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:

Answer 1

You can use by functions as by(df1$Year, df1$Month, count) that will produce a list of needed aggregation.

The output will look like,

df1$Month: Feb
     x freq
1 2012    1
2 2013    1
3 2014    5
--------------------------------------------------------------- 
df1$Month: Jan
     x freq
1 2012    5
2 2013    2
--------------------------------------------------------------- 
df1$Month: Mar
     x freq
1 2012    1
2 2013    3
3 2014    2
>