First chance exception at $7C81EB33 - delphi

Question

I have an application that when run at home works fine, however when ran on school computers(Windows XP) i get the following message. (This is recompiling it, not just running the .exe)- In Delphi 2005
First chance exception at $7C81EB33. Exception class EAccessViolation with message 'Access violation at address 0045E5E2 in module 'Project2.exe'. Read of address 00000198'. Process Project2.exe (440)
Code: Ignoring unneeded stuff.
Image1: TImage; // Image(all the way to 72)
Timer1: TTimer; Timer2: TTimer;
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure SomeOtherProcedure(Sender: TImage);
procedure Timer1Timer(Sender: TObject);
procedure Timer2Timer(Sender: TObject);
private
{ private declarations }
public
{ public declarations }
end;
var
Form1: TForm1;
left : integer;
top : integer;
gap : integer;
type
coordinates = record
row : integer ;
col : integer;
end;
var
picarray : array[0..5,0..5] of timage;
thiscover, midcover, lastcover : timage;
imageindex : array[0..5,0..5] of integer;
picloc: array[0..3] of coordinates;
clickcount, pairsfound, attemptcount : integer;
implementation
{$R *.lfm}
procedure initialise();
var
i, j, whichcol, whichrow : integer;
begin
for i := 0 to 5 do
for j := 0 to 5 do
imageindex[i,j] := -1; // not used
randomize;
for i := 0 to 11 do
for j := 1 to 3 do
begin
repeat
begin
whichcol := random(6) ;
whichrow := random(6) ;
end;
until imageindex[whichcol, whichrow] = -1;
picarray[whichcol, whichrow].Picture.LoadFromFile('C:\Users\Hayden\Pictures\'+ inttostr(I+1) +'.jpg');
imageindex[whichcol, whichrow] := I ;
end;
clickcount := 0 ; //
pairsfound := 0 ;
attemptcount := 0 ;
end;
procedure TForm1.FormCreate(Sender: TObject);
var
cpic : tcomponent;
whichcol: integer;
whichrow : integer;
begin
gap := image2.left - image1.left;
top := image1.Top;
left := image1.left;
for cpic in form1 do
begin
if (cpic.ClassType = timage) and (cpic.Tag = 10) then
begin
whichcol := (timage(cpic).left - left) div gap;
whichrow := (timage(cpic).Top - top) div gap;
picarray[whichcol, whichrow] := timage(cpic) ;
end;
end;
initialise;
end;
Line >>> picarray[whichcol, whichrow].Picture.LoadFromFile('C:\Users\Hayden\Pictures\'+ inttostr(I+1) +'.jpg');
seems to cause the error. And if it is a coding error, what is the correct way to do this?

Answer 1

First, I'm going to clean up your code a little, because as it stands, it's very difficult to figure what's going on. I highly recommend you get into the habit of taking a few minutes to keep your code clearly formatted - it will save you hours of debugging.
I've applied only the following simple changes: Indentation, Blank lines, and liberal use of begin .. end;
var
picarray : array[0..5,0..5] of timage;
thiscover, midcover, lastcover : timage;
imageindex : array[0..5,0..5] of integer;
picloc: array[0..3] of coordinates;
clickcount, pairsfound, attemptcount : integer;
implementation
{$R *.lfm}
procedure initialise();
var
i, j, whichcol, whichrow : integer;
begin
for i := 0 to 5 do
begin
for j := 0 to 5 do
begin
//It's clear you're initialising the 36 entries of imageindex to -1
imageindex[i,j] := -1; // not used
end;
end;
randomize;
for i := 0 to 11 do
begin
for j := 1 to 3 do
begin
//This loop also runs 36 times, so it fills the whole of imageindex with new values
//It also loads all 36 entries of picarray with an image specfied by the current value of i
//The approach is dangerous because it depends on the 'loop sizes' matching,
//there are much safer ways of doing this, but it works
repeat
begin //This being one of the only 2 begin..end's you provided inside this is routine is pointless because repeat..until implies it.
whichcol := random(6) ;
whichrow := random(6) ;
end;
until imageindex[whichcol, whichrow] = -1;
//This line itself will throw an access violation if picarray[whichcol, whichrow] doesn't
//contain a valid TImage instance... we have to check other code to confirm that possibility
picarray[whichcol, whichrow].Picture.LoadFromFile('C:\Users\Hayden\Pictures\' + inttostr(I+1) + '.jpg');
imageindex[whichcol, whichrow] := I ;
end;
end;
clickcount := 0 ; //
pairsfound := 0 ;
attemptcount := 0 ;
end;
Moving on to the next piece of code:
procedure TForm1.FormCreate(Sender: TObject);
var
cpic : tcomponent;
whichcol: integer;
whichrow : integer;
begin
gap := image2.left - image1.left;
top := image1.Top;
left := image1.left;
for cpic in form1 do
begin
//This loop attempts to assign existing TImage instances to picarray
//However, the way you're going about it is extremely dangerous and unreliable.
//You're trying to use the position of a component on the form to determine its
//position in the array.
//There are many things that could go wrong here, but since this seems to be a
//homework excercise, I'll just point you in the right direction - you need
//to debug this code.
if (cpic.ClassType = timage) and (cpic.Tag = 10) then
begin
whichcol := (timage(cpic).left - left) div gap;
whichrow := (timage(cpic).Top - top) div gap;
picarray[whichcol, whichrow] := timage(cpic) ;
end;
end;
//Here you call initialise, which as I said before, will
//cause an Access Violation if picarray is not correctly 'set up'
//The previous code in this method certainly has a bug which is
//preventing one or more picarray entries from being assigned a
//valid TImage instance.
//You could write a simple for I := 0 to 5, for J := 0 to 5 loop
//here to check each of picarray entries and pinpoint which is
//incorrect to aid your debugging of the pevious loop.
initialise;
end;

Answer 2

The critical section is the initialization of picarray. You can't be sure that every array element is assigned with a TImage component. If at least one Image has a wrong left or top you have a double assignment to one element and another is left nil. This will result in an Access Violation when you use it for the first time e.g. in picarray[whichcol, whichrow].Picture.LoadFromFile.
I would recommend to redesign the picarray initalization with for loops for every dimension. To get the correct TImage I would name them like 'Image_2_3' and get the instances in the loop by name.

Answer 3

you can check if the file exists and try to catch the exception to display a meaningful message
try
if FileExists('C:\Users\Hayden\Pictures\'+ inttostr(I+1) +'.jpg') then
picarray[whichcol, whichrow].Picture.LoadFromFile('C:\Users\Hayden\Pictures\'+ inttostr(I+1) +'.jpg');
else
ShowMessage("File not found");
except
on E : Exception do
ShowMessage(E.ClassName+' error raised, with message : '+E.Message);
end;