?read.table

#copy and paste data from another source:
hotdogs=read.table("clipboard",header=T)
head(hotdogs)

#change directory to data file first:
hotdogs2=read.table("hotdogs.txt",header=T)
head(hotdogs2)

#extract the column variables:
type=hotdogs$Type
calories=hotdogs$Calories
sodium=hotdogs$Sodium

#numerical summary measures:
summary(type)
summary(calories)
summary(sodium)

#statistical summary of calories by types:
summary(calories[type=="Beef"])
summary(calories[type=="Poultry"])
summary(calories[type=="Meat"])

#statistical summary of sodium by types:
summary(sodium[type=="Beef"])
summary(sodium[type=="Poultry"])
summary(sodium[type=="Meat"])

#Plot histograms of caolries by types:
par(mfrow=c(2,2),cex=0.5)
hist(calories[type=="Beef"],freq=F)
lines(density(calories[type=="Beef"]))
hist(calories[type=="Meat"],freq=F)
lines(density(calories[type=="Meat"]))
hist(calories[type=="Poultry"],freq=F)
lines(density(calories[type=="Poultry"]))
plot(calories ~ type)

#know more about the lm command:
?lm
#Fit a linear model for calories:
fit.calories=lm(calories ~ type)
summary(fit.calories)

#Plot histograms of sodium by types:
par(mfrow=c(2,2),cex=0.6)
hist(sodium[type=="Beef"],freq=F)
lines(density(sodium[type=="Beef"]))
hist(sodium[type=="Meat"],freq=F)
lines(density(sodium[type=="Meat"]))
hist(sodium[type=="Poultry"],freq=F)
lines(density(sodium[type=="Poultry"]))
plot(sodium ~ type)

#Fit a linear model sodium:
fit.sodium=lm(sodium ~ type)
summary(fit.sodium)

#How is calories related to sodium?
par(mfrow=c(2,2))
plot(calories ~ sodium)
abline(lsfit(sodium,calories))
plot(calories[type=="Beef"] ~ sodium[type=="Beef"])
abline(lsfit(sodium[type=="Beef"],calories[type=="Beef"]))
plot(calories[type=="Meat"] ~ sodium[type=="Meat"])
abline(lsfit(sodium[type=="Meat"],calories[type=="Meat"]))
plot(calories[type=="Poultry"] ~ sodium[type=="Poultry"])
abline(lsfit(sodium[type=="Poultry"],calories[type=="Poultry"]))

fit=lm(calories ~ sodium + type)
summary(fit