A hash table gets instantiated during execution time but we want to only instantiate the hash table once. "_n_" is an automatic variable which contains the count of the iteration of the data step (so 1 in the first iteration, 2 in the 2nd itereation and so on).
"if _n_=1" lets us do things only once in the very first iteration of the data step (here: instantiate the hash table).
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