Collision Problem in Array

[landfarm]

What is it you are trying to achieve?
I am trying to create collision detection.


What is the problem you are having?
The problem is that when I put "i" into the collision check, it says it has not been declared yet.


Describe what you have tried to solve this problem
I put a for loop before the collision check, clearly declaring the "i".


Post any relevant code (You may choose to attach the file instead of posting the code if it is too long)

Turing:

%General constants const bottom := 0 const movement := 10 const jump := 35 const grav := 2 %Constants for platforms const num := 4 %Total number of moving platforms at once type Platform :     record         platx, platy, speed : real     end record var platforms : array 1 .. num of Platform setscreen ("offscreenonly") var keys : array char of boolean % player position and velocity var dx, dy : int var velx, vely : real % platform positions var platx, platy : int dx := 200 velx := 0 dy := 400 vely := 0 platx := 100 platy := 30 %Randomizes starting locations of moving platforms for i : 1 .. num     platforms (i).platx := Rand.Int (0, maxx - 100)     platforms (i).platy := Rand.Int (20, 350)     platforms (i).speed := Rand.Int (1, 5) end for loop     Input.KeyDown (keys)     if keys ('q') then         exit     end if     % to make the player move     if keys (KEY_LEFT_ARROW) then         velx := -movement     elsif keys (KEY_RIGHT_ARROW) then         velx := movement     else         velx := 0     end if     if dy <= bottom then         vely := jump     end if     % constantly subtracting gravity from the dy position     vely -= grav     dx += round (velx)     dy += round (vely)     %Draws platforms     for i : 1 .. num         if platforms (i).platx < 0 then             platforms (i).speed *= -1         elsif platforms (i).platx + 100 > maxx then             platforms (i).speed *= -1         end if         platforms (i).platx += platforms (i).speed         drawfillbox (round (platforms (i).platx), round (platforms (i).platy), round (platforms (i).platx + 100), round (platforms (i).platy + 20), blue)     end for     % collision check, keeps the player bouncing if they hit the platforms     if dy < bottom or (dx + 30 < platforms (i).platx + 100 and dx - 30 > platforms (i).platx and dy + 30 < platforms (i).platy + 20 and dy - 10 > platforms (i).platy) then         dy := bottom         vely := 0     end if     %Draws player     drawfillbox (dx - 30, dy - 10, dx + 30, dy + 30, green)     %Draws the bottom, easier to see for testing     drawline (0, bottom, maxx, bottom, blue)     View.Update     delay (30)     cls end loop

Please specify what version of Turing you are using
Latest Version.

[Tony]

i is local to the score of the for-loop. That is, if it's created inside the for-loop, you can't use it outside. Though in this case, if you want to use the last value of i, you can use "num" instead.

[landfarm]

I replaced i with num and now it's working, but the player is not bouncing when it hits the platforms. Is there something wrong with my collision? I'm going to try using WhatDotColour now, but I would still like a clear explanation for my mistake. You guys are the best!

[Tony]

I don't really understand why you would check the collision for just one platform, when you have multiple ones.

landfarm @ Sat Dec 19, 2009 12:50 pm wrote:

I would still like a clear explanation for my mistake.

Uhh... perhaps it's...

landfarm @ Sat Dec 19, 2009 12:50 pm wrote:

I'm going to try using WhatDotColour now

[landfarm]

Wait, I'm trying to check the collision for all of them, but through the use of the array. I just tried using View.WhatDotColour to check if the points turn blue or not but it does not seem to work either. Here's what I changed.

Turing:

%General constants const bottom := 0 const movement := 10 const jump := 35 const grav := 2 %Constants for platforms const num := 4 %Total number of moving platforms at once type Platform :     record         platx, platy, speed : real     end record var platforms : array 1 .. num of Platform setscreen ("offscreenonly") var keys : array char of boolean % player position and velocity var dx, dy : int var velx, vely : real % platform positions var platx, platy : int dx := 200 velx := 0 dy := 400 vely := 0 platx := 100 platy := 30 %Randomizes starting locations of moving platforms for i : 1 .. num     platforms (i).platx := Rand.Int (0, maxx - 100)     platforms (i).platy := Rand.Int (20, 350)     platforms (i).speed := Rand.Int (1, 5) end for loop     Input.KeyDown (keys)     if keys ('q') then         exit     end if     % to make the player move     if keys (KEY_LEFT_ARROW) then         velx := -movement     elsif keys (KEY_RIGHT_ARROW) then         velx := movement     else         velx := 0     end if     %The collision check     if dy <= bottom or (View.WhatDotColour (dx - 20, dy - 15) = 1 and View.WhatDotColour (dx, dy - 15) = 1 and View.WhatDotColour (dx + 20, dy - 15) = 1) or (View.WhatDotColour (dx - 20, dy             - 15) = 1) then         vely := jump     end if     % constantly subtracting gravity from the dy position     vely -= grav     dx += round (velx)     dy += round (vely)     %Draws platforms     for i : 1 .. num         if platforms (i).platx < 0 then             platforms (i).speed *= -1         elsif platforms (i).platx + 100 > maxx then             platforms (i).speed *= -1         end if         platforms (i).platx += platforms (i).speed         drawfillbox (round (platforms (i).platx), round (platforms (i).platy), round (platforms (i).platx + 100), round (platforms (i).platy + 20), 1)     end for     % collision check, keeps the player above the ground     if dy < bottom then         vely := 0     end if     %Draws player     drawfillbox (dx - 30, dy - 10, dx + 30, dy + 30, green)     Draw.ThickLine (dx - 20, dy - 8, dx - 20, dy - 15, 5, black)     Draw.ThickLine (dx, dy - 8, dx, dy - 15, 5, black)     Draw.ThickLine (dx + 20, dy - 8, dx + 20, dy - 15, 5, black)     %Draws the bottom, easier to see for testing     drawline (0, bottom, maxx, bottom, 1)     View.Update     delay (30)     cls end loop

