How do I check if an integer is assigned

[deananorth]

I'm dealing with a problem trying to make sure an integer is assigned in an object using an if statement.

I tried two approaches that have fallen through.

First was to make sure it's not null (or 'nil' as pascal calls it).

So... if x <> nil then

But it doesn't compile. The compiler spits out Operator is not overloaded: "LongInt" = "Pointer"

So I google around and found out about the "Assigned" function.

So... if not assigned(x) then...

Again, the compiler's not having it and complains: "Error: Illegal parameter list"

So... How should I go about doing this?
Have I run into a compiler bug?

[rvk]

Have I run into a compiler bug?

No, an Integer always has a value. There is no nil value for Integer.

The best choice you have is to assign an "invalid" value of -1, -maxint or any other value you deem invalid and check for that.

[Thaddy]

Indeed, but I guess OP has a different problem and then he should post  example code.

[rvk]

Indeed, but I guess OP has a different problem and then he should post  example code.

Or She 

But you're correct. I read too fast over the "... complains: "Error: Illegal parameter list" part.

(S)he should indeed post some code.

[jamie]

Would be nice to know what type of OBJECT is being referred to here?

Record
OBJECT
CLASSS

 They all have their use, personally I like OBJECT model for somethings when I need what looks like a record
with a inheritance feature.., non-dynamic.. I know classes can do it but this is with far less over head.

Anyway I think if a Class/OBJECT is being used here maybe  INIT method for the object/Constructor for dynamic o
just a Constructor for the Class.


 In any and all cases, you need an INITIATION method. With classes you do it in the Constructor.. etc.

 I know at one time the compiler would generate a zero block of memory when creating dynamic objects, not sure if
that still holds true?

[Thaddy]

Programmer is male. Period. Even if their real name is the goddess of the hunt.
There are too many things taken away from men to allow such a thing to be common knowledge....

(I know a lot of good female programmers, btw)

[deananorth]

First, the object class in question.

type
    person = class
    private
           x,y,vx,vy : integer;
    public
           constructor create; overload; {example of overleading No params}
           constructor create(args : array of integer); overload; {Has params i.e. the thing I want to check.}
           procedure sayhi; {Error occurs in this method}
    end;

Person is just a test class while I try OOP in freepascal/Lazarus.

procedure person.sayhi;
begin
     writeln('Hi from person.');
     if x <> nil then {<- throws Error: Operator is not overloaded: "LongInt" = "Pointer"}
     begin
        writeln(format('I am at %d, %d',[x,y])); { Sysutils FTW! }
     end;
end;

Also... *Reads through thread.* Me: Oh snap I think I've started a flame war- oh, wait I haven't - Have I?

[rvk]

And you say, if you comment out the if, that your program crashes ?

I made a complete example that works fine here:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. uses {$IFDEF UNIX} {$IFDEF UseCThreads}
6.   cthreads, {$ENDIF} {$ENDIF}
7.   Classes, SysUtils;
8.  
9. type
10.   person = class
11.   private
12.     x, y, vx, vy: integer;
13.   public
14.     constructor Create; overload; {example of overleading No params}
15.     constructor Create(args: array of integer); overload; {Has params i.e. the thing I want to check.}
16.     procedure sayhi; {Error occurs in this method}
17.   end;
18.  
19.   constructor person.Create;
20.   begin
21.     inherited Create;
22.   end;
23.  
24.   constructor person.Create(args: array of integer); overload;
25.   begin
26.     inherited Create;
27.   end;
28.  
29.   procedure person.sayhi;
30.   begin
31.     writeln('Hi from person.');
32.     // if x <> nil then {<- throws Error: Operator is not overloaded: "LongInt" = "Pointer"}
33.     begin
34.       writeln(format('I am at %d, %d', [x, y])); { Sysutils FTW! :) }
35.     end;
36.   end;
37.  
38. var
39.   P: person;
40. begin
41.   P := person.Create;
42.   P.sayhi;
43.   readln;
44. end.

Result:

Code: [Select]

Hi from person.
I am at 0, 0
You mentioned a compiler error: "Error: Illegal parameter list"

Error: Illegal parameter list
You are calling a function with parameters that are of a different type than the declared parameters of the function.

So it must be something else in your code.

Please post some code which reproduces your problem.

[jamie]

Integers do not have an unassigned value.

 you must come up with a different plan...

 when using NIL it is expected that you are referring to a POINTER type.

 So you have some choices here:

 1. Use -1 for the Integer as a not set value yet and then you can simply test for -1

 2. If -1 is needed then you can create another field of Boolean that informs you that your
 integer has a valid value in it..

  In your Class
 
    FIsValid:Boolean;

this needs to get set at some point in your code to tree..

so when you are test for "X" to be a valid value...

 if fIsValid then Use_X_For_SOMETHING...

 of course you need to set fIsValid at some point..

[rvk]

you must come up with a different plan...

Yes, that's what I thought too.

But OP is probably just making a simple coding error.
The result of the compiler error "Error: Illegal parameter list" is an indication of that.
We should get the real code which reproduces this error.

