preventing a box from leaving the screen

[Solar_Projectile]

code:

%Declaration section var startpointx, startpointy : int := 0 var endpointx, endpointy : int := 50 var key : string (1) %set screen mode and size setscreen ("graphics:400;400") setscreen ("noecho") setscreen ("nocursor") setscreen ("offscreenonly") procedure display     drawfillbox (startpointx, startpointy, endpointx, endpointy, white)     drawfillbox (startpointx + 1, startpointy + 1, endpointx - 1, endpointy - 1, blue)     getch (key)     if key = chr (200) then         startpointy := startpointy + 1         endpointy := endpointy + 1     elsif key = chr (208) then         startpointy := startpointy - 1         endpointy := endpointy - 1     elsif key = chr (205) then         startpointx := startpointx + 1         endpointx := endpointx + 1     else         if key = chr (203) then             startpointx := startpointx - 1             endpointx := endpointx - 1         end if     end if     View.Update end display %Main Program loop     display     exit when key = chr (27) end loop

my question is this: how do i modify the code so that the box doesn't go outside the size of the screen or distort?

if anybody could help, id be very thankful.

[Solar_Projectile]

I think i may have found a solution on my own, but i dont know if this is the proper/best way to do it

code:

%Declaration section var startpointx, startpointy : int := 0 var endpointx, endpointy : int := 50 var key : string (1) var reply : string (1) %set screen mode and size setscreen ("graphics:400;400") setscreen ("noecho") setscreen ("nocursor") setscreen ("offscreenonly") procedure display     drawfillbox (startpointx, startpointy, endpointx, endpointy, white)     drawfillbox (startpointx + 1, startpointy + 1, endpointx - 1, endpointy - 1, blue)     getch (key)     if startpointx = 0 then         startpointx := startpointx + 1         endpointx := endpointx + 1     elsif startpointy = 0 then         startpointy := startpointy + 1         endpointy := endpointy + 1     elsif endpointx = 400 then         startpointx := startpointx - 1         endpointx := endpointx - 1     elsif endpointy = 400 then         startpointy := startpointy - 1         endpointy := endpointy - 1     elsif key = chr (200) then         startpointy := startpointy + 1         endpointy := endpointy + 1     elsif key = chr (208) then         startpointy := startpointy - 1         endpointy := endpointy - 1     elsif key = chr (205) then         startpointx := startpointx + 1         endpointx := endpointx + 1     else         if key = chr (203) then             startpointx := startpointx - 1             endpointx := endpointx - 1         end if     end if     View.Update end display %Main Program loop     display     exit when key = chr (27) end loop

[Cervantes]

Pretty much, though there's no need to keep two variables (start and end) when you could just have end depend on start. endx is always = startx + width, and endy is always = starty + height. width and height are constants.