编辑代码

mydata = data.frame(x1=c(1,3,2,5), x2=c(38,2,12,3))
mydata$sum = mydata$x1 + mydata$x2
mydata$mod = mydata$x2 %% mydata$x1
mydata$d = mydata$x2 %/% mydata$x1
mydata
mydata$x1[mydata$x2>10&mydata$x1>1]
names(mydata)[2] = c('x121212')
mydata
y= c(1,2,3,NA, NA, NA)
y[is.na(y)] = 0
y
strDates<-c("01/05/1965", "08/16/1975")
Dates <-as.Date(strDates, "%m/%d/%Y")
Dates
cbind(mydata, mydata)
Sys.Date()
date()
mydata= mydata[order(as.vector(mydata$x1)),1:5]
mydata
newdata<-mydata[mydata$x1 > 3, ]
newdata<-subset(mydata, x1>=4 & x1<=6,select=c("sum"))
newdata
nrow(mydata)
ncol(mydata)
length(mydata)
a=matrix(1:20,c(4,5))
i=1
for(i in 1:20){
    print(sample(mydata, 3, replace=T))
}