Pascal - doubly linket list - list

Question

I'm trying compile Doubly linked list but I have one small problem with this part of code :
It's a "insert after"
When I insert data to the list : 1 2 3 4 5 and want insert after number 4 for example number 9, number 9 is inserted like this : 1 9 2 3 4 5. I really dont know where I have wrong code:(
everytime i insert new number to second position..
procedure insertAfter(var L:Plist; n_p:integer);
var novy_za:Plist;
begin
new(novy_za);
novy_za^.info:=n_p;
novy_za^.next:=L^.next;
novy_za^.prev:=L;
L^.next:=novy_za;
if novy_za^.next<>nil
then novy_za^.next^.prev:=novy_za;
end;
procedure call :
begin
writeln;
write('which number insert : '); readln(x);
writeln;
write('insert after : '); readln(Y);
InsertAfter(P,x);
end;

Answer 1

Here's a corrected version of your routine. Your original failed to accept the value you want to insert after as an argument. I made it a function so that it returns the pointer to the new added element if it was able to find the one you wanted to insert after. Otherwise, it returns nil. You call it with a second argument which tells it what value you want to insert after.
function insertAfter(var L: Plist; n_a, n_p: integer): Plist;
var novy_za, cur_za: Plist;
begin
cur_za := L;
novy_za := nil;
{ Traverse the list, looking for value n_a and, if found, insert the new
element n_p after it }
while cur_za <> nil do begin
{ Insert the new element after the one with value n_a }
if cur_za^.info = n_a then begin
new(novy_za);
novy_za^.info := n_p;
novy_za^.next := cur_za^.next;
novy_za^.prev := cur_za;
cur_za^.next := novy_za;
if novy_za^.next <> nil then
novy_za^.next^.prev := novy_za;
break; { Exit the loop }
end;
cur_za := cur_za^.next;
end;
insertAfter := novy_za;
end;
Function call (this assumes P is defined somewhere as a valid Plist, and x and y are defined as info type):
begin
writeln;
write('which number insert : '); readln(x);
writeln;
write('insert after : '); readln(y);
{ Insert the value x after the number y }
if insertAfter(P, y, x) = nil then
writeln('The value ', y, ' was not found')
else
writeln('The value ', x, ' was successfully inserted after ', y);
end;