If-then constraints in integer linear programming

I have a 30*40 matrix. Lets say the components in the matrix are specified with "P" and the related number of the row and column of each "P" is specified by "X" and "Y" accordingly. I have a model that the output should give us the P, X and Y. How can I define constraints (for solving a simplex) which connect P with it's exact X and Y? I want to say for example:

if X=1 and Y=1 then P= 0.1

if X=1 and Y=2 then P= 0.5

if X=1 and Y=3 then P= 0.8 and so on.

I dont want the model to return a P that does not match it's location in the matrix. How can I achieve this?