Pascal Programming/Appendix
Jumps
editUsage of jumps are deemed bad practice. This topic has deliberately not been covered in any of the previous chapters, but for the sake of completeness it is explained in the appendix.
Using labels
editStandard Pascal includes the infamous "goto" statement. It redirects the computer to a labeled statement somewhere else in the program. Labels are unsigned integers, although some compilers allow them to be words.
1: write('Enter an even number please: '); (*Try to get an even number*)
readln(number);
even := (number mod 2) = 0;
if even then goto 2; (*Skip ahead*)
writeln('That was an odd number.');
goto 1; (*Try again*)
2: writeln('Thanks!'); (*Finished!*)
Declaring labels
editIf you use any labels, you are required to declare them ahead of time just as you would variables or constants.
program GetEvenNumber;
label 1, 2;
var
number: integer;
even: Boolean;
Avoiding labels
editIn some early programming languages, labeled jumps were the primary way of controlling the flow of execution. Too many labels and "goto" statements will read to unreadable code. Today, we would generally be expected to rewrite such code with the more specific control flow structures, like so:
repeat
write('Enter an even number, please: ');
readln(number);
even := (number mod 2) = 0;
if not even then writeln('That was an odd number.')
until even;
writeln('Thanks!');
Noteworthy types
edit| type | definition | size (in bytes) | availability |
|---|---|---|---|
| AnsiChar | one ANSI-standard textual character | 1 | |
| AnsiString | an array of ANSI-standard characters of indefinite length with an optional size constraint | 1 * number of characters | |
| Boolean | true or false | 1 | standard |
| Byte | whole number ranging from 0 to 255 | 1 | |
| Cardinal | synonym depending on processor type (16 bit=Word, 32 bit=LongWord, 64 bit=QWord) | varies (2, 4, 8) | |
| Char | one textual character (likely ASCII) | 1 | standard |
| Comp | a floating point number ranging 19-20 digits that is effectively a 64-bit integer | 8 | |
| Currency | a floating point number ranging 19-20 digits that is a fixed point data type | 8 | |
| Double | a floating point number ranging 15-16 digits | 8 | |
| DWord | whole number ranging from 0 to 4,294,967,295 | 4 | |
| Extended | a floating point number ranging 19-20 digits | 10 | |
| Int64 | whole number ranging from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 | 8 | |
| Integer | synonym depending on processor type (16 bit=SmallInt, 32 bit=LongInt, 64 bit=Int64) | varies (2, 4, 8) | standard |
| LongInt | whole number ranging from -2,147,483,640 to 2,147,483,647 | 4 | |
| LongWord | whole number ranging from 0 to 4,294,967,295 | 4 | |
| Pointer | an untyped pointer holding an address | 32 bit=4, 64 bit=8 | standard |
| PtrUInt | a pointer type implicitly convertable to an unsigned integer | 32 bit=4, 64 bit=8 | |
| QWord | whole number ranging from 0 to 18,446,744,073,709,551,615 | 8 | Free Pascal |
| Real | a floating point number whose range is platform dependent | 4 or 8 | standard |
| ShortInt | whole number ranging from -128 to 127 | 1 | |
| ShortString | an array of textual characters of up to 255 elements (likely ASCII) with an optional size constraint | 1 * number of characters (max 255) | standard |
| Single | a floating point number ranging 7-8 digits | 4 | |
| SmallInt | whole number ranging from -32,768 to 32,767 | 4 | |
| String | synonym for ShortString (or AnsiString with the $H preprocessor directive turned on) | 1 * number of characters (max 255) | standard |
| UInt64 | whole number ranging from 0 to 18,446,744,073,709,551,615 | 8 | Free Pascal, Delphi 8 or later |
| WideChar | one UTF-8 textual character | 2 | |
| WideString | an array of UTF-8 characters of indefinite length with an optional size constraint | 2 * number of characters | |
| Word | whole number ranging from 0 to 65,535 | 2 |
Noteworthy preprocessor directives
edit| directive | description | value(s) | example | availability |
|---|---|---|---|---|
| $COPERATORS | allows use of C-style operators | OFF or ON | {$COPERATORS ON}
i += 5;
i -= 5;
i *= 5;
i /= 5;
|
Free Pascal |
| $DEFINE | defines a symbol for the preprocessor (if '$macro on', can have a value assigned) | symbol name (:= value if '$macro on') | {$DEFINE Foo}
{$DEFINE Bar := 5}
|
standard |
| $H | implies whether the String type is a ShortString or AnsiString | - or + | Delphi, Free Pascal | |
| $I | inserts a file's contents into the current source code | filename | {$I hello.txt}
|
standard |
| $IF | begins a preprocessor conditional statement | compile-time boolean expression | {$IF DEFINED(DELPHI) OR DECLARED(Foo)}
|
standard |
| $IFDEF | begins a preprocessor conditional statement depending if a preprocessor symbol is defined | preprocessor symbol | {$IFDEF MSWINDOWS}
|
standard |
| $IFNDEF | begins a preprocessor conditional statement depending if a preprocessor symbol is not defined | preprocessor symbol | {$IFNDEF UNIX}
|
standard |
| $IFOPT | begins a preprocessor conditional statement depending on the status of a preprocessor switch | compiler option | {$IFOPT D+}
|
standard |
| $INCLUDE | inserts a file's contents into the current source code | filename | {$INCLUDE hello.txt}
|
standard |
| $INLINE | allows inline functions and procedures | OFF or ON | unit Foo;
{$INLINE ON}
interface
function Give5: integer; inline;
implementation
function Give5: integer;
begin
Give5 := 5;
end;
end.
|
Free Pascal |
| $MACRO | allows defined symbols to hold values | OFF or ON | {$MACRO ON}
{$DEFINE Foo := 7}
|
Free Pascal |
| $MODE | sets the Pascal dialect | DELPHI, FPC, MACPAS, OBJFPC, TP | Free Pascal | |
| $R | embeds a resource file into the code | a file name | {$R *.dfm}
|
Delphi, Free Pascal |
| $STATIC | allows use of the 'static' keyword | OFF or ON | unit Foo;
{$STATIC ON}
{$MODE OBJFPC}
interface
type
Bar = class
function Baz: string; static;
end;
implementation
function Bar.Baz: string;
begin
Result := 'This function is not part of a Bar instance.';
end;
end.
|
Free Pascal |