Let's replicate the fireplace/housing price example from class. First, let's load the data:
houses <- read.csv("https://raw.githubusercontent.com/brianlukoff/sta371g/master/data/houses.csv")
Now let's run a regression predicting house price from living space and whether the house has a fireplace:
model1 <- lm(Price ~ Living.Area + Fireplace, data=houses)
summary(model1)
Note that R automatically creates FireplaceYes as a dummy variable and adds it to the model.
Now let's add an interaction term between Fireplace and Living.Area:
model2 <- lm(Price ~ Living.Area * Fireplace, data=houses)
summary(model2)
Note that by specifying the formula as Living.Area * Fireplace, R automatically includes both main effects plus the interaction effect.