About the inventory system

Hi friends, I have a question about how to create an inventory system, the idea which I want to use is I have a block sprite, here named A, and according to that, A has 4 instance on the scenes. And about A, they will have two variables, one is index, initial num = 0, the second one is item , initial str = “”. About each instance, their instance variables are 0,1,2,3. also their item variables are all “”. I also create two scene variables, which are: itemPosition =>num = 0, full =>num = 3.
My plan is if I put something, for example, there is already one item in the box, so the scene variable itemPosition now is 1;and then I want put a sword, first of all check the scene variable itemPosition <full,if true, then change A[1]'s item = “sword”, means it has been stored, and then itemPosition + 1.
My problem is how do I get A[1] and put them to the condition and action in the event page. because there is nothing can really access the instance which i’ve found.

Please help me solve this.
Thank you!