Count number of rows per group and add result to original data frame

rr-faqaggregatecount
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Counting unique / distinct values by group in a data frame

rr-faqdataframedistinct-values
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Convert continuous numeric values to discrete categories defined by intervals

rr-faq
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Why does summarize or mutate not work with group_by when I load `plyr` after `dplyr`?

rr-faqplyrdplyr
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Subset rows corresponding to max value by group using data.table

rgreatest-n-per-groupdata.table
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