Count the number of rows where all columns have identical values

Question

I have a dataframe and I want to count the number of rows which have the same value for all the columns, within each row.

For example, I have this data:

cmp <- read.table(text = "
A B C D
1 1 1 0
1 1 1 1
2 2 2 2
3 3 3 0", header = TRUE)

Here, the count is 2, because the second row and third row have only one unique value each, only 1s, and only 2s, respectively.

Thanks in advance.

Answer 1

This, which uses apply() to count the number of distinct elements in each row, should do the trick:

sum(apply(cmp, 1, function(x) length(unique(x))==1))
## [1] 2