R | Slicing and Extracting Data

Indexing Vectors

  • x[n] nth element
  • x[-n] all but the nth element
  • x[1:n] first n elements
  • x[-(1:n)] elements from n+1 to the end
  • x[c(1,4,2)] specific elements
  • x[“name”] element named “name”
  • x[x > 3] all elements greater than 3
  • x[x > 3 & x < 5] all elements between 3 and 5
  • x[x %in% c(“a”,”and”,”the”)] elements in the given set

Indexing Lists

  • x[n] list with elements n
  • x[[n]] nth element of the list
  • x[[“name”]] element of the list named “name”
  • xname</strong> id.</li> </ul> <h3>Indexing Matrices</h3> <ul> 	<li><strong>x[i,j]</strong> element at row i, column j</li> 	<li><strong>x[i,]</strong> row i</li> 	<li><strong>x[,j]</strong> column j</li> 	<li><strong>x[,c(1,3)]</strong> columns 1 and 3</li> 	<li><strong>x["name",]</strong> row named "name"</li> </ul> <h3>Indexing Data Frames</h3> <ul> 	<li><strong>x[["name"]]</strong> column named "name"</li> 	<li><strong>xname id

R Objects: Data Types

  • OBJECTS
  • R has five basic classes of objects:
  1. Character
  2. Numeric
  3. Integer
  4. Complex
  5. Logical

However, the most basic object is a vector.

  • There are two things which you should remember when dealing with vectors.
    1. A vector can only contain objects of the same class.
    2. AND there is an exception to this, a list. A list looks like a vector, but can have different classes.

To create an empty vector use the following function:

vector()
Vectors can be created by using the following:
c() # used to concatenate individual values together
: # to create a sequence, such as 1:10
seq() # to create more complex sequences
rep() # replicates values
sort() # ordering elements in a vector
order() # ordering elements in a vector

 

An example of using rep()

rep(5,2) #a vector of two fives
[1] 5 5

 

An example of using c()

c(3,2,1) # vector of three numeric elements in that order
[1] 3 2 1

 

An example of using seq()

seq(4,20, by = 2)
[1]  4  6  8 10 12 14 16 18 20
seq(1,length = 20, by =4)
 [1]  1  5  9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 77