This function filters a data frame by the specified document ID.
If the ID is 0, the entire data frame is returned.
Usage
f5(UPIP, I)
Value
Returns a subset of the input data frame (`UPIP`) containing only
the rows where the 'doc_id' column matches the specified document ID `I`.
If `I` is 0, the function returns the entire data frame unmodified. The
output is a data frame with the same structure as the input but potentially
fewer rows, depending on the presence and frequency of the specified ID.