jump with gravity

[Cervantes]

Question: why are you using such strange and confusing variable names? You're using z to store the second ball's x position, and you're using x1 to store the second ball's y position. This isn't three dimensional.

Some general coding improvements:

You only need to use Input.KeyDown once inside your loop. You don't need two different arrays (char of boolean) to search for input from two different keys on the keyboard. Rather, just use one array char of boolean and just use one Input.KeyDown. Use if statements to determine if the key you are questioning is being pressed.
Try to keep all your drawing commands together. That means only have one cls; follow that by drawing your two ovals (though when the program is corrected you'll only have one ball); follow that by your delays.

I'm not sure if you've updated your code since Martin's post (I don't see a message edited by note, but I often don't see those when people have edited their posts. Dan?) but I think you may have, since you are using vx.

Right now, you've kinda got a program that simulates a person jumpin off of a black hole. The black hole is located at (maxx / 2, 100). Regardless of which direction the person jumps in, he is always pulled back to the black hole. If you are trying to make a program like what new was trying to do, you want to be on Earth. For the purposes of the program, we will assume that Earth is flat and that gravity pulls everything directly downwards with a constant acceleration regardless of location.

When you're on the surface of the Earth, you can move north, south, east, and west. For the purposes of this two-dimensional program, let's just say we can move east and west. We can, however, also jump up into the air. When we do this, we jump off the surface at a specific velocity. Gravity pulls back at us (it causes us to accelerate downwards).

Let's take a look at some code. It uses only one Input.KeyDown and only one section of drawing commands. It allows the player to jump and to walk around on the surface of the Earth.

code:

setscreen ("graphics:400;300,offscreenonly") const gravity := 0.1 var x, y, vy, vx : real x := maxx / 2 y := 10 var jumping := false var keys : array char of boolean loop     Input.KeyDown (keys)     if keys (KEY_UP_ARROW) and ~jumping then         jumping := true         vy := 5     end if     if keys (KEY_RIGHT_ARROW) then         x += 1     end if     if keys (KEY_LEFT_ARROW) then         x -= 1     end if     if jumping then         vy -= gravity         y += vy     end if     if y < 10 then         y := 10         jumping := false     end if     cls     drawfilloval (round (x), round (y), 10, 10, black)     View.Update     delay (5) end loop

Now, the only problem with this is that the ball moves instantly. ie. there is no acceleration. It's velocity is instantly set. For some types of games, this effect can be desired. But let's explore acceleration in the x plane.

It's essentially the same as jumping. Let's look at some more code:

code:

setscreen ("graphics:400;300,offscreenonly") const gravity := 0.1 var x, y, vy, vx : real x := maxx / 2 y := 10 vx := 0 var jumping := false var keys : array char of boolean loop     Input.KeyDown (keys)     if keys (KEY_UP_ARROW) and ~jumping then         jumping := true         vy := 5     end if     if keys (KEY_RIGHT_ARROW) then         vx += 0.1     end if     if keys (KEY_LEFT_ARROW) then         vx -= 0.1     end if     if jumping then         vy -= gravity         y += vy     end if     if y < 10 then         y := 10         jumping := false     end if     x += vx     cls     drawfilloval (round (x), round (y), 10, 10, black)     View.Update     delay (5) end loop

note that the ~jumping is the same as not jumping which is the same as jumping = false.

Simple enough, I hope. However, There are two problems with this. First, the ball rolls with the same volcity forever, unless the right / left arrow keys are pressed. We're not travelling in empty space here, so we'll add some friction to the object. To do this, create a constant for friction and assign it a value of 0.99. Then, just before (or after, it doesn't matter too much) you add vx to x, multiply vx by friction. However, since friction when jumping through the air is negligable, we need an if statement to make sure that the player is on the ground. Like this:

code:

const Friction := 0.99 %---------- x += vx if ~jumping then     vx *= Friction end if

Good enough. The second problem is that the player can change directions in mid-air. Once again, this can be a desired feature of some games, but let's explore how to fix it so it's more like the real world.

To fix it, wrap an if statement around the keys (KEY_RIGHT_ARROW) and keys (KEY_LEFT_ARROW) if statements. The outer if statement should make sure that the ball is not jumping. If he's not, then it will enter the if statement and allow horizontal movement.
Like this:

code:

if ~jumping then         if keys (KEY_RIGHT_ARROW) then             vx += 0.1         end if         if keys (KEY_LEFT_ARROW) then             vx -= 0.1         end if     end if

So, let's look at our final code:

code:

setscreen ("graphics:400;300,offscreenonly") const gravity := 0.1 const Friction := 0.99 var x, y, vy, vx : real x := maxx / 2 y := 10 vx := 0 var jumping := false var keys : array char of boolean loop     Input.KeyDown (keys)     if keys (KEY_UP_ARROW) and ~jumping then         jumping := true         vy := 5     end if     if ~jumping then         if keys (KEY_RIGHT_ARROW) then             vx += 0.1         end if         if keys (KEY_LEFT_ARROW) then             vx -= 0.1         end if     end if     if jumping then         vy -= gravity         y += vy     end if     if y < 10 then         y := 10         jumping := false     end if     x += vx     if ~jumping then         vx *= Friction     end if     cls     drawfilloval (round (x), round (y), 10, 10, black)     View.Update     delay (5) end loop

And there you have it.

It should be noted, however, that a Mario style game, where the player is a man and is walking / running around, would probably benefit most from immediate acceleration (ie. don't use vx). A racing game, on the other hand, where the player drives a car, would benefit most from using acceleration (ie. using vx). Mind you, it would be a minghty strange racing game, given that we're seeing it from the side and that the car can jump into the air .

-Cervantes

[MihaiG]

heres the code for boundries..

code:

if x < 15 then         x := 15     end if     if x > maxx - 15 then         x := maxx - 15     end if

put it before the cls
draw circle... :-)otherwise it will be slow

[new]

hi guys..thx for your help.....
i 'm adding this jump to ma game mario...in that game...how to make :when he jumps under a block and when he hit it..he sud fall back and not go up any more using wat dot colour..? any ideas??

i'm using this code for the jump part


setscreen ("offscreenonly,graphics:600;350")
var jumping := false
var up : int := 0
const gravity := .15
var value : real
var x, y : real := 33
var chars : array char of boolean

loop

Input.KeyDown (chars)

if chars (KEY_UP_ARROW) and y <= 40 then
up := 1
end if

if up = 1 then

jumping := true
value := 7
end if
if jumping then
value := value - gravity
y := y + value
up := 0
end if
Draw.FillBox (round (x), round (y), round (x + 10), round (y + 10), yellow)
View.Update
end loop

[Cervantes]

I explained it all in your Mario thread when I linked to a past help topic about jumping and whatdotcolour collision detection. Have you read through that topic?

edit: code tags, please