Data frames

 A data frame is a list with class data.frame. There are restrictions on lists that may be made into data frames, namely

• The components must be vectors (numeric, character, or logical), factors, numeric matrices, lists, or other data frames.
• Matrices, lists, and data frames provide as many variables to the new data frame as they have columns, elements, or variables, respectively.
• Numeric vectors and factors are included as is, and non-numeric vectors are coerced to be factors, whose levels are the unique values appearing in the vector.
• Vector structures appearing as variables of the data frame must all have the same length, and matrix structures must all have the same row size.

Data frames may in many ways be regarded as a matrix with columns possibly of differing modes and attributes. It may be displayed in matrix form, and its rows and columns extracted using matrix indexing conventions.