[landfarm]

Ok, I put the collision check for the platforms inside the for loop like this. The problem is that the player will sometimes drop through the platforms. What can I do to fix this? And I'm sorry from jumping from problem to problem.

Turing:

%Draws platforms     for i : 1 .. num         if platforms (i).platx < 0 then             platforms (i).speed *= -1         elsif platforms (i).platx + 100 > maxx then             platforms (i).speed *= -1         end if         platforms (i).platx += platforms (i).speed         drawfillbox (round (platforms (i).platx), round (platforms (i).platy), round (platforms (i).platx + 100), round (platforms (i).platy + 20), 1)         %Checks if the player hits any platforms         if View.WhatDotColour (dx - 20, dy - 15) = 1 or View.WhatDotColour (dx, dy - 15) = 1 or View.WhatDotColour (dx + 20, dy - 15) = 1 or (View.WhatDotColour (dx - 20, dy - 15) = 1 and             View.WhatDotColour (dx, dy - 15) = 1 and View.WhatDotColour (dx + 20, dy - 15) = 1) then             vely := jump         end if     end for

[Tony]

I like how you posted just the relevant part of the code

But... why is that inside a for-loop? And, for bonus points (read as: "required question"), why was the array-based collision outside of for-loop originally?

[TheGuardian001]

My assumption would be that your character is simply moving too fast. When you change the characters position, it doesn't hit every point in between where it was and where it will be. This means if it moves fast enough, your character will pass right by the platform without ever appearing to hit it.

you will either have to limit the characters downward speed, or find a way to check farther below the player if they get above a certain speed.

[landfarm]

@ Tony, I just thought if it right now, and it's in the for loop because it doesn't seem to work when it's outside of it. Very strange.
Thanks Guardian, I just made the platforms longer so it shouldn't have that problem.

[Tony]

But the if-statement doesn't even rely on the value of i.

[landfarm]

I'm not sure, but it doesn't work if it's outside the for loop. Doesn't the i represent each platform though?

[TheGuardian001]

landfarm wrote:

I'm not sure, but it doesn't work if it's outside the for loop. Doesn't the i represent each platform though?

Yes, i does represent that. But you aren't ever using i in your collision detection, since you're using whatdotcolour. The computer doesn't care which block dy-15 belongs to, it just cares if it's blue.

Lets walk through your main loop before you put the collisions inside:

code:

loop start     check for input         respond to input         check for colours     for loop         draw colours         clear screen loop end

Can you see anything wrong with where you're checking the colours in relation to where you're drawing the colours?

[landfarm]

Oh, I see. The check is before the for loop actually draws them. There shouldn't be a problem though if the boxes are thick enough though? And also, out of a question of laziness, is it okay if I just the colour check inside the for loop?

[TheGuardian001]

landfarm wrote:

Oh, I see. The check is before the for loop actually draws them.

Exactly.

landfarm wrote:

There shouldn't be a problem though if the boxes are thick enough though?

The thickness of the box is irrelevant. if you check before you draw them, there won't be anything to check, regardless of how thick they will be when you actually draw them.

landfarm wrote:

And also, out of a question of laziness, is it okay if I just the colour check inside the for loop?

For the sake of you not getting into bad practices, No, it's not okay. All you need to do is move it outside of the loop, it's really not that much trouble.
Something like this might not present much of a problem in this specific program, but what if you had 10 platforms to draw? what if you had 100? whatdotcolour itself is a relatively costly procedure to call, and if you're calling it repeatedly in a for loop, there's a good chance you'll see some performance issues.

[landfarm]

TheGuardian001 @ Sat Dec 19, 2009 2:30 pm wrote:

My assumption would be that your character is simply moving too fast. When you change the characters position, it doesn't hit every point in between where it was and where it will be. This means if it moves fast enough, your character will pass right by the platform without ever appearing to hit it.

you will either have to limit the characters downward speed, or find a way to check farther below the player if they get above a certain speed.

That's why I increased the thickness of the platforms. Thanks for your help though. I'm gonna fix it right now.