How do I keep the speed of items in a table constant and make them stop moving after I release a button?

Question

I am following a shoot-em-up game tutorial,and I ran into this problem where I want the speed of the stars go faster when I press the up button while they keep their separate colors. Also, the stars should slow down after I release the button.Right now the game starts out fine, with all the stars having correct color, but once the stars start moving faster, they lose their color. ALso, I am unable to slow down the stars after I press the up button.

function _init()

    --variables
    x=63
    y=110
    xv=1.5
    yv=1.5
    bulx=63
    buly=110
    bulv=0
    bullet_moving = 0 
    ship_sprite=2
    exhaust_sprite=5
    muzzle_flash=0
    score=0
    lives=3
    bombs=3
     
    starx={}
    stary={}
    starspd={}
    for i=1,100 do
        add(starx,rnd(127))
        add(stary,rnd(127))
        add(starspd,rnd(1.2))
 end
 
 starspd_init={}
 
 for i=1,#starspd do
        add(starspd_init,starspd[i])
 end
 
end

function _update()
    
    --player movement
    
    if btn(0) and not btn(2) and not btn(3) then 
        ship_sprite=1
        if x>40 then
            x=x-xv
        end
            
        for i=1,#starx do
            if starx[i]<128 then
                starx[i]=starx[i]+starspd[i]
            else
                starx[i]=starx[i]-128
            end
        end
        
 end
    
    if btn(1) and not btn(2) and not btn(3) then 
        ship_sprite=3
        if x<86 then
            x=x+xv
        end
        
        for i=1,#starx do
            if starx[i]>0 then
                starx[i]=starx[i]-starspd[i]
            else
                starx[i]=starx[i]+128
            end
        end
        
 end
 
 if not btn(1) and not btn(0) then
    ship_sprite=2
    end
    
    if btn(2) then 
        for i=1,#starspd do
            starspd[i]=2
        end
    else
        for i=1,#starspd do
            if not starspd[i]==starspd_init[i] then
                for i=1,#starspd do
                    starspd[i]=starspd_init[i]
                end
            end
        end
    end
    
    if btn(3) then
        print("debug")
        for i=1,#starspd do
            if not starspd[i]==starspd_init[i] then
                    starspd[i]=starspd_init[i]
            end
        end
    end
    --player loop
    
    if x<-5 then 
        x=128
    end
    
    if x>132 then 
        x=-1
    end
    
    --shooting
    if btnp(5) and bullet_moving==0 then
        sfx (0)
        muzzle_flash=4
    end 
    
    if btnp(5) or bullet_moving==1 then
        buly=buly-2
        bullet_moving=1
    else
        bulx=x
        buly=y+1
    end
    
    if buly<-5 then 
        bullet_moving=0
        bulx=x
        buly=y+1
    end
    
    if muzzle_flash>0 then
        muzzle_flash=muzzle_flash-1
    end
    
    --ship_exhaust
    if exhaust_sprite>8 then
            exhaust_sprite=5
        elseif btn(3) then
            exhaust_sprite=4
        else
            exhaust_sprite=exhaust_sprite+1
    end

end

--background 

function background()

    for i=1,#starx do
        local star_opacity=6
        if starspd[i]<0.4 then
            star_opacity=1
        elseif starspd[i]<0.7 then
            star_opacity=13
        elseif starspd[i]<0.73 then
            star_opacity=10
        end
        pset(starx[i],stary[i],star_opacity)
    end

end

function _draw()
    
    cls(0)
    
    background()
    
    spr(16,bulx,buly)
    
    spr(ship_sprite,x,y)
    
    spr(exhaust_sprite, x, y+6)
    
    if muzzle_flash>0 then
        circfill(x+3,y+2,muzzle_flash,7)
    end
    
    print("score: "..score,46,3)
    
    spr(13,1,1)
    
    for i=1,3 do
        if lives>=i then
            spr(11,i*9-6,2)
        else
            spr(12,i*9-6,2)
        end
    end
    
    for i=1,3 do
        if bombs>=i then
            spr(14,i*9+90,2)
        else
            spr(15,i*9+90,2)
        end
    end
    
    for i=1,#stary do
            if stary[i]<128 then
                stary[i]=stary[i]+starspd[i]
            else
                stary[i]=stary[i]-128
            end
        end
    
end

Answer 1

In the way the background() function is written, the stars change color depending on how fast they move. The star_opacity is the defined color the stars will get.

So if you speed up the stars, they'll all move faster, likely all having the star_opacity=1 at the end.

And in the section where it should revert the star speed back to it's initial value, I've noticed you're doing the for loop twice there (for i=1,#starspd do). I think removing the second for-loop would work. like this:

if btn(2) then 
    for i=1,#starspd do
        starspd[i]=2
    end
else
    for i=1,#starspd do
        if not starspd[i]==starspd_init[i] then
            starspd[i]=starspd_init[i]
        end
    end
end

This is unrelated, but personally, I would put the stars in a seperate object in a new tab, so each star holds their own values and color. And it would also make the main script more readable. But for now, I'd suggest to keep sticking to the tutorial. Maybe they'll talk more about that later on.