Summarise a data frame.
Summarise works in an analogous way to
instead of adding columns to an existing data frame, it creates a new
data frame. This is particularly useful in conjunction with
ddply as it makes it easy to perform group-wise summaries.
- the data frame to be summarised
- further arguments of the form var = value
Be careful when using existing variable names; the corresponding columns will be immediately updated with the new data and this can affect subsequent operations referring to those variables.
# Let's extract the number of teams and total period of time # covered by the baseball dataframe summarise(baseball, duration = max(year) - min(year), nteams = length(unique(team))) # Combine with ddply to do that for each separate id ddply(baseball, "id", summarise, duration = max(year) - min(year), nteams = length(unique(team)))