Creating vectors

Creating named vectors

Named vector can be created in several ways. With c:

xc <- c('a' = 5, 'b' = 6, 'c' = 7, 'd' = 8)

which results in:

> xc
a b c d 
5 6 7 8

with list:

xl <- list('a' = 5, 'b' = 6, 'c' = 7, 'd' = 8)

which results in:

> xl
$a
[1] 5

$b
[1] 6

$c
[1] 7

$d
[1] 8

With the setNames function, two vectors of the same length can be used to create a named vector:

x <- 5:8
y <- letters[1:4]

xy <- setNames(x, y)

which results in a named integer vector:

> xy
a b c d 
5 6 7 8

As can be seen, this gives the same result as the c method.

You may also use the names function to get the same result:

xy <- 5:8
names(xy) <- letters[1:4]

With such a vector it is also possibly to select elements by name:

> xy["c"]
c 
7 

This feature makes it possible to use such a named vector as a look-up vector/table to match the values to values of another vector or column in dataframe. Considering the following dataframe:

mydf <- data.frame(let = c('c','a','b','d'))

> mydf
  let
1   c
2   a
3   b
4   d

Suppose you want to create a new variable in the mydf dataframe called num with the correct values from xy in the rows. Using the match function the appropriate values from xy can be selected:

mydf$num <- xy[match(mydf$let, names(xy))]

which results in:

> mydf
  let num
1   c   7
2   a   5
3   b   6
4   d   8

Expanding a vector with the rep() function

The rep function can be used to repeat a vector in a fairly flexible manner.

# repeat counting numbers, 1 through 5 twice
rep(1:5, 2)
[1] 1 2 3 4 5 1 2 3 4 5

# repeat vector with incomplete recycling
rep(1:5, 2, length.out=7)
[1] 1 2 3 4 5 1 2

The each argument is especially useful for expanding a vector of statistics of observational/experimental units into a vector of data.frame with repeated observations of these units.

# same except repeat each integer next to each other
rep(1:5, each=2)
[1] 1 1 2 2 3 3 4 4 5 5

A nice feature of rep regarding involving expansion to such a data structure is that expansion of a vector to an unbalanced panel can be accomplished by replacing the length argument with a vector that dictates the number of times to repeat each element in the vector:

# automated length repetition
rep(1:5, 1:5)
 [1] 1 2 2 3 3 3 4 4 4 4 5 5 5 5 5
# hand-fed repetition length vector
rep(1:5, c(1,1,1,2,2))
[1] 1 2 3 4 4 5 5

This should expose the possibility of allowing an external function to feed the second argument of rep in order to dynamically construct a vector that expands according to the data.


As with seq, faster, simplified versions of rep are rep_len and rep.int. These drop some attributes that rep maintains and so may be most useful in situations where speed is a concern and additional aspects of the repeated vector are unnecessary.

# repeat counting numbers, 1 through 5 twice
rep.int(1:5, 2)
[1] 1 2 3 4 5 1 2 3 4 5

# repeat vector with incomplete recycling
rep_len(1:5, length.out=7)
[1] 1 2 3 4 5 1 2

seq()

seq is a more flexible function than the : operator allowing to specify steps other than 1.

The function creates a sequence from the start (default is 1) to the end including that number.

You can supply only the end (to) parameter

seq(5)
# [1] 1 2 3 4 5

As well as the start

seq(2, 5) # or seq(from=2, to=5)
# [1] 2 3 4 5

And finally the step (by)

seq(2, 5, 0.5) # or seq(from=2, to=5, by=0.5)
# [1] 2.0 2.5 3.0 3.5 4.0 4.5 5.0

seq can optionally infer the (evenly spaced) steps when alternatively the desired length of the output (length.out) is supplied

seq(2,5, length.out = 10)
# [1] 2.0 2.3 2.6 2.9 3.2 3.5 3.8 4.1 4.4 4.7 5.0

If the sequence needs to have the same length as another vector we can use the along.with as a shorthand for length.out = length(x)

x = 1:8
seq(2,5,along.with = x)
# [1] 2.000000 2.428571 2.857143 3.285714 3.714286 4.142857 4.571429 5.000000

There are two useful simplified functions in the seq family: seq_along, seq_len, and seq.int. seq_along and seq_len functions construct the natural (counting) numbers from 1 through N where N is determined by the function argument, the length of a vector or list with seq_along, and the integer argument with seq_len.

seq_along(x)
# [1] 1 2 3 4 5 6 7 8

Note that seq_along returns the indices of an existing object.

