PASCAL: AN INTRODUCTION

POINTERS
The previous program can be rewritten to make it easier to read, understand and maintain. To do this, we will use a dedicated pointer to maintain and initialise the list, rather than get into the long notation that we used in the previous program, eg,


	startrecord^.nextrecord^.nextrecord^.number := 30;

The modified program now looks like,


	program PointerRecordExample2( output );

	type  rptr = ^recdata;
		recdata = record
			number : integer;
			code   : string;
			nextrecord : rptr
		end;

	var  startrecord, listrecord : rptr;

	begin
		new( listrecord );
		if listrecord = nil then
		begin
			writeln('1: unable to allocate storage space');
			exit
		end;
		startrecord := listrecord;
		listrecord^.number := 10;
		listrecord^.code := 'This is the first record';
		new( listrecord^.nextrecord );
		if listrecord^.nextrecord = nil then
		begin
			writeln('2: unable to allocate storage space');
			exit
		end;
		listrecord := listrecord^.nextrecord;
		listrecord^.number := 20;
		listrecord^.code := 'This is the second record';
		new( listrecord^.nextrecord );
		if listrecord^.nextrecord = nil then
		begin
			writeln('3: unable to allocate storage space');
			exit
		end;
		listrecord := listrecord^.nextrecord;
		listrecord^.number := 30;
		listrecord^.code := 'This is the third record';
		listrecord^.nextrecord := nil;
		while startrecord <> nil do
		begin
			listrecord := startrecord;
			writeln( startrecord^.number );
			writeln( startrecord^.code );
			startrecord := startrecord^.nextrecord;
			dispose( listrecord )
		end
	end.

In this example, the pointer listrecord is used to create and initialise the list. After creation of the first record, it is saved in the pointer startrecord.

The lines of code


	new( listrecord );
	if listrecord = nil then
	begin
		writeln('1: unable to allocate storage space');
		exit
	end;
	startrecord := listrecord;
	listrecord^.number := 10;
	listrecord^.code := 'This is the first record';

creates the first record and initialises it, then remembers where it is by saving it into startrecord. Pictorially, it looks like,

The lines of code


	new( listrecord^.nextrecord );
	if listrecord^.nextrecord = nil then
	begin
		writeln('2: unable to allocate storage space');
		exit
	end;
	listrecord := listrecord^.nextrecord;
	listrecord^.number := 20;
	listrecord^.code := 'This is the second record';

add a new record to the first by linking it into listrecord^.nextrecord, then moving listrecord to the new record. Pictorially, it looks like,

The lines of code


	new( listrecord^.nextrecord );
	if listrecord^.nextrecord = nil then
	begin
		writeln('3: unable to allocate storage space');
		exit
	end;
	listrecord := listrecord^.nextrecord;
	listrecord^.number := 30;
	listrecord^.code := 'This is the third record';
	listrecord^.nextrecord := nil;

add the last record to the previous by linking it into listrecord^.nextrecord, then moving listrecord to the new record. Pictorially, it looks like,

Note how much easier the code looks than the previous example.