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To access the contents, click the chapter and section titles.
Learn Pascal in a Three Days (2nd Ed.)
Because the “PreviousPointer” follows the “CurrentPointer” and its value is expected to change, it has to be preceded by the keyword VAR. The following is the “DelRecord” procedure:
{ ----------------------- Procedure DelRecord ------------------------ }
PROCEDURE DelRecord(VAR FirstPointer:ListPointer);
VAR
CurrentPointer, PreviousPointer:ListPointer;
Found :BOOLEAN;
SSNumber: SSNstring;
BEGIN
Found := FALSE;
WRITELN(Separator);
WRITE('Enter the SSN of the employee to be removed:');
READLN(SSNumber);
SearchList(FirstPointer, CurrentPointer, PreviousPointer,
SSNumber, Found);
IF NOT Found THEN
WRITELN('SSN: ', SSNumber, ' Not Found')
ELSE
BEGIN
IF PreviousPointer = NIL THEN
{ The node to be deleted is the first node. }
FirstPointer:= FirstPointer^.NextField
ELSE
{ The node to be deleted has a predecessor. }
PreviousPointer^.NextField:= CurrentPointer^.NextField;
DISPOSE(CurrentPointer);
WRITELN('The record has been deleted from the list.')
END;
WRITE('Hit any key to continue...');
READLN
END;
The changes made on the “SearchList” procedure will affect other procedures. Any call to the “SearchList” must include the new pointer parameter “PreviousPointer” as shown in the call below:
SearchList(FirstPointer, CurrentPointer, PreviousPointer,
SSNumber, Found);
In the previous program (10-14.pas) the “DisplayRec” procedure calls “SearchList” using two pointer parameters only. This is obvious because “PreviousPointer” is not used in the “DisplayRec” procedure. In order to incorporate the new “Search List” procedure into the program, the calls to the “SearchList” must be modified. In the procedure “DisplayRec,” you may declare a dummy pointer which has no work to do except being passed as a parameter to the search procedure. This is an example of the new call:
SearchList(FirstPointer, CurrentPointer, DummyPointer,
SSNumber, Found);
In the following program, the employee database is almost completed. These are the main features of the program:
{ -------------------------- figure 11-17 ---------------------------- }
PROGRAM LinkedListDB(INPUT, OUTPUT, MyListFile);
CONST
FileName = 'emplist.bin';
Header = '------------- Main Menu --------------';
Separator = '--------------------------------------';
TYPE
{Declaration of data type }
SSNstring = STRING[11];
DataRecord = RECORD
ID :STRING[5];
Name, Position :STRING[20];
SSN :SSNstring;
Rate :REAL
END;
{Declaration of the list }
ListPointer = ^ListRecord;
ListRecord = RECORD
DataField :DataRecord;
NextField :ListPointer
END;
EmpFile = FILE OF DataRecord;
VAR
FirstPointer :ListPointer;
MyListFile :EmpFile;
EmpRecord :DataRecord;
{ ----------------------- Procedure SearchList ----------------------- }
{ This procedure searches the linked list for an employee's SSN.
If found, the value of the boolean flag "Found" becomes TRUE}
PROCEDURE SearchList(FirstPointer :ListPointer;
VAR CurrentPointer :ListPointer;
VAR PreviousPointer :ListPointer;
SSNumber :SSNstring;
VAR Found :BOOLEAN);
BEGIN
PreviousPointer := NIL;
CurrentPointer := FirstPointer;
WHILE (CurrentPointer <> NIL) AND (NOT Found) DO
IF CurrentPointer^.DataField.SSN = SSNumber THEN
Found := TRUE
ELSE
BEGIN
PreviousPointer := CurrentPointer;
CurrentPointer := CurrentPointer^.NextField
END
END;
{ ----------------------- Procedure BuildList ------------------------ }
PROCEDURE BuildList(VAR FirstPointer:ListPointer;
DataItem :DataRecord);
