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Summarise works in an analogous way to mutate, except
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.
summarise(.data, ...)the data frame to be summarised
further arguments of the form var = value
# 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)))
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