Matlab: How to establish a relationship between two elements belonging to the same table?

I have three matrix like these:

categories1 =

     1     2   NaN
     3   NaN   NaN
     4   NaN   NaN

categories2 =

     1     2     3   NaN
     4     5     6   NaN
     4     5     6     7

that put beside give:

all_categories =

     1     2   NaN     1     2     3   NaN
     3   NaN   NaN     4     5     6   NaN
     4   NaN   NaN     4     5     6     7

Then, I have two vectors like:

obj1 = [4 2 1]

obj2 = [2 3 5]

How can I establish a condition that permits a certain operation between an element of obj1 and one of obj2, if and only if their correspondent category belong to the same line in all_categories? In other words, I would like to start a if block in the form:

if (a specified element of array `obj1` is on the same line of another specified element of the array`obj2`)
    do something
end

For instance, the condition would be verified in case of obj1(2) and obj2(1), because their correspondent value are both on the 1st line of all_categories).

Basically, I am trying to implement the condition that only if there is compatibility between and object belonging to a certain category and another object belonging to another category than an operation can happen. Hope this has not been too long to explain. Many thanks.