EXAMPLE OF COMAL ON-LINE SUPPORT

Subj: string func?
From: Icarus

Can someone explain strings and string functions,
particularly placement of the colons?

Subj: strings
From: Captain C

Since BASIC doesn't have user defined string
functions, they are unfamiliar to most. Compare
them to a built-in function. We talk about SPC$(5)
returning 5 spaces:
 
  A$=SPC$(5) 
  PRINT "HI",SPC$(5),"JIM" 
  HI     JIM

See the 5 spaces in the printout. And A$ now is 5
spaces. Next we can write a string function: 

  FUNC jim$(num) CLOSED 
    DIM temp$ OF num*3 
    temp$="" // start with nothing 
    FOR x=1 TO num DO temp$:+"JIM" 
    RETURN temp$ 
  ENDFUNC jim$ 
 
Now, try it:

  PRINT jim$(3) 
  JIMJIMJIM 

Now, about the colons. Colons are used to specify
substrings:
 
  A$="BIRTHDAY" 
  A$(1:3)  would be "BIR" 
  A$(2:5)  would be "IRTH" 
 
The colon just separates the start position from
the end position. 

Subj: colons
From: Comalite J

Think of a string as a one dimensional array of
characters:

  DIM mystring$ OF 6 

At first, it looks like this: 

  +---+---+ 
  |   6   |  --MAX (DIMmed Length)
  +---+---+ 
  |   0   |  --Current-Length
  +---+---+---+---+---+---+ 
  | @ | @ | @ | @ | @ | @ | 
  +---+---+---+---+---+---+ 
    1   2   3   4   5   6 
 
Now execute this:

  mystring$:="Joel"

  +---+---+ 
  |   6   |  -- MAX-LEN 
  +---+---+ 
  |   4   |  -- CUR-LEN 
  +---+---+---+---+---+---+ 
  | J | o | e | l | @ | @ | 
  +---+---+---+---+---+---+ 
    1   2   3   4   5   6 

A string's current length can never exceed the max
length (specified in the DIM statement). Unlike a
numeric array, a string can have an apparent
current dimension that is smaller than its actual
maximum dimension. Also unlike a numeric array, you
can reference the entire string or any contiguous
part of it containing 0 or more characters with
indexing. Instead of a single number specifying
which item of a numeric array to index, we use a
RANGE to specify a SUBSTRING:

  mystring$(start : end)

START and END are expressions which evaluate to a
value between 1 and MAX-LEN. See the result here:

  PRINT mystring$(2:3)
  oe

Characters 2 through 3 of mystring$ ("Joel"),
output oe. The expression MYSTRING$(2:3) does this:

  +---+---+ 
  |   6   |  -- MAX-LEN 
  +---+---+ 
  |   4   |  -- CUR-LEN 
  +---+---+---+---+---+---+ 
  | J | o | e | l | @ | @ | 
  +---+---+---+---+---+---+ 
    1   2   3   4   5   6 
        ^   ^ 

Now type:

  mystring$:+" Rea"

The ":+" assignment appends " Rea" onto the end of
mystring$. The result is a string with the value
"Joel Rea", but since mystring$ has a MAX-LEN of 6,
characters after the 6th character are cut off. So
the result is "Joel R". Thus: 
 
  +---+---+ 
  |   6   |   -- MAX-LEN 
  +---+---+ 
  |   6   |   -- CUR-LEN 
  +---+---+---+---+---+---+ 
  | J | o | e | l |   | R | 
  +---+---+---+---+---+---+ 
    1   2   3   4   5   6 
 
Now type:

  PRINT mystring$(2:4)

The result is "oel", characters 2 through 4 of the
string "Joel R". If I just want character 3, I can
specify the START and END to both be 3:

  PRINT mystring$(3:3)

The result is "e".
 
To reference a substring starting with the first
character of a string, you normally specify a START
of 1. COMAL has a shortcut:  simply omit the START.
Just specify the colon and the END. The next two
lines are equivalent:
 
  PRINT mystring$(:4) 
  PRINT mystring$(1:4) 

A similar shortcut is available for substrings
which end with the last character. Just specify the
first character followed by a colon. The next two
lines are the same:
 
  PRINT mystring$(3:)
  PRINT mystring$(3:LEN(mystring$))
 
which, in this case, is:
 
  PRINT mystring$(3:6) 
 
The result is "el R". 

For a one character substring, just specify the
START and END as the same:

  PRINT mystring$(3:3)
  e

In COMAL, you can ASSIGN to a substring! For
example, assuming mystring$ equals "Joel R", then: 
 
  mystring$(2:4):="ack" 
  PRINT mystring$
  Jack R

Characters 2-4 were changed without affecting the
rest of the string. Substrings are padded with
spaces to the right if needed:
 
  mystring$(2:4):="an" 
  PRINT mystring$ 
  Jan  R

  mystring$(2:):="im V" 
  PRINT mystring$ 
  Jim V

The above is an example of on-line COMAL support.
In addition to message boards, there are upload /
download libraries of COMAL programs and national
monthly COMAL meetings: every first Sunday and
second Thursday on QLink; every third Wednesday on
People Link (and CIS pending). Meetings are at 9pm
Central time.

People Link - to get to our COMAL Support Section
just /GO COMAL. To sign up on PLink call 1-800-524-
0100. EMail to CAPTAIN C.

Quantum Link - to get to our COMAL Support Section
go to CIN; Computing Support; Programmers Workshop.
To sign up on QLink call 1-800-392-8200. EMail to
CAPTAIN C.

CompuServe (coming soon) - to get to our COMAL
Support Section ... GO AMIGAV then LN to find COMAL
listed. To sign up on CIS call 1-800-848-8199.