r - Count number of values in row using dplyr

Question

This question should have a simple, elegant solution but I can't figure it out, so here it goes:

Let's say I have the following dataset and I want to count the number of 2s present in each row using dplyr.

set.seed(1)
ID <- LETTERS[1:5]
X1 <- sample(1:5, 5,T)
X2 <- sample(1:5, 5,T)
X3 <- sample(1:5, 5,T)

df <- data.frame(ID,X1,X2,X3)
library(dplyr)

Now, the following works:

df %>%
  rowwise %>%
  mutate(numtwos = sum(c(X1,X2,X3) == 2))

But how do I avoid typing out all of the column names?

I know this is probably easier to do without dplyr, but more generally I want to know how I can use dplyr's mutate with multiple columns without typing out all the column names.

Answer 1

Try rowSums:

> set.seed(1)
> ID <- LETTERS[1:5]
> X1 <- sample(1:5, 5,T)
> X2 <- sample(1:5, 5,T)
> X3 <- sample(1:5, 5,T)
> df <- data.frame(ID,X1,X2,X3)
> df
  ID X1 X2 X3
1  A  2  5  2
2  B  2  5  1
3  C  3  4  4
4  D  5  4  2
5  E  2  1  4
> rowSums(df == 2)
[1] 2 1 0 1 1

Alternatively, with dplyr:

> df %>% mutate(numtwos = rowSums(. == 2))
  ID X1 X2 X3 numtwos
1  A  2  5  2       2
2  B  2  5  1       1
3  C  3  4  4       0
4  D  5  4  2       1
5  E  2  1  4       1