```
x <- 1:12
m <- matrix(1:6, nrow = 2, dimnames = list(c("a", "b"), LETTERS[1:3]))
li <- list(pi = pi, e = exp(1))
x[10] # the tenth element of x
x <- x[-1] # delete the 1st element of x
m[1,] # the first row of matrix m
m[1, , drop = FALSE] # is a 1-row matrix
m[,c(TRUE,FALSE,TRUE)]# logical indexing
m[cbind(c(1,2,1),3:1)]# matrix numeric index
ci <- cbind(c("a", "b", "a"), c("A", "C", "B"))
m[ci] # matrix character index
m <- m[,-1] # delete the first column of m
li[[1]] # the first element of list li
y <- list(1, 2, a = 4, 5)
y[c(3, 4)] # a list containing elements 3 and 4 of y
y$a # the element of y named a
## non-integer indices are truncated:
(i <- 3.999999999) # "4" is printed
(1:5)[i] # 3
## named atomic vectors, compare "[" and "[[" :
nx <- c(Abc = 123, pi = pi)
nx[1] ; nx["pi"] # keeps names, whereas "[[" does not:
nx[[1]] ; nx[["pi"]]
## recursive indexing into lists
z <- list(a = list(b = 9, c = "hello"), d = 1:5)
unlist(z)
z[[c(1, 2)]]
z[[c(1, 2, 1)]] # both "hello"
z[[c("a", "b")]] <- "new"
unlist(z)
## check $ and [[ for environments
e1 <- new.env()
e1$a <- 10
e1[["a"]]
e1[["b"]] <- 20
e1$b
ls(e1)
## partial matching - possibly with warning :
stopifnot(identical(li$p, pi))
op <- options(warnPartialMatchDollar = TRUE)
stopifnot( identical(li$p, pi), #-- a warning
inherits(tryCatch (li$p, warning = identity), "warning"))
## revert the warning option:
if(is.null(op[[1]])) op[[1]] <- FALSE; options(op)
```

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