# counting numbers 1 through 10
seq_len(10)
[1]  1  2  3  4  5  6  7  8  9 10
# indices of existing vector (or list) with seq_along
letters[1:10]
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j"
seq_along(letters[1:10])
[1]  1  2  3  4  5  6  7  8  9 10

seq.intis the same as seq maintained for ancient compatibility.

There is also an old function sequencethat creates a vector of sequences from a non negative argument.

sequence(4)
# [1] 1 2 3 4
sequence(c(3, 2))
# [1] 1 2 3 1 2
sequence(c(3, 2, 5))
# [1] 1 2 3 1 2 1 2 3 4 5

Sequence of numbers

Use the : operator to create sequences of numbers, such as for use in vectorizing larger chunks of your code:

x <- 1:5
x
## [1] 1 2 3 4 5

This works both ways

10:4
# [1] 10  9  8  7  6  5  4

and even with floating point numbers

1.25:5
# [1] 1.25 2.25 3.25 4.25

or negatives

-4:4
#[1] -4 -3 -2 -1  0  1  2  3  4

Vectors

Vectors in R can have different types (e.g. integer, logical, character). The most general way of defining a vector is by using the function vector().

vector('integer',2) # creates a vector of integers of size 2.
vector('character',2) # creates a vector of characters of size 2.
vector('logical',2) # creates a vector of logicals of size 2.

However, in R, the shorthand functions are generally more popular.

integer(2) # is the same as vector('integer',2) and creates an integer vector with two elements
character(2) # is the same as vector('integer',2) and creates an character vector with two elements
logical(2) # is the same as vector('logical',2) and creates an logical vector with two elements

Creating vectors with values, other than the default values, is also possible. Often the function c() is used for this. The c is short for combine or concatenate.

c(1, 2) # creates a integer vector of two elements: 1 and 2.
c('a', 'b') # creates a character vector of two elements: a and b.
c(T,F) # creates a logical vector of two elements: TRUE and FALSE.

Important to note here is that R interprets any integer (e.g. 1) as an integer vector of size one. The same holds for numerics (e.g. 1.1), logicals (e.g. T or F), or characters (e.g. 'a'). Therefore, you are in essence combining vectors, which in turn are vectors.

Pay attention that you always have to combine similar vectors. Otherwise, R will try to convert the vectors in vectors of the same type.

c(1,1.1,'a',T) # all types (integer, numeric, character and logical) are converted to the 'lowest' type which is character.

Finding elements in vectors can be done with the [ operator.

vec_int <- c(1,2,3)
vec_char <- c('a','b','c')
vec_int[2] # accessing the second element will return 2
vec_char[2] # accessing the second element will return 'b'

This can also be used to change values

vec_int[2] <- 5 # change the second value from 2 to 5
vec_int # returns [1] 1 5 3

Finally, the : operator (short for the function seq()) can be used to quickly create a vector of numbers.

vec_int <- 1:10
vec_int # returns [1] 1 2 3 4 5 6 7 8 9 10

This can also be used to subset vectors (from easy to more complex subsets)

vec_char <- c('a','b','c','d','e')
vec_char[2:4] # returns [1] "b" "c" "d"
vec_char[c(1,3,5)] # returns [1] "a" "c" "e"

Vectors from build in constants: Sequences of letters & month names

R has a number of build in constants. The following constants are available:

  • LETTERS: the 26 upper-case letters of the Roman alphabet
  • letters: the 26 lower-case letters of the Roman alphabet
  • month.abb: the three-letter abbreviations for the English month names
  • month.name: the English names for the months of the year
  • pi: the ratio of the circumference of a circle to its diameter

From the letters and month constants, vectors can be created.

1) Sequences of letters:

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

> LETTERS[7:9]
[1] "G" "H" "I"

> letters[c(1,5,3,2,4)]
[1] "a" "e" "c" "b" "d"

2) Sequences of month abbreviations or month names:

> month.abb
 [1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"

> month.name[1:4]
[1] "January"  "February" "March"    "April"   

> month.abb[c(3,6,9,12)]
[1] "Mar" "Jun" "Sep" "Dec"


2016-07-21
2016-07-24
R Language Pedia
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