library(ISLR)
library(boot)
data(Auto)

 glm.fit=glm(mpg~poly(horsepower,2),data=Auto)

#Default cost (squared error loss)
 set.seed(17)
 cv.err.default = cv.glm(Auto, glm.fit, K=10)$delta[1] 
 cv.err.default

#Custom cost (intentinally same as squared error loss)
 set.seed(17)
 cost.rs = function(obs,pred=0) mean((obs-pred)^2)
 cv.err.custom = cv.glm(Auto, glm.fit, cost=cost.rs, K=10)$delta[1]  
 cv.err.custom

#Custom binary cost
 set.seed(17)
 cost.bin = function(obs,pred=0) mean(abs(obs-pred)>0.5)
 cv.err.binary = cv.glm(Auto, glm.fit, cost=cost.bin, K=10)$delta[1] 
 cv.err.binary


 cv.err.default
 cv.err.custom
 cv.err.binary