templat<typename T, typename I = unsigned long>
struct NSmallestVisitor;
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This functor class calculates the N smallest values of a column. I runs in O(N*M), where N is the number of largest values and M is the total number of all values.
If N is relatively small this is better than O(M*logM). The constructor takes a single optional Boolean argument to whether skip NaN values. The default is True.
See this document and dataframe_tester.cc for examples.
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N: Number of smallest values
T: Column data type
I: Index type
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