Select Next Empty Inventory Slot?

You'll need to make this, and it's a bit complicated.

you could do something like this:

when you collect a new resource, add a character to represent it on an alterable string.
f.ex "1" = wood, and "2" rock. so when you pick a wood set alterable string A to:
alterable string A + "1".
then add 1 to alterable value (by index), at index: Find(Alterable String A( "Active" ), "1", 0)

this finds where in the string "1" is located on the string, which is the slot number.
and the amount of each resource is stored on each alterable value.

to know if the resource has been added compare if Find(Alterable String A( "Active" ), "1", 0) is equal to -1, that means it's not there.
So if it's equal to -1 then add "1" to alterable string A (then the number to value at index), otherwise skip the character and just add 1 to alterable value at index, since you already added that kind of resource.

f.ex if you collect rock "2", then wood "1", then water "3", and another water "3".
your string would be "213"
and the alterable values would be 1,1,2.

the reason to put numbers in a string like this, is so you can extract the number and use that to display the icon for your resource. f.ex by just setting the animation direction. so direction 1 = wood, and direction 2 = rock.

now to update the display, run a fastloop len(Alterable String A( "Active" )) times, then the loop runs for each different kind of resource collected, which is the number of characters in the string, lets call this loop "x".
on this loop, create the icons by setting the direction to val(Mid$( Alterable String A( "Active" ), LoopIndex("x"), 1))
this picks out each character one by one and sets the icon correctly.

now you just need to set the number, which is AltValN( "Active", LoopIndex("x") ) <- alterable value at index

done, the only hard part now is if you need to remove a resource when it's spent down to 0, but it's managable! i'll explain how if you managed to follow what i've said so far and want to add that.

the amount of each resource, f.ex rock, is: AltValN( "Active", Find(Alterable String A( "Active" ), "2", 0) ) just replace "2" with the icon number for each resource.

if you want to spend a resource, you'll need to subtract from alterable value by expression, using the same: Find(Alterable String A( "Active" ), "2", 0)

the above example supports 10 different kinds of resources, and any number of slots, sorry if this is way above your head, but to make it exactly like you want, with the flexible slots, it needs to be almost this complicated.