# Data Types

R has a wide variety of data types including scalars, vectors (numerical, character, logical), matrices, data frames, and lists.

## Vectors

``` a <- c(1,2,5.3,6,-2,4) # numeric vector b <- c("one","two","three") # character vector c <- c(TRUE,TRUE,TRUE,FALSE,TRUE,FALSE) #logical vector ```

Refer to elements of a vector using subscripts.

` a[c(2,4)] # 2nd and 4th elements of vector`

## Matrices

All columns in a matrix must have the same mode(numeric, character, etc.) and the same length. The general format is

mymatrix <- matrix(vector, nrow=r, ncol=c, byrow=FALSE,
dimnames=list(
char_vector_rownames, char_vector_colnames))

byrow=TRUE indicates that the matrix should be filled by rows. byrow=FALSE indicates that the matrix should be filled by columns (the default). dimnames provides optional labels for the columns and rows.

``` # generates 5 x 4 numeric matrix y<-matrix(1:20, nrow=5,ncol=4) # another example cells <- c(1,26,24,68) rnames <- c("R1", "R2") cnames <- c("C1", "C2") mymatrix <- matrix(cells, nrow=2, ncol=2, byrow=TRUE,   dimnames=list(rnames, cnames)) ```

Identify rows, columns or elements using subscripts.

``` x[,4] # 4th column of matrix x[3,] # 3rd row of matrix x[2:4,1:3] # rows 2,3,4 of columns 1,2,3 ```

## Arrays

Arrays are similar to matrices but can have more than two dimensions. See help(array) for details.

## Data Frames

A data frame is more general than a matrix, in that different columns can have different modes (numeric, character, factor, etc.). This is similar to SAS and SPSS datasets.

```d <- c(1,2,3,4) e <- c("red", "white", "red", NA) f <- c(TRUE,TRUE,TRUE,FALSE) mydata <- data.frame(d,e,f) names(mydata) <- c("ID","Color","Passed") # variable names ```

There are a variety of ways to identify the elements of a data frame .

```myframe[3:5] # columns 3,4,5 of data frame myframe[c("ID","Age")] # columns ID and Age from data frame myframe\$X1 # variable x1 in the data frame ```

## Lists

An ordered collection of objects (components). A list allows you to gather a variety of (possibly unrelated) objects under one name.

```# example of a list with 4 components - # a string, a numeric vector, a matrix, and a scaler w <- list(name="Fred", mynumbers=a, mymatrix=y, age=5.3) # example of a list containing two lists v <- c(list1,list2) ```

Identify elements of a list using the [[]] convention.

``` mylist[] # 2nd component of the list mylist[["mynumbers"]] # component named mynumbers in list```

## Factors

Tell R that a variable is nominal by making it a factor. The factor stores the nominal values as a vector of integers in the range [ 1... k ] (where k is the number of unique values in the nominal variable), and an internal vector of character strings (the original values) mapped to these integers.

```# variable gender with 20 "male" entries and # 30 "female" entries gender <- c(rep("male",20), rep("female", 30)) gender <- factor(gender) # stores gender as 20 1s and 30 2s and associates # 1=female, 2=male internally (alphabetically) # R now treats gender as a nominal variable summary(gender) ```

An ordered factor is used to represent an ordinal variable.

```# variable rating coded as "large", "medium", "small' rating <- ordered(rating) # recodes rating to 1,2,3 and associates # 1=large, 2=medium, 3=small internally # R now treats rating as ordinal ```

R will treat factors as nominal variables and ordered factors as ordinal variables in statistical proceedures and graphical analyses. You can use options in the factor( ) and ordered( ) functions to control the mapping of integers to strings (overiding the alphabetical ordering). You can also use factors to create value labels. For more on factors see the UCLA page.

## Useful Functions

```length(object) # number of elements or components str(object)    # structure of an object class(object)  # class or type of an object names(object)  # names c(object,object,...)       # combine objects into a vector cbind(object, object, ...) # combine objects as columns rbind(object, object, ...) # combine objects as rows object     # prints the object ls()       # list current objects rm(object) # delete an object newobject <- edit(object) # edit copy and save as newobject fix(object)               # edit in place ```

## To Practice

To explore data types in R, try this free interactive introduction to R course