Path: utzoo!attcan!uunet!mcsun!ukc!tcdcs!swift.cs.tcd.ie!david
From: david@swift.cs.tcd.ie
Newsgroups: comp.sys.mac.programmer
Subject: Problems with THINK Pascal
Message-ID: <1991Jan11.190458.7714@swift.cs.tcd.ie>
Date: 11 Jan 91 19:04:58 GMT
Organization: Computer Science Department, Trinity College Dublin
Lines: 59

I have just purchased a new version of THINK Pascal to run on a Mac II si,
and have encountered two problems when dealing with PACKED ARRAYs of BOOLEAN.

 i. The mapping for the elements of "A: PACKED ARRAY [0..7] OF BOOLEAN" is
    not the same as the mapping for "B: PACKED ARRAY [1..8] OF BOOLEAN"

    If A and B are viewed as bytes, then

       setting A[0]:=TRUE corresponds to setting the byte value to 1,
       ...
       setting A[6]:=TRUE corresponds to setting the byte value to 64, and
       setting A[7]:=TRUE corresponds to setting the byte value to 128

    while

       setting B[1]:=TRUE corresponds to setting the byte value to 2,
       ...
       setting B[7]:=TRUE corresponds to setting the byte value to 128, and
       setting B[8]:=TRUE corresponds to setting the byte value to 1

    yet the manual states that the elements of a packed data structure are
    allocated sequentially

ii. Assigning a boolean value based on the comparison of elements from packed
    boolean data structures, will not always produce the correct result

    Given the declarations

       VAR
          RESULT: BOOLEAN;
          LOGICAL1,LOGICAL2: PACKED ARRAY[0..7] OF BOOLEAN;

    the statement

       RESULT:=LOGICAL1[3]=LOGICAL2[3];

    will produce the following results

       LOGICAL[1]    LOGICAL[2]    RESULT
       ----------    ----------    ------
             TRUE          TRUE      TRUE
             TRUE         FALSE      TRUE    !!!
            FALSE          TRUE     FALSE
            FALSE         FALSE      TRUE

This problem can be solved by using an if statement

    IF LOGICAL1[3] = LOGICAL2[3]
       THEN RESULT:=TRUE
       ELSE RESULT:=FALSE;

I would like to know if I am missing something obvious, if there are any other
known problems in this area, etc.

David M. Abrahamson,
Dept. of Computer Science,
Trinity College, Dublin 2, Ireland.

E-Mail to David@CS.TCD.IE