Swap two nodes in a singly linked list - linked-list

Question

How to switch two nodes in a singly linked list in pascal?
procedure List.switch(var n1,n2 : pNode);
var aux : pNode;
begin
aux := n1;
p1:= n2;
n2 := aux;
end;
(Here pNode is a pointer on a node in the list.)
Nodes are defined like this:
pNode = ^Node;
Node = record
data : data;
next : pNode;
end;
This code doesn't work. It either doesn't compile, saying "Can't take the address of constant expressions", or just doesn't do anything. I guess it has to do with how pointers work...
I found relevant information here, but I don't read C.
Thanks for any advice!

Answer 1

I think something like this would work:
function SwapNodes(first: pNode): pNode;
begin
Result := first.next;
first.next := Result.next;
Result.next := first;
end;

Answer 2

Something like this should work for you. It uses a local variable of type Node (which is presumably what PNode is a pointer to) as a placeholder.
procedure List.Switch(NodeA, NodeB: PNode);
var
Temp: Node;
begin
Temp.Data := NodeB^.Data;
Temp.Next := NodeB^.Next;
NodeB^.Data := NodeA^.Data;
NodeB^.Next := NodeA^.Next;
NodeA^.Data := Temp.Data;
NodeA^.Next := Temp.Next;
end;
Here's a version of it that isn't an object method, with a console app that tests it:
program Project1;
uses
System.SysUtils;
type
PNode = ^Node;
Node = record
Data: Integer;
Next: PNode;
end;
procedure Swap(NodeA, NodeB: PNode);
var
Temp: Node;
begin
Temp.Data := NodeB^.Data;
Temp.Next := NodeB^.Next;
NodeB^.Data := NodeA^.Data;
NodeB^.Next := NodeA^.Next;
NodeA^.Data := Temp.Data;
NodeA^.Next := Temp.Next;
end;
var
A, B: Node;
pA, pB: PNode;
begin
New(pA);
pA^.Data := 1;
pA^.Next := nil;
New(pB);
pB^.Data := 2;
pB^.Next := #A;
WriteLn('Before - pA^.Data: ', pA^.Data, ' pB^.Data: ', pB^.Data);
Swap(pA, pB);
WriteLn('After - pA^.Data: ', pA^.Data, ' pB^.Data: ', pB^.Data); // Outputs 2 and 1
Readln;
Dispose(pA);
Dispose(pB);
end.