Enemy Movement - GameCreators Forum

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Darth Gazak

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Joined: 13th Jan 2009

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Posted: 29th Jan 2009 11:26

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Hello again,

I managed to overcome my main program loop problem, and spawn ten enemies along the right edge of the screen at random y coordinates (Thanks to BN2 Productions and Ashingda for their assistance)

However, i am now having trouble making the enemy sprites move. I use a loop to spawn the sprites (numbered from 2 - 11, as sprite number 1 is the player ship) yet when i try and make them move using another loop (to correspond with the numbering) i get an error, stating 'BOB DOES NOT EXIST AT LINE 115'. I know this is something to do with the numbering being inaccurate, and DB trying to move a sprite which doesn't exist. The code will show it better:

+ Code Snippet

   load image "h:\projectmedia\drone.bmp",5
   flag=0
   enemy_speed=1
   current_enemy=2

DO

REM to spawn creatures every 50 iterations
   IF flag=0
      i=0
      GOSUB Spawn_Enemies
      flag=flag+1
   ENDIF

   FOR movement=2 to current_enemy
         SPRITE movement,SPRITE X(movement)-enemy_speed,Sprite Y(movement),5
      ENDIF
   NEXT movement

LOOP

Spawn_Enemies:
REM Spawns 10 enemies
   Y=RND(600)
   X=700
   FOR i=1 TO 10
     sprite current_enemy,X,Y,5
     INC current_enemy
   NEXT i
RETURN

I hope you can understand what i am trying to achieve here. Thanks in advance for any help you can offer.

Darth Gazak

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Ashingda 27

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Posted: 29th Jan 2009 15:05 Edited at: 29th Jan 2009 15:09

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The value of current_enemy is too large. First it's asigned = 2 then it's inc in a loop of 10. When looping to move enemies, it is 12 and that is 2 more than there are sprites.

Remember to reset your increasing values. Inside the Spawn_Enemies subroutine:

+ Code Snippet

REM Spawns 10 enemies
   Y=RND(600)
   X=700

   current_enemy = 0

   FOR i=1 TO 10
     sprite current_enemy,X,Y,5
     INC current_enemy
   NEXT i

You seem to have an ENDIF out of place?

+ Code Snippet

   FOR movement=2 to current_enemy
         SPRITE movement,SPRITE X(movement)-enemy_speed,Sprite Y(movement),5
      ENDIF <---
   NEXT movement

It might be that you may have had a check there befor it was removed. Anyhow a IF SPRITE EXIST() would be good there as well to check if the sprite exist befor attempting to move it and not being sure if it still exist, it might have been shot and died.

Another thing, the 2 should be 1, it's best to loop through the same amount of times while moving as it took to spawn it. current_enemy can also simply be 10 or a variable that holds the max for both loops such as Max_Enemy for example.

+ Code Snippet

   FOR movement=1 to current_enemy
      If SPRITE EXIST(movement)
         SPRITE movement,SPRITE X(movement)-enemy_speed,Sprite Y(movement),5
      ENDIF
   NEXT movement

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Darth Gazak

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Joined: 13th Jan 2009

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Posted: 30th Jan 2009 12:29

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Hello,
Thanks for your help, i took what you said into account and implemented it into my program, yet I still have a problem; none of the enemy sprites appear on the screen, never mind moving! I realise this may be because i haven't used the sync command, but i'm still unsure about how to use it correctly.

Here's the code, it is error free but doesn't display the enemy sprites:

cls
   stop music 2
   load bitmap "h:\projectmedia\approach.bmp"

load music "h:\projectmedia\theme.mid",1
   play music 1
   load image "h:\projectmedia\grad2.bmp", 1
   sprite 19,50,300,1

spritewidth=(sprite width(19)/2)
   spriteheight=(sprite height(19)/2)
   offset sprite 19,spritewidth,spriteheight

load image "h:\projectmedia\drone.bmp",5
   flag=0
   enemy_speed=1

REM main program loop

DO

if mouseclick()=0
      targetacross=mousex()
      targetdown=mousey()
   endif
   if mouseclick()=1
      targetacross=mousex()
      targetdown=mousey()
   endif
   if mouseclick()=2
      targetacross=mousex()
      targetdown=mousey()
   endif

if mouseclick()=3
      targetacross=mousex()
      targetdown=mousey()
   endif

if acrossscreen<targetacross:acrossscreen=acrossscreen+1:endif
   if acrossscreen>targetacross:acrossscreen=acrossscreen-1:endif
   if downscreen<targetdown:downscreen=downscreen+1:endif
   if downscreen>targetdown:downscreen=downscreen-1:endif

sprite 19,acrossscreen,downscreen,1

REM to spawn creatures every 50 iterations
   IF flag=0
      i=0
      GOSUB Spawn_Enemies
      flag=flag+1
   ENDIF

FOR movement=1 to current_enemy
      If SPRITE EXIST(movement)
         SPRITE movement,SPRITE X(movement)-enemy_speed,Sprite Y(movement),5
      ENDIF
   NEXT movement

LOOP

Spawn_Enemies:
REM Spawns 10 enemies
   Y=RND(600)
   X=700

current_enemy=1

FOR i=1 TO 10
     sprite current_enemy,X,Y,5
     INC current_enemy
   NEXT i
RETURN

