I encounters some codes in ggplot2 manual and confused with one of its lm syntax. The code is here:


library(ggplot2) d <- subset(diamonds, carat < 2.5 & rbinom(nrow(diamonds), 1, 0.2) == 1) d$lcarat <- log10(d$carat) d$lprice <- log10(d$price) detrend <- lm(lprice ~ lcarat, data = d) d$lprice2 <- resid(detrend) mod <- lm(lprice2 ~ lcarat * color, data = d) # ***
what puzzled me is the last statement marked with ***. How does R deal with lcarat * color, since color is not of numeric value. If this is ok, how can I write the mathematical formula of this regression model ?
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