A data frame is used for storing data tables. It is a list of vectors of equal length. For example, the following variable df is a data frame containing three vectors n, s, b.
n = c(2, 3, 5)
s = c("aa", "bb", "cc")
b = c(TRUE, FALSE, TRUE)
df = data.frame(n, s, b)
a <- data.frame(x = c(1,2,3), y = c(4,5,6), z = c(7,8,9))
x <- as.matrix(a)
idx <- which(x > 5, arr.ind=TRUE)