rules.   In basic,  where there are no local  variables,
scoping is global.   In Pascal,  where a variable name refers  to
the name most closely linked to its own structural shell, scoping
is  termed static.   In general scoping means,  "If I define  two
variables,  both  with the name 'x' which one will I be referring
to  in this procedure?"  APL uses a concept of scoping  which  it
has in common with LISP.  If the variable is not defined locally,
it is checked for locality to the procedure which called it,  and
if not there, the process is repeated until the variable is found
by  unwinding the calling stack.   This type of scoping is  named
'dynamic'.
     As  you must assume from the preceding discussion,  APL  has
its own way of declaring internal variables.  These variables are
available  to all functions called by the function which  defined
them, until the name is redefined.

     The  form  for  the declaration of a local  variable  is  as
follows:

     G fn ; lv1 ; lv2 ; lv 3

     There  may  be as many local variables  as  necessary,  each
appended  to the list with a new semicolon.   Old values for  the
variables  named are restored when the function finally  returns.
The local values are not retained.