This function applies a specified function to each group defined by an index, and returns a summary of the results. It is useful for summarizing data by group when the groups are defined by an index rather than a named column.
groupStatI(f, x, idx)
A data frame or matrix containing the summary statistics for each group.
A function that takes a single argument and returns a summary of the data.
A data frame or matrix containing the data to be summarized.
A vector of indices or group names that define the groups.