What is the syntax of the formula argument in R?

Question

In the package nnet, the following example is given:

# or
ird <- data.frame(rbind(iris3[,,1], iris3[,,2], iris3[,,3]),
        species = factor(c(rep("s",50), rep("c", 50), rep("v", 50))))
ir.nn2 <- nnet(species ~ ., data = ird, subset = samp, size = 2, rang = 0.1,
               decay = 5e-4, maxit = 200)
table(ird$species[-samp], predict(ir.nn2, ird[-samp,], type = "class"))

I do not understand how this part works: species ~ ., I understand it is some kind of formula that is passed as argument but I do not know where to search for more information about the syntax of formulaes and what the . will represent.

Please close this question if it is a duplicate, I could not find the same question.

do you want to add a full answer explaining what the syntax is for a formula in R ? — BlueTrin, Apr 25 '13 at 16:28

score 2 · Accepted Answer · answered Apr 25 '13 at 16:40

. represents all the features/columns except the outcome (which is written on the RHS of ~). More info can be found here ?formula

Basically, for the iris3 data set, the formula

species ~ .

is equivalent to

species ~ Sepal L. + Sepal W. + Petal L. + Petal W.

What is the syntax of the formula argument in R?

1 Answers1