Checkbox does not work! Help please!

My code does not work! :(/>/>
(Checkbox)

-- Checkbox does not work!
-- Help please
term.clear()
term.setCursorPos(1,1)
term.setTextColor(colors.yellow)
print("Checkbox does not work")
term.setCursorPos(1,2)
print("Help please")
term.setCursorPos(40,1)
print("Toggle --> X")
term.setTextColor(colors.white)


---------------- Code --------------------
while true do
local event, button, X, Y = os.pullEvent("mouse_click")
XY = X..","..Y

local V = -5
local Z = 0
if XY == "51,1" and button == 1 then
if Z > V then
V = V + 10
term.clear()
term.setCursorPos(1,1)
print("+")
term.setCursorPos(51,1)
print("X")

end
end
if XY == "51,1" and button == 1 then
if Z < V then
V = V - 10
term.clear()
term.setCursorPos(1,1)
print("-")

term.setCursorPos(51,1)
print("X")

end
end
end

Help please!

Ok, hold on.
You need an endif in there, and you can remove one if.

Corrected

-- Checkbox does not work!
-- Help please

-- We will define V outside of the while loop so that it doesn't keep getting set each time the code loops.
-- That's partly why it wasn't changing: You added 10, but as soon as the while loop started over again, you had
-- "local V = -1", resetting it BACK to -5. Oh and that's aside from the fact that it was a local variable.
local V = -5

term.clear()
term.setCursorPos(1,1)
term.setTextColor(colors.yellow)
print("Checkbox does not work")
term.setCursorPos(1,2)
print("Help please")
term.setCursorPos(40,1)
print("Toggle --> X")
term.setTextColor(colors.white)


---------------- Code --------------------
while true do
local event, button, X, Y = os.pullEvent("mouse_click")
XY = X..","..Y

local Z = 0
if XY == "51,1" and button == 1 then
if Z > V then
V = V + 10
term.clear()
term.setCursorPos(1,1)
print("+")
term.setCursorPos(51,1)
print("X")

elseif Z < V then

V = V - 10
term.clear()
term.setCursorPos(1,1)
print("-")

term.setCursorPos(51,1)
print("X")

end
end
end

Note: I have not made any other changes to make it simpler or anything of the sort. I've only changed it enough to make it work.

You should do to string(X)..","..tostring(Y)

Also it would be better if you do checks seperately without converting them to a string
Ex: if X == 51 and Y == 1 then