Count number of rows by group using dplyr

There's a special function n() in dplyr to count rows (potentially within groups):

library(dplyr) mtcars %>% group_by(cyl, gear) %>% summarise(n = n()) #Source: local data frame [8 x 3] #Groups: cyl [?] # # cyl gear n # (dbl) (dbl) (int) #1 4 3 1 #2 4 4 8 #3 4 5 2 #4 6 3 2 #5 6 4 4 #6 6 5 1 #7 8 3 12 #8 8 5 2 

But dplyr also offers a handy count function which does exactly the same with less typing:

count(mtcars, cyl, gear) # or mtcars %>% count(cyl, gear) #Source: local data frame [8 x 3] #Groups: cyl [?] # # cyl gear n # (dbl) (dbl) (int) #1 4 3 1 #2 4 4 8 #3 4 5 2 #4 6 3 2 #5 6 4 4 #6 6 5 1 #7 8 3 12 #8 8 5 2 

I think what you are looking for is as follows.

cars_by_cylinders_gears <- mtcars %>% group_by(cyl, gear) %>% summarise(count = n()) 

This is using the dplyr package. This is essentially the longhand version of the count () solution provided by docendo discimus.

Another approach is to use the double colons as this will help avoid potential conflicts with functions with similar names from other packages.

mtcars %>% dplyr::group_by(cyl, gear) %>% dplyr::summarise(length(gear)) 

Another option, not necesarily more elegant, but does not require to refer to a specific column:

mtcars %>% group_by(cyl, gear) %>% do(data.frame(nrow=nrow(.))) 

This is equivalent to using count():

library(dplyr, warn.conflicts = FALSE) all.equal(mtcars %>% group_by(cyl, gear) %>% do(data.frame(n=nrow(.))) %>% ungroup(), count(mtcars, cyl, gear), check.attributes=FALSE) #> [1] TRUE 

One could simply do this using dplyr:

library(dplyr) # Use the function add_count() and name the new variable as "count" mtcars %>% add_count(cyl, gear, name = "count") 

This way you do not need to group the columns as the add_count() function does that for you when you mention variables. Also, this allows you to retain other variables in the data frame (if any).

Another option is using the function tally from dplyr. Here is a reproducible example:

library(dplyr) mtcars %>% group_by(cyl, gear) %>% tally() #> # A tibble: 8 × 3 #> # Groups: cyl [3] #> cyl gear n #> <dbl> <dbl> <int> #> 1 4 3 1 #> 2 4 4 8 #> 3 4 5 2 #> 4 6 3 2 #> 5 6 4 4 #> 6 6 5 1 #> 7 8 3 12 #> 8 8 5 2 

^{Created on 2022-09-11 with reprex v2.0.2}