library(dplyr)
set.seed(1234)
mydata<-data.frame(class = sample(3,10, replace=T),
                   id = sample(30,10, replace=T),
                   mid = as.integer(rnorm(10, mean=60, sd=10)),
                   final = as.integer(rnorm(10, mean=70, sd=8)))
mydata

filter(mydata, class==1)
filter(mydata, class==3, id==30)
filter(mydata, class==1 | class ==2)
filter(mydata, class %in% c(1,2))
filter(mydata, class %in% c(1,2), final==70)
filter(mydata, ((class==1) | (class==2)) & final==70)

mutate(mydata, total=mid+final, mean=total/2)
mydata<-mutate(mydata, total=mid+final, mean=total/2)

transmute(mydata, total=mid+final, mean=total/2)
mydata_trans<-transmute(mydata, total=mid+final, mean=total/2)