Can you pass a variable through to an API?

Or are there better ways to handle this:

function tryForward_track(fd)
	while not turtle.forward() do
		if turtle.detect() then
			turtle.dig()
		end
	end
	fd = fd + 1
end

Is it possible to take a local variable - pass it to a function (in an API)?

Basically the above function is in my API; and my program calls that to keep track of how far it's moving forward - but I get the error: "attempt to perform arithmetic function __add on nil and number"

Moved to Ask a Pro.

Can you post the code that uses your API function? The error message states that 'fd' is nil.

I'm pretty sure numbers are passed byval, not byref, which would mean that:

local something = 123
tryForward_track(something)
print(something) -- will print 123

Of course, this is not the cause of the problem. I just wanted to say that you will have to add a return including fd, and then allocate the return to the base variable again, like this:

local something = 123
something = tryForward_track(something)
print(something) -- will print 124

For your first problem, why the arithmetical error ocurrs, you have to post the whole code.

Sorry Lyqyd - wasn't paying attention to what forum I was in! :(/>/>

I actually just wiped out the code and went a different direction - but the line in my program calling that function was simply:

fd = tryForward_track(fd)

the very first line of the code was:

local fd = 0

I did do a work around tho; I simply wrote a small function called 'tryForward_track' - and then said 'fd=fd+1' and 'tryForward()'; it works out the same… But I'd like to know what I did wrong in that code above to get the error…

Your code above will cause the error if you use it the second time. You forgot to return fd

function tryForward_track(fd)
		while not turtle.forward() do
				if turtle.detect() then
						turtle.dig()
				end
		end
		fd = fd + 1
		return fd
end

Another possible way: If tryForward_track is in the same file as fd variable, you could just remove the fd argument

function tryForward_track()
		while not turtle.forward() do
				if turtle.detect() then
						turtle.dig()
				end
		end
		fd = fd + 1
end

(this way, fd is an upvalue, meaning it is declared in a block above it,
the old way, fd would become local, and when you would assign a new value to it, you would edit the local value, not the upvalue.)

Ah, I wasn't sure if I should use return in that case or not; got it! Thanks :)/>

You can also put your value into a table, because tables are passed by reference

local x = {val=2}

function foo(x)
  x.val = x.val + 1
end

foo(x)

print(x.val) --prints 3

Maybe alot of people answered my question, but I am going to take a shot to clarify it. Maybe to give you a little bit of clarification about variables.

foo = "bar" --#Global variable, can be used everywhere

if 1 ~= 0 then
  local foo = "bar" --#Local variable that can only (!) be used in the if-statement
end

function foo()
  local bar = "foobar" --#Local variable that can only be used in this specific block
end

Now, blocks can be if-statement, while- and for-loops and functions. That last part is where you are interested in. You can pass variables through by giving the function variables separated by comma's in the brackets. These variables can range from strings to numbers. Not to sure about integers. These variables that you put in between the brackets only apply to the function (that block). Demonstration:

function calculate(foo, bar)
  a = foo
  b = bar
end