iris$group <- as.factor(rep(c("a","b")))
str(iris)

#model without interaction
mod <- lm(Sepal.Length ~ Sepal.Width+Species+group, iris)
summary(mod)

#model with interaction(Species*group)
mod2 <- lm(Sepal.Length ~ Sepal.Width+Species*group, iris)
summary(mod2)

#dummy variables for interaction
int1 <- ifelse(iris$Species=="versicolor" & iris$group=="b", 1, 0)
int2 <- ifelse(iris$Species=="virginica" & iris$group=="b", 1, 0)

#model with dummy variables
mod3 <- lm(Sepal.Length ~ Sepal.Width+Species+group+int1+int2, iris)
summary(mod3)
summary(mod2)