Lists

List in R can hold elements of different types. There is no coercion. A list can contain numeric, characters, boolean, matrices, vectors, arrays, lists stc.

To create list, use list() argument.

list_data <- list('green', 'yellow', 1, 2, c(4,5,6))
print(list_data)

## [[1]]
## [1] "green"
## 
## [[2]]
## [1] "yellow"
## 
## [[3]]
## [1] 1
## 
## [[4]]
## [1] 2
## 
## [[5]]
## [1] 4 5 6

To give names to each entry of list, use names() argument.

names(list_data) <- c("A", "B", "C", "D", "E")

To access a particular entry in list use $

print(list_data$A); print(list_data$B) ### Access values of entries with name A and B

## [1] "green"

## [1] "yellow"

print(list_data[1]) ### Access first label and its value

## $A
## [1] "green"

print(list_data[[1]]) ### Access first value

## [1] "green"

To merge two or more lists, use c()

a <- list(1,2,3,4); b <- list(5,6,7,8); c <- c(a,b); print(c)

## [[1]]
## [1] 1
## 
## [[2]]
## [1] 2
## 
## [[3]]
## [1] 3
## 
## [[4]]
## [1] 4
## 
## [[5]]
## [1] 5
## 
## [[6]]
## [1] 6
## 
## [[7]]
## [1] 7
## 
## [[8]]
## [1] 8

Some predefined lists in R

print(letters); print(LETTERS); print(month.abb); print(month.name)

##  [1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s"
## [20] "t" "u" "v" "w" "x" "y" "z"

##  [1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S"
## [20] "T" "U" "V" "W" "X" "Y" "Z"

##  [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"

##  [1] "January"   "February"  "March"     "April"     "May"       "June"     
##  [7] "July"      "August"    "September" "October"   "November"  "December"

Vectors

To create a vector, we use c() function. It basically concatenates things like a list in python.

x <- c(1,2,3,4,5.4,'hello',TRUE,FALSE); print(x)

## [1] "1"     "2"     "3"     "4"     "5.4"   "hello" "TRUE"  "FALSE"

As we can see, a vector can have any data type, be it number, character or boolean. But we notice something. All the elements in the vector are coerced to character type because the vector contains a string "hello". This is the effect of implicit coercion.

For strictly making a numeric vector, use vector() function.

x <- vector("numeric", length = 20); print(x)

##  [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

We can use such a vector to preallocate a vector which can be used for appending values from a for loop which is faster than appending values to an empty vector since every time a value is appended in an empty vector, R makes a copy of it thus slowing the whole process.

Coercion — Objects like vectors, data frames etc. can be coerced to different classess using as.class function.

x <- c(1,2,3,4); print(class(x))

## [1] "numeric"

y <- as.character(x); print(class(y))

## [1] "character"

y <- as.logical(x); print(class(y))

## [1] "logical"

Matrices

Matrix is same as a vector except it has an additional attribute of dimension. It has two dimensional data structure.

a <- matrix(c(6,2,6,8,3,2,6,8,0), nrow = 3, ncol = 3); print(a); print(attributes(a))

##      [,1] [,2] [,3]
## [1,]    6    8    6
## [2,]    2    3    8
## [3,]    6    2    0

## $dim
## [1] 3 3

Matrices start filling row wise. Whereas in python, a matrix starts filling columnwise.

In R, we can pass the names of rows and columns.

a <- matrix(c(6,2,6,8,3,2,6,8,0), nrow = 3, ncol = 3,
            dimnames = list(c('a','b','c'), c('x','y','z'))); print(a)

##   x y z
## a 6 8 6
## b 2 3 8
## c 6 2 0

print(colnames(a)); print(rownames(a))

## [1] "x" "y" "z"

## [1] "a" "b" "c"

To access the elements of a matrix, use square brackets.

a1 <- matrix(c(6,2,6,8,3,2,6,8,0), nrow = 3, ncol = 3,
            dimnames = list(c('a','b','c'), c('x','y','z'))); print(a)

##   x y z
## a 6 8 6
## b 2 3 8
## c 6 2 0

print(a1[2,2])              ### select 2nd row and 2nd column element 

## [1] 3

print(a1[c(2,3),c(1,2)])    ### select rows 2 and 3 and columns 1 and 2

##   x y
## b 2 3
## c 6 2

But a[2,] (2nd row) or a[,2] (2nd column) gives a vector. To avoid this i.e. to get a matrix, use drop = FALSE

print(a[2,]); print(dim(a[2,]))

## x y z 
## 2 3 8

## NULL

print(a[2,,drop = FALSE]); print(dim(a[2,,drop = FALSE]))

##   x y z
## b 2 3 8

## [1] 1 3

Specific indexing can also be done.

print(a[c(1,2,4,6)])

## [1] 6 2 8 2

You can also do indexing using logical vectors.

print(a[c(TRUE,FALSE,TRUE), c(TRUE,TRUE,FALSE)])

##   x y
## a 6 8
## c 6 2

To transpose a matrix use t(a)

To combine vectors or matrices, use rbind or cbind

Dimension of matrix can also be changed (reshape)

dim(a) <- c(1,9); print(dim(a))

## [1] 1 9

Arrays, factors and data frames will be covered in next post.