Structure and Elements of a Pascal Program

 { } : multiple statements - 0 or more
< > : indicates token
 [ ] : optional

• Form of a Pascal Program
   program <program-name> (INPUT,OUTPUT);
      {<specification section>}
      procedure A;
      procedure B;
      begin
           {<executable-statements>}
          A;
           {<executable-statements>}
          B;
           {<executable-statements>}
      end.
  
  (the Free Pascal compiler does not require (INPUT,OUTPUT); other compilers may; as always, refer to your compiler documentation)
  
  
• Form of a Pascal Unit
   unit <unit-name>
      interface
        {<public specification section>}
        {<External Declaration section>}
        {<public Procedure Header section>}
      implementation
        {<private specification section>}
        {<functions & procedures section>}
    end.
  
  (the <functions & procedures section> consists of the full Pascal Procedures and Functions bodies;
  see NumRecGlbl.pas in D .dll Calling Free Pascal .dll - Pascal Files and Lazarus, Free Pascal, Silverfrost FTN95, and Numerical Recipes for examples of an Object Pascal Unit)
  
  fnameIO.pas (see below) is another example of a Pascal unit.  It uses three units (Windows for MAX_PATH, HWnd;  SysUtils for IntToStr (...);  and CommDlg for OPENFILENAMEA, GetOpenFileName (...), GetSaveFileName (...), CommDlgExtendedError);  has include file genetyp.inc¹;  has public variables ofn, szFileName, openInd, saveInd, fError;  calls procedure HWND_APPND_TEXT (...) in Object Pascal Library hwndio.dll;    has public procedures getOpenFName( ), getSaveFName(), fileOpnStat (...), IOerrMsg (...), trnsfrMsg (...);   unit is used in two Object Pascal Windows applications.
  
  
  
• Form of a Pascal Library
   library <library-name>
      {<specification section>}
        {<External Declaration section>}
        {<functions & procedures section>}
      exports
      {<function-name> [, <procedure-name> [,...]];}
    end.
  
  (to avoid any confusion, <function-name>s do not need to be grouped together or before <procedure-name>s in the exports list or the main body; shown like this as a matter of convenience; functions and procedures in the exports list will need a <call-model> (see below); Object Pascal Librarys compiles into a Windows .dll; see ALlazrs.pas in D .dll Calling Free Pascal .dll - Pascal Files for an example of an Object Pascal Library with external declarations)
  
  
• Form of a Pascal Procedure Header
    procedure <procedure-name> [(<formal-parameter-list>)];
       - OR -
    function <function-name> [(<formal-parameter-list>)] : <result-type>;
  
  (placed in a Pascal Unit's Interface; those procedures and functions are then public to any Pascal Program, Library, or Unit that uses the Unit)
  
  
• Form of a Pascal External Declaration
     procedure <procedure-name> [(<formal-parameter-list>)]; <call-model>;
      external '<dll-file-name>';
       - OR -
    function <function-name> [(<formal-parameter-list>)] : <result-type>; <call-model>;
      external '<dll-file-name>';
  
  (do not include extension in '<dll-file-name>'; file must be in current directory or path)
  
  
• Form of a Pascal Procedure or Subroutine
     procedure <procedure-name> [(<formal-parameter-list>)]; [<call-model>;]
      {<local specification section>}
      begin
           {<executable-statements>}
      end;
  
  (<call-model> used in Pascal Libraries, together with the Export clause, to declare the functions and procedures to be exported; also used in <External Declaration>)
  
  
• Form of a Pascal Function
     function <function-name> [(<formal-parameter-list>)] : <result-type>; [<call-model>;]
      {<local specification section>}
      begin
           {<executable-statements>}
           <function-name>:=<expression>
      end;
  
  
• Form of a Call to a Procedure
      <procedure-name> [(<actull-parameter-list>)];
  
  
• Specification Statements
      Uses
          Windows, Strings;
  (use in Program, Unit, and Library <specification section>, not Functions'/Procedures' <local specification section>; Windows and Strings are external <unit>s )
  
      Label
          999
  (use only in Functions'/Procedures' <local specification section>)
  
      const
          Max = 10;
          Pi = 3.1415926535;
          Small = 1e-20;
  
      type
          Subs = 1..Max;
          VECTOR = array[1..5] of real;
           MATRIX = array[1..5,1..5] of real;
           POINTER = array[Subs] of integer;
           STATUS = (Done,Iterating,Working);
          WORDS = string[80];
           nrgSubString = array[0..255] of Char;
  
      var
          Column,I,J: integer;
          A0,B0,Max,X,Y: real;
          Flag: boolean;
          V,R: VECTOR;
          A,D: MATRIX;
          Row: POINTER;
          Name: string[80];
          Lines: WORDS;
          State: STATUS;
          iErr: smallint;
          sNR: single;
  
