Question

I am pretty “green” at programming and I’ve to bring assignment tomorrow. It is almost complete but there is a slight problem. I cannot remove the first element and if after trying to delete first element I input
a new one in its place mmm…. lets just say I input infinitely many of them instead. I cant seem to find what is the problem. Thanks in advance

program dvipsar;

type duomenys = integer;
     sarasas = ^elementas;
    elementas = record
       info: duomenys;
       anks: sarasas;
       tolsn: sarasas
     end;


procedure sukurtiTuscia(var s: sarasas);   {creates empty list}
begin
  s := nil
end;

function tuscias(s: sarasas): boolean;   {checks if list is empty}
begin
  tuscias := s = nil
end;

function elmSk(s: sarasas): integer;      {counts elements}
  var kiek: integer;
begin
  kiek := 0;
  while s <> nil do
  begin
    kiek := kiek + 1;
    s := s^.tolsn
  end;
  elmSk := kiek
end;

function gautiRodN(s: sarasas; n:integer): sarasas;     {Arrow to n-th element}
  var i: integer;
begin
  i := 1;
  while (s <> nil) and (i<n) do
  begin
    i := i + 1;
    s := s^.tolsn
  end;
  if i = n then gautiRodN := s
  else gautiRodN := nil
end;

function gautiN(s: sarasas; n:integer): duomenys;         {gets n-th element data}
  var elem: sarasas;
begin
  elem := gautiRodN(s,n);
  if elem <> nil then gautiN := elem^.info
end;

procedure iterptiPries(s:sarasas; n: integer; duom: duomenys);       {adds new element before n-th element}
  var nElem: sarasas;
      naujas: sarasas;
begin
  nElem := gautiRodN(s,n);
  if nElem <> nil then begin
    new (naujas);
    naujas^.info := duom;
    naujas^.tolsn := nElem;
    naujas^.anks := nElem^.anks;
    if nElem^.anks <> nil then nElem^.anks^.tolsn := naujas;
    nElem^.anks := naujas;
 end
end;

procedure panaikintiN(s: sarasas; n: integer);         {removes element from n-th place}
  var nElem: sarasas;
begin
  nElem := gautiRodN(s,n);
  if nElem <> nil then begin
    if nElem^.anks <> nil then nElem^.anks^.tolsn := nElem^.tolsn;
    if nElem^.tolsn <> nil then nElem^.tolsn^.anks := nElem^.anks;
    dispose(nElem);
  end;
end;

function rasti(s: sarasas; duom: duomenys): sarasas;       {finds element}
begin
  while (s <> nil) and (s^.info <> duom) do s := s^.tolsn;
  rasti := s
end;

procedure spausdinti(s: sarasas);                {prints list}
begin
  while (s <> nil) do begin
    write(s^.info,' ');
    s := s^.tolsn
  end;
  writeln
end;

procedure panaikintiP(var s: sarasas);   {removes first element}
  var pirmas: sarasas;
begin
  pirmas := s;
  s := s^.tolsn;
  dispose (pirmas)
end;

procedure panaikinti(var s: sarasas);                     {deletes list}
begin
  while s <> nil do panaikintiP(s)
end;


procedure prideti(var s: sarasas; duom: duomenys);      {add element at the end of the list}
  var kiek: integer;
      paskutinis,naujas: sarasas;
begin
  kiek := elmSk(s);
  paskutinis := gautiRodN(s,kiek);
  new(naujas);
  naujas^.info := duom;
  naujas^.tolsn := nil;
  naujas^.anks := paskutinis;
  if paskutinis <> nil then paskutinis^.tolsn := naujas
  else s := naujas
end;

procedure menu;
begin
  writeln;
  writeln;
  writeln ('1 Creat a list');
  writeln ('2 Count the elements');
  writeln ('3 Check if list is empty');
  writeln ('4 Print an element');
  writeln ('5 Print the list');
  writeln ('6 Remove an element')
  writeln ('7 Add an element');
  writeln ('8 Search in the list');
  writeln;
  writeln ('0 End');
  writeln;
  writeln;

end;
var s: sarasas;
    i,j: integer;
    t: sarasas;
    c: char;
    veiksmas: integer;
begin
  sukurtiTuscia(s);
  repeat
    menu;
    write('Input action number : ');
    readln(veiksmas);
    case veiksmas of
      1:
        repeat 
          write('input a number which you want to add to the list: ');
          readln(i);
          prideti(s,i);
          write('Add new number? (t/n)? ');
          read(c);
        until (c='N') or (c='n');
      2: writeln ('List is not empty: ',elmSk(s));
      3: if tuscias(s) then writeln ('List is empty')
                       else writeln ('List is not empty');
      4: begin
        write ('Which element to print?: ');
        readln(i);
        writeln(i,'-th list element?: ',gautiN(s,i));
      end;
      5: spausdinti(s);
      6: begin
        write ('which element to remove?: ');
        readln(i);
        panaikintiN(s,i);
      end;
      7:begin
        write ('What to add to the list?: ');
        readln(i);
        write ('Before which element?: ');
        readln(j);
        iterptiPries(s,j,i);
      end;
      8: begin
        write ('What element to look for?: ');
        readln(i);
        t := rasti(s,i);
        if t <> nil then writeln (i, ' exists in the list')
                    else writeln (i, ' does not exists in the list');
      end;
      0: writeln ('Ending');
    else writeln('Incorrect action');
    end;
  until veiksmas = 0;
  panaikinti(s);                    {deletes list}
end.

Answer 1

http://en.wikipedia.org/wiki/Doubly_linked_list#Removing_a_node

Removal of a node is easier than insertion, but requires special handling if the node to be removed is the firstNode or lastNode:

 function remove(List list, Node node)
   if node.prev == null
       list.firstNode := node.next
   else
       node.prev.next := node.next
   if node.next == null
       list.lastNode := node.prev
   else
       node.next.prev := node.prev
   destroy node

One subtle consequence of the above procedure is that deleting the last node of a list sets both firstNode and lastNode to null, and so it handles removing the last node from a one-element list correctly.

Notice that we also don’t need separate “removeBefore” or “removeAfter” methods, because in a doubly linked list we can just use “remove(node.prev)” or “remove(node.next)” where these are valid.

This also assumes that the node being removed is guaranteed to exist.

If the node does not exist in this list, then some error handling would be required.