Collapsing a data frame over one variable

Question

I have a data frame in the following format:

Site    Year    Month   Count1  Count2  Count3  Patch
1        1        May     15      12      10      1
1        1         


        

Answer 1

library(dplyr) 
dat %>% 
group_by(Site, Year, Month) %>% 
summarise_each(funs(sum=sum(., na.rm=TRUE)), Count1:Count3)
# Source: local data frame [3 x 6]
#Groups: Site, Year

#    Site Year Month Count1 Count2 Count3
#  1    1    1  July      4      0      3  
#  2    1    1  June     23     11      6
#  3    1    1   May     30     17     18