Chapter 2 Objects

2.1 Vectors

A vector is a simple, one-dimensional list of data, like a single column in Excel or in SPSS. Typically a single vector holds a single variable of interest. The data in a vector can be of various classes: numeric, character (strings of letters, always enclosed in double quotes), or logical (i.e., boolean, TRUE or FALSE, may be abbreviated to T or F).

  • c: Atomic data are combined into a vector by means of the c (combine, concatenate) operator.

  • seq The sequence operator, also abbreviated as a colon :, creates subsequent values.

x <- 1:5
print(x)
## [1] 1 2 3 4 5
2*(x-1)
## [1] 0 2 4 6 8

Computations are also done on whole vectors, as exemplified above. In the last example, we see that the result of the computation is not assigned to a new object. Hence the result is displayed — and then lost. This may still be useful however when you use R as a pocket calculator.

  • rep Finally, the repeat operator is very useful in creating repetitive sequences, e.g. for levels of an independent variable.
x <- rep( 1:5, each=2 )
x
##  [1] 1 1 2 2 3 3 4 4 5 5

2.2 Factors

Factors constitute a special class of variables. A factor is a variable that holds categorical, character-like data. R realizes that variables of this class hold categorical data, and that the values are category labels or levels rather than real characters or digits, as illustrated in the examples below.

x1 <- rep( 1:4, each=2 ) # create vector of numbers
print(x1) # numeric
## [1] 1 1 2 2 3 3 4 4
summary(x1) # of numeric vector
##    Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
##    1.00    1.75    2.50    2.50    3.25    4.00
x2 <- as.character(x1) # convert to char
print(x2) # character vector
## [1] "1" "1" "2" "2" "3" "3" "4" "4"
x3 <- as.factor(x1) # convert to factor
print(x3) # factor
## [1] 1 1 2 2 3 3 4 4
## Levels: 1 2 3 4
summary(x3) # compare against summary(x1) above 
## 1 2 3 4 
## 2 2 2 2

2.3 Complex objects

Simple objects, like the ones introduced above, may be combined into composite objects. For example, we may combine all pancake ingredients into a complex object of class list.

In R we often use a particular complex object, a data frame, to hold various data together. A data frame is a complex object like an Excel worksheet or SPSS data sheet. The columns represent variables, and the rows represent single observations — these may be “cases” or sampling units, or single measurements repeated for each sampling unit, depending on the study 3.

The easiest way to create a data object is to read it from a plain-text (ASCII) file, using the command read.table. (Windows users must remember to use double backslashes in the file specification string). An optional header=TRUE argument indicates whether the first line contains the names of the variables; argument sep specifies the character(s) that separate the variables in the input file. The file argument can be a string specifying a local file, or a url to a web-based file, or a call of function file.choose() to select a file interactively. Argument na.strings specifies the character string(s) that indicate missing values in the input file.

# in Windows system
myexp <- read.table(
  file="f:\\temp\\mydata.txt", header=T, sep="," )
nlspkr <- read.table(
  file=url("http://www.hugoquene.nl/emlar/intra.bysubj.txt"),
  header=TRUE, na.strings=c("NA","MISSING") )

It is also possible to read so-called CSV files (comma-separated values) saved from Excel or SPSS (read.csv), and it is also possible to read Excel or SPSS data files directly using extension packages (readxl::readxl, foreign::read.spss, see Chapter 8).

The basic R and extension packages already have many datasets pre-defined, for immediate use. To see a long(!) overview of these datasets, enter the command data().