$($define BALLS $\text{'}())$ ; inital empty list of balls

;To make multiple balls move, use map the next$-$ball function over all balls

;then to get their next positions accordingly.

$($define $($ListOfBall balls$)$ ; how this balls relate to b below???

$($cond

$[$null$?$ balls$]$

$[$else $($cons $($ListOfBall balls$))]))$

;create a first$-$ball to start the problem

$($define first$-$ball $($ListOfBall $($make$-$ball$(3002504-3))))$

;Check if ball near edges

$($define $($touch$-$top? b$)$

$(<=(+($ball$-$y b$)($ball$-$dy b$))$ TOP$))$

$($define $($touch$-$bot? b$)$

$(>=(+($ball$-$y b$)($ball$-$dy b$))$ BOT$))$

$($define $($touch$-$rig? b$)$

$(>=(+($ball$-$x b$)($ball$-$dx b$))$ RIG$))$

$($define $($touch$-$lef? b$)$

$(<=(+($ball$-$x b$)($ball$-$dx b$))$ LEF$))$

;Bounce the ball of the edges

$($define $($bounce$-$top b$)($make$-$ball $($ball$-$x b$)(+$ TOP $1)($ball$-$dx b$)(-($ball$-$dy b$))))$

$($define $($bounce$-$bot b$)($make$-$ball $($ball$-$x b$)(-$ BOT $1)($ball$-$dx b$)(-($ball$-$dy b$))))$

$($define $($bounce$-$lef b$)($make$-$ball $(+$ LEF $1)($ball$-$y b$)(-($ball$-$dx b$))($ball$-$dy b$)))$

$($define $($bounce$-$rig b$)($make$-$ball $(-$ RIG $1)($ball$-$y b$)(-($ball$-$dx b$))($ball$-$dy b$)))$

;Move the ball

$($define $($glide b$)($make$-$ball $(+($ball$-$x b$)($ball$-$dx b$))(+($ball$-$y b$)($ball$-$dy b$))($ball$-$dx b$)($ball$-$dy b$)))$

;Handle mouse click to creat a new ball

$($define $($handle$-$mouse b x y me$)$ ;how does this b relate to balls below?

$($cond $[($equal$?$ me "button$-$down"$)$

$($make$-$ball x y $(-5($random $11))(-5($random $11)))]$

$[$else b$]))$

; handle$-$mouse adds a new ball on mouse click

$($define $($handle$-$mouse balls x y event$)$

$($cond $[($mouse$=?$ event "button$-$down"$)$

$($cons $($make$-$ball x y $(-5($random $11))(-5($random $11)))$

balls$)]$

$[$else balls$]))$

;To handle key event, when the space bar is pressed, it will make the ListOfBall empty

$($define $($handle$-$key lob key$)$ ; how does this lob relate to balls below?

$($cond $[($key$=?$ key $"")$

$\text{'}()]$

$[$else lob$]))$

; handle$-$key clears all balls on space press

$($define $($handle$-$key balls key$)$

$($cond $[($key$=?$ key $"")$

$\text{'}()]$

$[$else balls$]))$

;Render the ball on scene with green background

$($define $($render$-$ball b$)$ ; how does this b relate to lob below?

$($place$-$image BALL $($ball$-$x b$)($ball$-$y b$)($empty$-$scene WIDTH HEIGHT "green"$)))$

;render$-$balls to render image of multiple balls, use map the render$-$ball function over all balls

;then place them on the background image.

$($define $($render$-$balls balls$)$

$($foldr $($lambda $($ball acc$)$ ;what is difference ball vs balls??

$($place$-$image BALL $($ball$-$x ball$)($ball$-$y ball$)$ acc$))$ ;How does acc do in this foldrprocedure??

MTS

lob$))$

; next$-$balls produces the next list of balls, moving each one

$($define $($next$-$balls balls$)$ ; how does this balls relate to b below?

$($map next$-$ball balls$))$ ; what does procedure of map doing??

;Determine the next state of the ball using current position

$($define $($next$-$ball b$)$

$($cond

$[($touch$-$top? b$)($bounce$-$top b$)]$

$[($touch$-$bot? b$)($bounce$-$bot b$)]$

$[($touch$-$lef? b$)($bounce$-$lef b$)]$

$[($touch$-$rig? b$)($bounce$-$rig b$)]$

$[$else $($glide b$)]))$

;Main program start here use B$1$ to initialize

$($big$-$bang B$1$ ; how to convert B$1$ to first ball below??

$($on$-$tick next$-$ball$)$

$($on$-$draw render$-$ball$)$

$($on$-$key handle$-$key$)$

$($on$-$mouse handle$-$mouse$))$

; main remains the same but now uses BALLS as the initial world state

$($define $($main first$-$ball$)$ ;use first$-$ball to start the program.

$($big$-$bang first$-$ball

$($on$-$draw render$-$balls$)$ ;ListOfBall $->$ Image

$($on$-$tick next$-$balls$)$ ;ListOfBall $->$ ListOfBall

$($on$-$mouse handle$-$mouse$)$ ;ListOfBall Integer Integer MouseEvent $->$ ListOfBall

$($on$-$key handle$-$key$)))$ ;ListOfBall KeyEvent $->$ ListOfBall ;questions:

;$1)$ how does lob relate to balls?

;$2)$ what are difference of lob vs ball vs balls?

;$3)$ How does acc do in this foldrprocedure??

;$4)$ what does map in procedure next$-$balls doing??