Extracting and replacing elements of a vector 

x <- c(10, 12, 30, 14, 50)

x[1]

[1] 10

x[5]

[1] 50

x[c(2,4,6)]

[1] 12 14 NA

x[c(1,1,1,2,2)]

[1] 10 10 10 12 12

x[-c(1,3,5)]

[1] 12 14

x[c(FALSE,TRUE,FALSE,TRUE,FALSE)]

[1] 12 14

x[x>=20]

[1] 30 50

names(x) <- c("a","b","c","d","e")

x[c("a","c")]

 a  c 
10 30 

set.seed(231)
y <- rnorm(n=12)

y[1:4] <- 0
y

 [1]  0.00000000  0.00000000  0.00000000  0.00000000 -0.47335746  0.21739728
 [7]  0.06292205 -0.87782986  0.56368979 -0.03432728 -0.22631292  1.38657787

y <- rnorm(n=12)

y[y < 0] <- 0
y

 [1] 0.0000000 0.0000000 1.4013312 0.3196224 1.0058453 0.0000000 0.0000000
 [8] 1.6502536 1.4374338 0.0000000 0.0000000 0.0000000