[SOLVED] NIL Pointer check Method
lazarus / 62-General

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nasprin · 2021-11-09 23:29

Greetings! First post here, I searched, I failed, I started a topic
someone somewhere, I'm sure, already answered  a 1000 times 

I'm just starting the FreePascal/Lazarus Journey for my study and seem
to have a NIL Pointer problem I'm stuck at, would appreciate a hint or
two on this. Basically, I am trying to create a linked list of records
Type "tKante", fill them, one by one with a value, stop the filling
when 0 is entered and then iterate over all linked list objects until
the last one has its 'next' pointed to nil - so create a singly linked
list. Here's the code:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i program
learningLinkedListsPainfully;uses crt; typetRefKante = ^tKante;tKante
= record           von: integer;           zu: integer;         
 x:boolean;           next: tRefKante         end; var A :
tRefKante;var startA,    lookupPointer : ^tKante; procedure
ElementAnhaengen (          inZahlVon : integer;          inZahlZu :
integer;          B :boolean;          var ioRefAnfang : tRefKante); 
       {joins a new object type tRefkante to the linked list}begin 
new (A);  if (ioRefAnfang<>nil) then  begin    A^.von :=inZahlVon;   
A^.zu := inZahlZu;    A^.x :=B;    A^.next :=ioRefAnfang;   
ioRefAnfang := A;  end  else  begin    A^.von :=inZahlVon;    A^.zu :=
inZahlZu;    A^.x :=B;    ioRefAnfang := A;  end; end; {End of
ElementAnhaengen} procedure ShowList (          var ioRefAnfang :
tRefKante);var y: ^word;         {iterates over all objects of the
linked list}begin  y :=nil;  writeln();  lookupPointer:=ioRefAnfang; 
while (lookupPointer<>nil) do  begin    write('Number von: ',
lookupPointer^.von,'; ');    write('Number zu: ', lookupPointer^.zu,';
');    write('X : ', lookupPointer^.x,'; ');    y :=
addr(lookupPointer);    writeln(',  At Adress: ',y^);   
lookupPointer:=lookupPointer^.next;  end; end; {End of
ShowList} procedure createList (var ioStartList:tRefKante);var input:
integer;    xInt, von, zu: integer;    x: boolean;      {Fills the
linked List trough user input and the procedure
ElementAnhaengen}Begin  ioStartList:=nil;  repeat  begin     
write('Enter "von": ');      read(von);      write('Enter "zu": ');  
  read(zu);      write('Enter "x": ');      read(xInt);      if
(xInt=1) then        x := true      else        x :=false;     
ElementAnhaengen(von, zu,x,ioStartList);      writeln('Do again? 0 to
Stop');      readln(input);  end  until (input=0); end;    begin  
createList(startA);  showlist(startA); readkey();end. 
The createlist() procedure seems to work as I would expect it to, but
the showlist() iterates trough all elements, created by createlist(),
and instead of stopping at the last one, tries to go one further...
Whisch leads to a SIGSEGV error, see the srceenshot attached.

It seems the code tries to follow a link to the next object, even
though the object was not created. I would expect a .next Entry in the
last record of the linked list to be NIL, because I didn't set it, but
it seems its actually pointing at some address that isn't NIL, when I
add a simple writeln to check it..

Sorry for a probably super-duper easy question, the search was
stronger then me =)

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dbannon · 2021-11-09 23:49

You are not setting 'next' the first time you add an element to the
list.

I used to love LL when I played with C.  But now you are walking in
the light, maybe consider fplist or even avllist ?

Davo

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nasprin · 2021-11-10 00:10

Quote from: dbannon on November 10, 2021, 12:49:49 amYou are not
setting 'next' the first time you add an element to the list.

I used to love LL when I played with C.  But now you are walking in
the light, maybe consider fplist or even avllist ?

Davo

Well, I knew it was easy, but yeah, I didn't think it was that
obvious, now that you've said it.. Thanks a lot, that really helped!

The linked lists are more a part of my study, but thanks for the
advice on other list types =) Hopefully, some day I will be spending
my coding time not trying to invent bicycles..