+ Code Snippet

   cls
   stop music 2
   load bitmap "h:\projectmedia\approach.bmp"

   load music "h:\projectmedia\theme.mid",1
   play music 1
   load image "h:\projectmedia\grad2.bmp", 1
   sprite 19,50,300,1

   spritewidth=(sprite width(19)/2)
   spriteheight=(sprite height(19)/2)
   offset sprite 19,spritewidth,spriteheight

   load image "h:\projectmedia\drone.bmp",5
   flag=0
   enemy_speed=1

REM main program loop

DO

   if mouseclick()=0
      targetacross=mousex()
      targetdown=mousey()
   endif
   if mouseclick()=1
      targetacross=mousex()
      targetdown=mousey()
   endif
   if mouseclick()=2
      targetacross=mousex()
      targetdown=mousey()
   endif

   if mouseclick()=3
      targetacross=mousex()
      targetdown=mousey()
   endif

   if acrossscreen<targetacross:acrossscreen=acrossscreen+1:endif
   if acrossscreen>targetacross:acrossscreen=acrossscreen-1:endif
   if downscreen<targetdown:downscreen=downscreen+1:endif
   if downscreen>targetdown:downscreen=downscreen-1:endif

   sprite 19,acrossscreen,downscreen,1

REM to spawn creatures every 50 iterations
   IF flag=0
      i=0
      GOSUB Spawn_Enemies
      flag=flag+1
   ENDIF


   FOR movement=1 to current_enemy
      If SPRITE EXIST(movement)
         SPRITE movement,SPRITE X(movement)-enemy_speed,Sprite Y(movement),5
      ENDIF
   NEXT movement

LOOP

Spawn_Enemies:
REM Spawns 10 enemies
   Y=RND(600)
   X=700

   current_enemy=1

   FOR i=1 TO 10
     sprite current_enemy,X,Y,5
     INC current_enemy
   NEXT i
RETURN

Could someone try and solve the problem, or help me implement the sync command correctly.

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Caleb1994

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Joined: 10th Oct 2008

Location: The Internet you idiot!

Posted: 31st Jan 2009 03:48

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in your original thing the reason you were getting that error was you had it inc the current_Enemy variable after it makes the sprite every time witch means that all you have to do to make the old one work is instead of inc before you call the sprite command

so the starting value of current_Enemy would need to be one instead of 2 or make it zero for your second variation of the code.

then you wouldn't get that error.

the reason it's doing that is because when it makes the last sprite it inc again so that number is 2 extra then how many are out there because that equals 12 when moving you encounter problems because your using 12 as the number of sprites. though sprite exist command does solve thta and other problems when you add other sprites that are past the sprite number of the enemies

and when you moved sprites you start with sprite one which you said was the player so I'm not sure how that works

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Ashingda 27

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Posted: 31st Jan 2009 05:31 Edited at: 31st Jan 2009 05:41

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In your Spawn routine you can use the variable in the FOR..Next statement instead of current_enemy.

+ Code Snippet

for i = 1 to 10
  sprite i,x,y,5
next i

At the end of that loop, the variable i will be 10 thus going from 1 to 10. You can then simply current_enemy = i.

While setting a random value for the Y, it will assign Y a random value only one time. So if you create 10 enemies after Y=RND(x) then all the enemies will have the same Y. You must put Y inside the For..Next loop to assign a new random Y for each enemy.

+ Code Snippet

for i = 1 to 10
  X = 600
  Y = RND(480)
  sprite i,x,y,5
next i

The default display mode will be set to 640x480 16bit if you dont manually set it yourself. Your previous value of Y=RND(600) X=700 was too high.

While moving the sprites you can use the same For..Next as when you spawn them.

+ Code Snippet

   FOR i = 1 to 10
      If SPRITE EXIST(i)
         SPRITE i,SPRITE X(i)-enemy_speed,Sprite Y(i),5
      ENDIF
   NEXT i

You do not need to reset a variable befor using it in a For..Next. That statement will set and change the variable.

To have your Grand2 have the same x,y position as the mouse it's easier to do it this way.

+ Code Snippet

	mx = mousex()
	my = mousey()
	mc = mouseclick()

   sprite 19,mx,my,1

Note that you can also use the mouse functions directly:

+ Code Snippet

	mc = mouseclick()

   sprite 19,mousex(),mousey(),1

To sync properly make sure you have this at or near the top of the page:

+ Code Snippet

sync on
sync rate 0

Then in your main loop have it right befor you loop. You may also want a cls at the very end or very beginning.

+ Code Snippet

Do
  cls <-- can go here

  `**Code Stuff**
  `**Code Stuff**

  sync
  cls <-- or go here
Loop

Lastly you might want to go here. Could save you alot of time and headaches.
http://forum.thegamecreators.com/?m=forum_view&t=99497&b=10

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Darth Gazak

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Joined: 13th Jan 2009

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Posted: 5th Feb 2009 11:06

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I cannot thank you enough for your help Ashingda, you have truly helped me out of a hole here.

After a bit of fiddling, i managed to implement the sync command. I couldn't get my background to appear, but that isn't a problem, i just removed it and it worked perfectly. Now 10 enemy sprites spawn at random Y coordinates (albeit they overlap, but i will look at your previous suggestions on ways to counter this) and move across the screen. I also used your mouse control advice, which solved the problem of sluggish movement. Genius!

These are the things that remain to be implemented:

1. Non overlapping enemy sprites
2. Sprites spawning in waves
3. Collision detection
4. Timer to derive a score from

Thanks again for your assistance, i won't hesitate to post again if i encounter problems in any of these remaining tasks.

Thank you!
Darth Gazak

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PackRat

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Location: U.S.

Posted: 14th Feb 2009 11:55

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@Darth Gazak

You can use make object plain object number,width,height and texture it with your background image then adjust it to the camera. Doing this gives you some pretty cool options for level changes, etc. (e.g. rolling the completed level out of view then texturing it with your next level background and rolling it back into view).

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DarkBASIC Discussion / Enemy Movement