Computer Science Canada

Help With Array Subscripts
Author:  Slaivis [ Wed Jun 28, 2006 9:57 pm ]
Post subject:  Help With Array Subscripts
Now, I'm not really new to arrays, but this one's got me stumped.

I'm writing a game, and this is supposed to be the background.

The goal is to draw lines of a length of 100 pixels, that start at the centre of the screen, grow to their full length, then radiate outwards. The effect is supposed to give the impression that the player is flying through space at warp speeds.

However, what happens is that I get an array subscript is out of range order. I've tried following what the program does, but I don't understand how it could possibly be out of range because all of my arrays have maximum values either defined by variables, or values that aren't reached by the program at the time of the error.

I don't know... Perhaps if you run this, you will have either the experience to know what the problem is, or an eye keen enough to spot it.

Here's the code:

code:

setscreen ("graphics:800;600,offscreenonly")
/*
lc - Light Create?
lmax - Light Max
le - Light Exists
lang - Light Angle
ldc - Light Distance from Centre
ldci - Light Distance from Centre Initiated
lx - X value of Light Distance from Centre
ly - Y value of Light Distance from Centre

*/

var lc : int
var lmax : int := 10
var le : array 1 .. lmax of boolean
var lang, ldc, ldci : array 1 .. lmax of int
for i : 1 .. lmax
le (i) := false
ldci (i) := 0
ldc (i) := 0
end for

%Initialize the values coresponding to lcdi, so that the program doesn't have to make repetetive calculations while drawing.
var sina, cosa : array 1 .. 360, 1 .. 100 of int
for i : 1 .. 360
for j : 1 .. 100
sina (i, j) := round (sind (i) * j)
cosa (i, j) := round (cosd (i) * j)
end for
end for

%Initialize the X and Y values while the light is acutally moving, coresponding to ldc
var lx, ly : array 1 .. 360, 1 .. (maxx * 2) of int
for i : 1 .. 360
for j : 1 .. (maxx * 2)
lx (i, j) := round (sind (i) * j)
ly (i, j) := round (cosd (i) * j)
end for
end for


%Declare the procedures so that the order of their actual declaration does not matter
forward proc createlight (n : int)
forward proc createlightQ
forward proc light (n : int)


%Set all the values of a new 'particle' of light to their starting position.
body proc createlight (n : int)
le (n) := true
randint (lang (n), 1, 360)
ldc (n) := 0
ldci (n) := 0
lx (lang (n), 1) := 0
ly (lang (n), 1) := 0
end createlight

%Generate a random number to decide whether or not another 'particle' of light should be created.
body proc createlightQ
randint (lc, 1, 100)
%If it is decided that another 'particle' of light should be created...
if lc > 95 then
for i : 1 .. lmax
%Find a 'slot' that is not taken.
if le (i) = false then
%Create the 'particle' in an untaken 'slot'.
createlight (i)
exit
end if
end for
end if
end createlightQ

%Draw any 'particle'.
body proc light (n : int)
%If the length of the 'particle' is not full.
if ldci (n) <= 98 then
drawline ((lx (lang (n), 1) + (maxx div 2)), (ly (lang (n), 1) + (maxy div 2)), (lx (lang (n), 1) + sina (lang (n), ldci (n)) + (maxx div 2)),
(ly (lang (n), 1) + cosa (lang (n), ldci (n)) + (maxy div 2)), 0)
%If the length of the particle is full.
else
drawline ((lx (lang (n), ldc (n)) + (maxx div 2)), (ly (lang (n), ldc (n)) + (maxy div 2)), (lx (lang (n), ldc (n)) + sina (lang (n), ldci (n)) + (maxx div 2)),
(ly (lang (n), ldc (n)) + cosa (lang (n), ldci (n)) + (maxy div 2)), 0)
end if

%If the 'particle' is past the visible horizon.
if ldci (n) >= 99 then
%Free up the slot.
%le (n) := false
end if
end light



drawfillbox (0, 0, maxx, maxy, 7)
loop
%drawfillbox (0, 0, maxx, maxy, 7)
createlightQ
for i : 1 .. lmax
%Move each particle accordingly.
if ldci (i) < 97 then
ldci (i) += 3
elsif ldci (i) > 97 then
ldc (i) += 3
end if
if le (i) = true then
light (i)
end if
end for

View.Update
end loop



If you've made it this far, I salute you.

- Thanks
Author:  Mazer [ Wed Jun 28, 2006 10:04 pm ]
Post subject: 
I haven't got Turing with me, but a couple of things you may want to consider in order for us to help you:
1) Tell us where you get the error (ie, line number), this makes it easier on people who (like myself) don't have Turing available
2) Indent! For the same reason as above and more.
Author:  Slaivis [ Wed Jun 28, 2006 10:45 pm ]
Post subject: 
Alright, I was mostly counting on people running the program in Turing and hitting F2 to find both of those things, but I'll paste the information into the forum.

