Question: Optimization with binary constraint {0,1}

I am attempting to minimize an objective function that is related to a p-median problem (facility location). Can somebody please tell me how to impose a constraint that comprises 0 or 1? For instance, I wish to assign the condition, x[i,j] = 0 or 1; in other words, x[i,j] = 1 if the point i is assigned to facilty located at point j; 0 otherwise.

 

Thanks in advance!

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