Count number of rows within each group

Question

I have a dataframe and I would like to count the number of rows within each group. I reguarly use the aggregate function to sum data as follows:

df2 <- aggregate(x ~ Year + Month, data = df1, sum)

Now, I would like to count observations but can't seem to find the proper argument for FUN. Intuitively, I thought it would be as follows:

df2 <- aggregate(x ~ Year + Month, data = df1, count)

But, no such luck.

Any ideas?

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Some toy data:

set.seed(2)
df1 <- data.frame(x = 1:20,
                  Year = sample(2012:2014, 20, replace = TRUE),
                  Month = sample(month.abb[1:3], 20, replace = TRUE))

Answer 1

Current best practice (tidyverse) is:

require(dplyr)
df1 %>% count(Year, Month)