(It's not an "I want null for Integer" problem at all)

[balazsszekely]

I cannot comment on the "Illegal parameter list" error, but you can use variants and assign null if this is really necessary, like this:

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. var
3.   V: Variant;
4.   I: Integer;
5. begin
6.   V := null;
7.   if V = null then
8.     ShowMessage('null'); //is this what are you after?
9.   V := 3;
10.   ShowMessage(v);
11.   I := V;   //assign variant to an integer
12.   ShowMessage(IntToStr(I));
13. end;

[Thaddy]

, but you can use variants and assign null if this is really necessary,

Oh, well...
Plz explain variants are evil.....

[deananorth]

You mentioned a compiler error: "Error: Illegal parameter list"

When I check for assignment using "assigned(x)" instead of if "x <> nil", the illegal parameter list error showed up.

Code: Pascal  [Select][+][-]

1. {A test procedure}
2. procedure person.sayhi;
3. begin
4.      writeln('Hi from person.');
5.      if not assigned(x) then { <- Error: Illegal parameter list. I suspect It doesn't like anything but pointers. }
6.      begin
7.         writeln(format('I am at %d, %d',[x,y]));
8.      end;
9. end;

Also it's good to know that when I declare variables in classes, they're just set to zero before I do anything with them.

Maybe  I should...:

Make a special "extended" boolean...

Code: Pascal  [Select][+][-]

1. type
2.     person = class
3.     private
4.            x,y,vx,vy : integer;
5.            extended : boolean; {Added this to tell the class if I'm using one constructor or the other}
6.     public
7.            constructor create; overload;
8.            constructor create(args : array of integer); overload; {set extended to true here and check for it}
9.            procedure sayhi;
10.     end;

...and change it depending on which constructor I used. (Overloading FTW.)

Code: Pascal  [Select][+][-]

1. {Constructor with data passed in.}
2. constructor person.create(args : array of integer);
3. begin
4.      x := args[0];
5.      y := args[1];
6.      vx := args[2];
7.      vy := args[3];
8.      writeln(format('Person created with (%d,%d,%d,%d)',[args[0],args[1],args[2],args[3]]));
9.                  extended := true; {set extended here}
10. end;

Code: Pascal  [Select][+][-]

1. {A test procedure}
2. procedure person.sayhi;
3. begin
4.      writeln('Hi from person.');
5.      if extended then {check extended here.}
6.      begin
7.         writeln(format('I am at %d, %d',[x,y]));
8.      end;
9. end;

Sooooooo it's just me being a derp. I feel very stupid.

It kinda goes to show how helpful the freepascal/Lazarus community is. Thanks guys.

Edits: How many typing blunders and oversights do I have to make today?

[rvk]

Yes, Assigned() only accepts pointers.

In your last code you have vy := args[3]; etc.
You should check if you really provided an array with 4 elements.

Or create a special array-type with 4 elements so you can pass that.

Code: Pascal  [Select][+][-]

1. type
2.   ArrayForPerson = array[0..3] of Integer;
3.  
4. //...
5.   constructor person.create(args: ArrayForPerson);

or

Code: Pascal  [Select][+][-]

1. constructor person.create(args: array of integer);
2. begin
3.   if Length(args) = 4 then
4.   begin
5.     // assign the values
6.   end;

[deananorth]

You should check if you really provided an array with 4 elements.

Nice catch.

My first option is using that if statement in the constructor, but I wanna see of using a special array also works.

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. {
4.  A quick Object Oriented Programming Sanity test.
5. }
6.  
7. uses sysutils;
8.  
9. type
10.     person = class
11.     private
12.            x,y,vx,vy : integer;
13.            hasattr : boolean;
14.     public
15.            constructor create; overload;
16.            constructor create(args : array of integer); overload;
17.            procedure sayhi;
18.     end;
19.  
20. {
21.  Plain "Vanilla" constructor.
22.  It allows me to make the computer do something whenever I'm
23.  creating an object.
24. }
25. constructor person.create;
26. begin
27.      writeln('Person created.');
28.      extended := false; {set extended here}
29. end;
30.  
31. {Constructor with data passed in.}
32. constructor person.create(args : array of integer);
33. begin
34.      if length(args) = 4 then
35.      begin
36.        x := args[0];
37.        y := args[1];
38.        vx := args[2];
39.        vy := args[3];
40.        writeln(format('Person created with (%d,%d,%d,%d)',[args[0],args[1],args[2],args[3]]));
41.        extended := true; {set extended here}
42.      end
43.      else
44.      begin
45.          writeln('The array I need to create this object needs 4 and only 4 elements.');
46.      end;
47. end;
48.  
49. {A test procedure}
50. procedure person.sayhi;
51. begin
52.      writeln('Hi from person.');
53.      if extended then {check extended here.}
54.      begin
55.         writeln(format('I am at %d, %d',[x,y]));
56.      end;
57.  
58.      if (vx <> 0) or (vy <> 0) then
59.      begin
60.         writeln(format('I'' moving (%d, %d) units/tick',[x,y]));
61.      end;
62. end;
63.  
64. var
65.    a: Person;
66.    b: Person;
67. begin
68.      a := Person.create;
69.      a.sayhi;
70.      b := Person.create([1,2,3,4]);
71.      b.sayhi;
72.      readln();
73. end.
74.  

edit: One of my variable names turned out to be a reserved word. Whoops!