How to count the number of observations in R like Stata command count

Question

aaa<- data.frame(sex=c(1,1,2,2,1,1), group1=c(1,2,1,2,2,2),group2=c(\"A\",\"B\",\"A\",\"B\",\"A\",\"B\"))

stata command:

count if se         


        

Answer 1

The with function will let you use shorthand column references and sum will count TRUE results from the expression(s).

sum(with(aaa, sex==1 & group1==2))
## [1] 3

sum(with(aaa, sex==1 & group2=="A"))
## [1] 2

As @mnel pointed out, you can also do:

nrow(aaa[aaa$sex==1 & aaa$group1==2,])
## [1] 3

nrow(aaa[aaa$sex==1 & aaa$group2=="A",])
## [1] 2

The benefit of that is that you can do:

nrow(aaa)
## [1] 6

And, the behaviour matches Stata's count almost exactly (syntax notwithstanding).

Answer 2

You can also use the filter function from the dplyr package which returns rows with matching conditions.

> library(dplyr)

> nrow(filter(aaa, sex == 1 & group1 == 2))
[1] 3
> nrow(filter(aaa, sex == 1 & group2 == "A"))
[1] 2