  (see Variable Storage Compatibility and Equivalency for more information on data types;
  often const, type, and occasionally var are grouped together into include files - *.inc - and are included with the statement {$include <include-file-name>}; see genetyp.inc in D .dll Calling Free Pascal .dll - Pascal Files for example)
  
  
• Assignment Statements
      X := A[1] + 3;
      Y := A*X*X + B*X + C;
      Flag := true;
      Name := 'Fran';
  
  
• Arithmetic Operations
  

  +	Addition
  -	Subtraction
  *	Multiplication
  /	Division
  div	Integer Division
  mod	Remainder (Integer)

  
  (unlike FORTRAN, there is not a ** exponentiation operator)
  
  
• Relational Operators
  

  =	Equal To
  <>	Not equal to
  <	Less than
  >	Greater than
  <=	Less than or equal to
  >=	Greater than or equal to

  
  
  
• Logical Operators
  

  not	Complement
  and	True if both operands are true
  or	True if either (or both) operands are true

  
  
  
• Logical Constants
      true
      false
  
  
• Remark Statement
      {This is a comment.}
      // Also a comment
  
  
• Place-holder Statement (or label)
      100:
  
  (labels must be declared in a <specification-statement>)
  
  
• Unconditional Transfer
      goto 100;
  
  Warning. Do not use the goto statement to transfer into a If (Block) Control Statement, Indexed Loop Control Statement, While Loop Structure, or Case Statement from outside the block
  
  
• If (Block) Control Statement
  Performs the series of {<executable-statements>} following it or transfers control to an else, or end statement, depending on the value of the <logical-expression>.
  
      if <logical-expression> then
         <executable-statement>;
  
      if <logical-expression> then
         begin
           {<executable-statements>}
         end;
  
      if <logical-expression> then
         begin
           {<executable-statements>}
         end
       else
         begin
           {<executable-statements>}
         end;
  
      if ((argc[1] = 'T') or (argc[1] = 't'))
       and ((argc[2] = 'H') or (argc[2] = 'h')) then
        begin
            s:=treeHtSwitch;
            k:=3
        end;
  
  
• Indexed Loop Control Statement
      for <index-variable> := <start> to <end> do
         <executable-statement>;
  
      for <index-variable> := <start> to <end> do
         begin
           {<executable-statements>}
         end;
  
      for i:=1 to N do
         begin
           x:=R[i];
           B[i]:=YBELL (x, AVE, SDEV)
         end;
  
  
• While Loop Structure
  The <logical-expression> is evaluated first. If the <logical-expression> is true, then the loop body will be executed. The sequence is repeated as long as the expression remains true. If the expression is false, then the loop body will be skipped, and execution will continue with the statement following end;.
  
      while <logical-expression> do
         begin
           {<executable-statements>}
         end;
  
      while SendMessage (hEditOutput, WM_GETTEXTLENGTH, 0, 0) > 8192 do
         begin
           rmv0_1023 (hEditOutput);
         end;
  
  
• Case Statement
      case <logical-expression> of
         <constant> : {<executable-statements>}
         <constant> : {<executable-statements>}
         <constant> : {<executable-statements>}
       else
          {<executable-statements>}
      end;
  
  (<constant> can be of the following types : enumeration types, Ordinal types (except boolean), and chars or string types;
  this also includes smallint)
  
  (extreme Object Pascal example demonstrating <case-statement>;
  <logical-expression> is the result of function NRGcmdLnFname (...) in Object Pascal Unit NumRecGlbl.pas;
  note that <constant> can also be a range - minSmallint..-1, or list of individual values - 0, InputOnly, FileSwitch)
  
  
  
  
• Input and Output
  (console I/O)
      readln(x,y);
      readln(A[i,j]);
      writeln;
      writeln('x = ',x,' y = ',y);
      write ('A(i,j) = ',A[i,j]:15:5);
  
  (file I/O)
      Assign (<file-variable>, <file-name>);            (assign name of external file to file variable)
      Reset (<file-variable>, <record-size>);          (open existing file)
      BlockRead (<file-variable>, <record-buffer>, <records-to-read-size>, <records-read-size>);
      Rewrite (<file-variable>, <record-size>);        (open file for writing)
      BlockWrite (<file-variable>, <record-buffer>, <records-to-write-size>, <records-written-size>);
  
  (there are additional procedures availble for file I/O; refer to your compiler documentation)
  
  
• Mathematical Functions

  cos(x)	cosine (radians)
  sin(x)	sine (radians)
  exp(x)	exponential  exp(x)
  arctan(x)	inverse tangent (radians)
  ln(x)	natural logarithm base e
  sqrt(x)	square root
  sqr(x)	the square of a number
  abs(x)	absolute value
  int(x)	conversion to integer
  trunc(x)	conversion to integer; no rounding
  round(x)	the closest integer value