How to tally occurrences of same value across columns in R?

Question

I have a dataframe like so:

exit	x1	x2	x3	x4
3	4	6	6	NA
4	4	6	6	NA
5	4	6	6	NA
6	4	6	6	NA

I am only interested when exit = xi, where i = 1, 2, 3, 4 as shown in the data. I want to count the number of times exit = xi. So, the ideal output I am looking for is:

exit	x1	x2	x3	x4	n
3	4	6	6	NA	0
4	4	6	6	NA	1
5	4	6	6	NA	0
6	4	6	6	NA	2

Ideally I would like to be using ifelse() in R, but it seems I am unable to figure out a good logical path.

Answer 1

A base solution:

df$n = rowSums(apply(df, 2, function(col) col == df$exit)[, -1], na.rm = T)

> df
  exit x1 x2 x3 x4 n
1    3  4  6  6 NA 0
2    4  4  6  6 NA 1
3    5  4  6  6 NA 0
4    6  4  6  6 NA 2

Data:

> dput(df)
structure(list(exit = 3:6, x1 = c(4L, 4L, 4L, 4L), x2 = c(6L, 
6L, 6L, 6L), x3 = c(6L, 6L, 6L, 6L), x4 = c(NA, NA, NA, NA), 
    n = c(0, 1, 0, 2)), row.names = c(NA, -4L), class = "data.frame")