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A data frame, a matrix-like structure whose columns may be of
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differing types (numeric, logical, factor and character and so on).
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How the names of the data frame are created is complex, and the rest
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of this paragraph is only the basic story. If the arguments are all
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named and simple objects (not lists, matrices of data frames) then the
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argument names give the column names. For an unnamed simple argument,
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a deparsed version of the argument is used as the name (with an
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enclosing \code{I(...)} removed). For a named matrix/list/data frame
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argument with ore than one named column, the names of the columns are
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the name of the argument followed by a dot and the column name inside
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the argument: if the argument is unnamed, the argument's column names
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are used. For a named or unnamed matrix/list/data frame argument that
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contains a single column, the column name in the result is the column
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name in the argument. Finally, the names are adjusted to be unique
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and syntactically valid unless \code{check.names = FALSE}.
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A data frame is a list of variables of the same length with unique row
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names, given class \code{"data.frame"}. If there are zero variables,
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the row names determine the number of rows.
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Duplicate column names are allowed, but you need to use
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\code{check.names=FALSE} for \code{data.frame} to generate such a data
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frame. However, not all operations on data frames will preserve