{ This procedure builds the liked list, or adds nodes to it.}
{Note: The FirstPointer is passed using the VAR keyword as
it will be updated by this procedure }
VAR
ToolPointer :ListPointer;
BEGIN
NEW(ToolPointer);
ToolPointer^.DataField := DataItem;
ToolPointer^.NextField := FirstPointer;
FirstPointer := ToolPointer
END;
{ ----------------------- Procedure ReadList ------------------------- }
PROCEDURE ReadList(FirstPointer:ListPointer);
{ This procedure reads and displays the contents of the list }
VAR
CurrentPointer :ListPointer;
BEGIN
CurrentPointer := FirstPointer;
WHILE CurrentPointer <> NIL DO
BEGIN
WITH CurrentPointer^.DataField DO
BEGIN
WRITE(ID :7);
WRITE(Name :22);
WRITE(Position :22);
WRITE(SSN :13);
WRITELN(' $',Rate :0:2)
END;
CurrentPointer:= CurrentPointer^.NextField
END;
WRITELN
END;
{ ---------------------- Procedure DelRecord ------------------------- }
PROCEDURE DelRecord(VAR FirstPointer:ListPointer);
{ This procedure deletes a node from the list. If the node to be
deleted is the first node. The "FirstPointer" is moved to the next
node; otherwise, the pointer field of the previous node is updated
to point to the next node. In both cases the "CurrentPointer" is
disposed. }
VAR
CurrentPointer, PreviousPointer:ListPointer;
Found:BOOLEAN;
SSNumber: SSNstring;
BEGIN
Found:= FALSE;
WRITELN(Separator);
WRITE('Enter the SSN of the employee to be removed:');
READLN(SSNumber);
SearchList(FirstPointer, CurrentPointer, PreviousPointer,
SSNumber, Found);
IF NOT Found THEN
WRITELN('SSN: ', SSNumber, ' Not Found')
ELSE
BEGIN
IF PreviousPointer = NIL THEN
{ The node to be deleted is the first node. }
FirstPointer := FirstPointer^.NextField
ELSE
{ The node to be deleted has a predecessor. }
PreviousPointer^.NextField:= CurrentPointer^.NextField;
DISPOSE(CurrentPointer);
WRITELN('The record has been deleted from the list.')
END;
WRITE('Hit any key to continue...');
READLN
END;
{ ------------------------ Procedure GetData ------------------------- }
PROCEDURE GetData(VAR FirstPointer:ListPointer);
{ This procedure receives the employee data from the keyboard,
and passes the record information to the procedure "BuildList"
to be added to the linked list }
VAR
CurrentPointer, DummyPointer:ListPointer;
Item :DataRecord;
SSNumber: SSNstring;
Found :BOOLEAN;
BEGIN
Found := FALSE;
WRITE('Please enter the SSN of the employee: ');
READLN(SSNumber);
SearchList(FirstPointer, CurrentPointer, DummyPointer,
SSNumber, Found);
IF NOT Found THEN
BEGIN
WRITELN('Please enter the employee information:');
WITH Item DO
BEGIN
SSN := SSNumber;
WRITE('ID: '); READLN(ID);
WRITE('Name: '); READLN(Name);
WRITE('Position: '); READLN(Position);
WRITE('Rate: '); READLN(Rate);
WRITELN(Separator)
END;
BuildList(FirstPointer, Item);
WRITELN('The employee has been added to the list.')
END
ELSE
WRITELN('The SSN: ', SSNumber, ' is already in the list.');
WRITE('Hit any key to continue...');
READLN
END;
{ ----------------------- Procedure DisplayItAll --------------------- }
PROCEDURE DisplayItAll(FirstPointer:ListPointer);
{ This procedures displays the headers of the fields in the
proper format and calls the procedure "ReadList" to display
the contents of the list }
BEGIN
WRITELN(Separator);
WRITELN('The contents of the list: ');
WRITELN('ID':7, 'Name':22, 'Position':22, 'SSN':13,
'Rate':7);
WRITELN;
ReadList(FirstPointer);
WRITE('Hit any key to continue...');
READLN
END;
{ ----------------------- Procedure DisplayRec ----------------------- }
PROCEDURE DisplayRec(FirstPointer:ListPointer);
{ This procedure displays the information for a specific employee.
