x <- seq(-1,1,length=12)
x
x < 0 | x > 0.8
x < 0 & x > 0.8

x
x[3]    # extract  the third element
x[c(1,2,5)]   # extract the first, second and fifth elements.
x[-(3:10)]    # extract all the elements except those in positions 3 to 10.
x[ x > 0]     # extract the elements that satisfy the condition.
x[ x > 0 & x < 0.5]

A <- cbind(c(1,2,-1), c(12,15,18), c(-1,-4,-9))
A
A[1,1]  #extracts the (1,1) element
A[1,3]  # extracts the (1,3) element
A[1:2,3]  #extracts the elements (1,3), (2,3)
A[1:2,2:3]   #extracts a two by two matrix
A[,2:3]      # omission of a dimension gives the corresponding columns
A[-1,2:3]   # use of negative indices

mylist <- list(x,A)
mylist
mylist[[1]]
mylist[[2]]

library(MASS)
is.data.frame(survey)
names(survey)
survey$Age[1:100]

standard.deviation <- function(x)
{
sqrt(var(x))
}
x <- rnorm(100, mean=0, sd=2) 
var(x)
standard.deviation(x)

random.gener <- function(n, distribution, shape)
{
if(distribution=="gamma") rgamma(n, shape) else
if(distribution=="exp")   rexp(n) else
if(distribution=="norm") rnorm(n) else
stop("Invalid Distribution")
}
random.gener(10, "gamma", 2)
random.gener(10, "unif", 2)

new.sign <-  function(x)
{
   for (i in 1:length(x)){
         if(x[i] > 0)
            x[i] <- 1
    else if(x[i] < 0)
            x[i] <- -1
   }
   x
}
new.sign(-10:5)

sgnfunction <- function(x)
{
ifelse(x > 0, 1, ifelse(x<0, -1, 0))
}
sgnfunction(-10:10)