by decreasing scores, starting from rank 1.
PRECISION(K) and RECALL(K) are the precison and recall when
samples of rank smaller or equal to K-1 are predicted to be
positive and the remaining to be negative. So for example
PRECISION(3) is the percentage of positive samples among the two
samples with largest score. PRECISION(1) is the precision when no
samples are predicted to be positive and is conventionally set to
the value 1.
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Set to zero the lables of samples that should be ignored in the
evaluati