how to compare two linear lists in pascal - dynamic

Question

The task is to create new list which contains 2 linear lists, and if elements of first list equal to elements of second list then we must delete the same element from new list
procedure CreateList(var t1,L1,t2,L2:plist);
var
tmp1, tmp2:plist;
begin
t1 := L1;
t2 := L2;
while t1 <> nil do
begin
write(t1^.Data, ' ');
t1 := t1^.Next;
while t2 <> nil do
begin
write(t2^.data, ' ');
tmp2:=t2;
t2 := t2^.Next;
if(t2 = tmp1^.next) then // here is the problem how to compare 2 elements
begin
tmp1 := t1;
t1 := t1^.Next;
Dispose(tmp1);
end;
end;
end;
Writeln;
readkey;
end;

Answer 1

I advise you to change type of you list for something like:
type
TData = integer;
PList = ^TList;
TList = record
data : TData;
next : PList;
copy : boolean;
end;
After you will use value .copy for checking copys. Here is simple code you need.
I tried to make all code as simple as possible.
procedure deleteCopys(var list:PList);
// make all TList.copy := false (after we will use it)
var
curList : PList;
begin
curList := list;
while (curList<>nil) do
begin
curList^.copy := false;
curList := curList^.next;
End;
End;
procedure createList(var list1, list2, list3:PList);
// we belive that: (list3 is empty) and (there is no copys in list1)
// and (there is no copys in list2)
var
curList1, curList2 : PList;
begin
deleteCopys(list1);
deleteCopys(list2);
deleteCopys(list3);
// make TList.copy := true for equal elements
// from curList1 and curList2
curList1 := list1;
while (curList1 <> nil) do
begin
curList2 := list2;
while (curList2 <> nil) do
begin
if (curList1^.data = curList2^.data) then
begin
curList1^.copy := true;
curList2^.copy := true;
break;
End;
curList2 := curList2^.next;
End;
curList1 := curList1^.next;
End;
// now we can put all elements from list1 and list2
// with state TLilt.copy=false
// and each of them will be different
// (Let's do it :)
curList1 := list1;
while (curList1 <> nil) do
begin
if (curList1^.copy = false) then
begin
//some procedure for new element
putNewElement(list3, curList1^.data);
End;
End;
curList2 := list2;
while (curList2 <> nil) do
begin
if (curList2^.copy = false) then
begin
//some procedure for new element
putNewElement(list3, curList2^.data);
End;
End;
end;
I hope it will help you!