This code should be indented, and the error first occurs at the variable that I've bolded. I haven't included the line number because the lines tend to get shifted over in the post.

This is really irritating. I can't bold anything with the 'code' option, and the 'quote' option removes all indentation. I also seem to be unable to edit my post here.

To make the problem code the most visible, I've surrounded it by three blank lines and commented on it.

code:

setscreen ("graphics:800;600,offscreenonly")
/*
lc - Light Create?
lmax - Light Max
le - Light Exists
lang - Light Angle
ldc - Light Distance from Centre
ldci - Light Distance from Centre Initiated
lx - X value of Light Distance from Centre
ly - Y value of Light Distance from Centre

*/

var lc : int
var lmax : int := 10
var le : array 1 .. lmax of boolean
var lang, ldc, ldci : array 1 .. lmax of int
for i : 1 .. lmax
    le (i) := false
    ldci (i) := 0
    ldc (i) := 0
end for

%Initialize the values coresponding to lcdi, so that the program doesn't have to make repetetive calculations while drawing.
var sina, cosa : array 1 .. 360, 1 .. 100 of int
for i : 1 .. 360
    for j : 1 .. 100
        sina (i, j) := round (sind (i) * j)
        cosa (i, j) := round (cosd (i) * j)
    end for
end for

%Initialize the X and Y values while the light is acutally moving, coresponding to ldc
var lx, ly : array 1 .. 360, 1 .. (maxx * 2) of int
for i : 1 .. 360
    for j : 1 .. (maxx * 2)
        lx (i, j) := round (sind (i) * j)
        ly (i, j) := round (cosd (i) * j)
    end for
end for


%Declare the procedures so that the order of their actual declaration does not matter
forward proc createlight (n : int)
forward proc createlightQ
forward proc light (n : int)


%Set all the values of a new 'particle' of light to their starting position.
body proc createlight (n : int)
    le (n) := true
    randint (lang (n), 1, 360)
    ldc (n) := 0
    ldci (n) := 0
    lx (lang (n), 1) := 0
    ly (lang (n), 1) := 0
end createlight

%Generate a random number to decide whether or not another 'particle' of light should be created.
body proc createlightQ
    randint (lc, 1, 100)
    %If it is decided that another 'particle' of light should be created...
    if lc > 95 then
        for i : 1 .. lmax
        %Find a 'slot' that is not taken.
            if le (i) = false then
            %Create the 'particle' in an untaken 'slot'.
                createlight (i)
                exit
            end if
        end for
    end if
end createlightQ

%Draw any 'particle'.
body proc light (n : int)
%If the length of the 'particle' is not full.
    if ldci (n) <= 98 then
        drawline ((lx (lang (n), 1) + (maxx div 2)), (ly (lang (n), 1) + (maxy div 2)), (lx (lang (n), 1) + sina (lang (n), ldci (n)) + (maxx div 2)),
        (ly (lang (n), 1) + cosa (lang (n), ldci (n)) + (maxy div 2)), 0)
        %If the length of the particle is full.
    else



%The error occurs in this line, in the first instance of [ldc(n)], and by
%logic, all other instances of the same variable.
        drawline ((lx (lang (n), [b]ldc (n)[/b]) + (maxx div 2)), (ly (lang (n), ldc (n)) + (maxy div 2)), (lx (lang (n), ldc (n)) + sina (lang (n), ldci (n)) + (maxx div 2)),
            (ly (lang (n), ldc (n)) + cosa (lang (n), ldci (n)) + (maxy div 2)), 0)



    end if
   
    %If the 'particle' is past the visible horizon.
    if ldci (n) >= 99 then
    %Free up the slot.
        %le (n) := false
    end if
end light



drawfillbox (0, 0, maxx, maxy, 7)
loop
    %drawfillbox (0, 0, maxx, maxy, 7)
    createlightQ
    for i : 1 .. lmax
    %Move each particle accordingly.
        if ldci (i) < 97 then
            ldci (i) += 3
        elsif ldci (i) > 97 then
            ldc (i) += 3
        end if
        if le (i) = true then
            light (i)
        end if
    end for

    View.Update
end loop
Author:  Mazer [ Thu Jun 29, 2006 7:56 am ]
Post subject: 
The problem isn't that n is out of the range of ldc, but that ldc(n) is out of the range of lx. Is ldc(n) ever greater than zero?
Author:  Slaivis [ Thu Jun 29, 2006 12:09 pm ]
Post subject: 
Mazer wrote:
The problem isn't that n is out of the range of ldc, but that ldc(n) is out of the range of lx. Is ldc(n) ever greater than zero?


ldc(n) is never less than zero.

I think you've led me to the solution.

The thing is that ldc(n) is set to zero to initialize it, but the subscripts for lx are set from a minium value of one.

Thus, my subscript is actually below the range, which is what completely threw me off because I'd never encountered something like that.

This means that I should initialize my value of ldc as 1.


- Thanks, Mazer!

:


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