It calls the procedure "SearchList" to search the list using the
Social Security Number of the employee.}
VAR
CurrentPointer, DummyPointer:ListPointer;
SSNumber :SSNstring;
Found :BOOLEAN;
{ Note: The "DummyPointer" is used to call the "SearchList"
procedure (which takes three pointers as parameters), but this
pointer is not required in this procedure. }
BEGIN
Found := FALSE;
WRITELN(Separator);
WRITE('Enter the SSN of the employee:'); READLN(SSNumber);
SearchList(FirstPointer, CurrentPointer,
DummyPointer, SSNumber, Found);
IF NOT Found THEN
WRITELN('SSN: ', SSNumber, ' Not Found')
ELSE
WITH CurrentPointer^.DataField DO
BEGIN
WRITELN('ID: ',ID);
WRITELN('Name: ',Name);
WRITELN('Position: ', Position);
WRITELN('Social Security Number: ', SSN);
WRITELN('Hourly Rate: $', Rate:2:2)
END;
WRITE('Hit any key to continue...');
READLN
END;
{ ----------------------- Procedure UpdateRec ------------------------ }
PROCEDURE UpdateRec(FirstPointer:ListPointer);
{ This procedure updates record information for a specific employee.
It calls the procedure "SearchList" to search the list using the
Social Security Number of the employee. The new information is
accepted from the user, otherwise a message "not found" is issued.}
VAR
CurrentPointer, DummyPointer:ListPointer;
SSNumber :SSNstring;
Found :BOOLEAN;
{ Note: The "DummyPointer" is used to call the "SearchList"
procedure (which takes three pointers as parameters), but this
pointer is not required in this procedure. }
BEGIN
Found := FALSE;
WRITELN(Separator);
WRITE('Enter the SSN of the employee:'); READLN(SSNumber);
SearchList(FirstPointer, CurrentPointer,
DummyPointer, SSNumber, Found);
IF NOT Found THEN
WRITELN('SSN: ', SSNumber, ' Not Found')
ELSE
WITH CurrentPointer^.DataField DO
BEGIN
WRITELN('Please enter the now information for',
' the employee (SSN: ', SSNumber,'):');
WRITE('ID: '); READLN(ID);
WRITE('Name: '); READLN(Name);
WRITE('Position: '); READLN(Position);
WRITE('Hourly Rate: '); READLN(Rate);
WRITELN('Record updated.')
END;
WRITE('Hit any key to continue...');
READLN
END;
{ ----------------------- Procedure SaveList ------------------------- }
PROCEDURE SaveList(FirstPointer:ListPointer;
VAR MyListFile: EmpFile);
{This procedure saves the data fields in the linked list to a file
of the type RECORD. }
VAR
CurrentPointer:ListPointer;
BEGIN
ASSIGN(MyListFile, FileName);
REWRITE(MyListFile);
CurrentPointer := FirstPointer;
WHILE CurrentPointer <> NIL DO
BEGIN
WRITE(MyListFile, CurrentPointer^.DataField);
CurrentPointer := CurrentPointer^.NextField
END;
CLOSE(MyListFile);
WRITELN('The list has been saved to the file.');
WRITE('Hit any key to continue...');
READLN
END;
{ ----------------------- Procedure ReadFile ------------------------- }
PROCEDURE ReadFile(VAR FirstPointer:ListPointer;
VAR MyListFile: EmpFile);
{This procedure reads data from the file "emplist.bin" and adds
the data to the linked list. }
VAR
Item :DataRecord;
BEGIN
ASSIGN(MyListFile, FileName);
RESET(MyListFile);
WHILE NOT EOF (MyListFile) DO
BEGIN
READ(MyListFile, Item);
BuildList(FirstPointer, Item);
END;
CLOSE(MyListFile);
WRITELN('The employee list is ready in memory.');
WRITE('Hit any key to continue...');
READLN
END;
{ ------------------------- Procedure Menu --------------------------- }
PROCEDURE Menu;
VAR
Option:INTEGER;
BEGIN
WRITELN(Header);
WRITELN('1. Add records to the list.');
WRITELN('2. Display the whole list.');
WRITELN('3. Display an employee record.');
WRITELN('4. Add records from file.');
WRITELN('5. Save the list to a file.');
WRITELN('6. Delete a record.');
WRITELN('7. Update a record.');
WRITELN('8. Exit.');
WRITELN(Separator);
WRITE('Make a choice and press a number: ');
READLN(Option);
CASE Option OF
1: GetData(FirstPointer);
2: DisplayItAll(FirstPointer);
3: DisplayRec(FirstPointer);
4: ReadFile(FirstPointer, MyListFile);
5: SaveList(FirstPointer, MyListFile);
6: DelRecord(FirstPointer);
7: UpdateRec(FirstPointer);
8: Exit
END;
Menu
END;
{ --------------------------- Main Program --------------------------- }
BEGIN
{ Initialize an empty List }
FirstPointer := NIL;
menu
END.
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