Lecture 3

Xiaotong Suo

Logis1cs

HW 1 due on Wednesday before the class. No late homework will be accepted. Note that you have to get at least 60% on each homework to pass this class.

Todays agenda

Matrix Factor List Dataframe

Matrix con1ued The func1on order returns the ascending (default) or descending order of elements of a vector. For example, order(0.5,5,-3.1) will return 3, 1, 2. Its sister func1on is sort. We always have x[order(x)]=sort(x).

The matrix stack.x is a readily available matrix in R. It has three names columns: Air.Flow, Water.Temp, Acid.Conc.. The meaning of these numbers are not very important here.

Order the rows of the matrix by increasing values of the column AirFlow.

Factor

In R a factor is a vector that hold categorical data. (for example, Yes-No answers). f1=factor(Yes,levels=c(Yes,No)) f1[2]=No f1[3]=Maybe f1[3]=No f1[4]=No table(f1)

Factor

Some1mes we want to keep an ordering between the levels of the factors data=sample(le]ers[1:10],size=50,replace=T) f2=factor(data,levels=le]ers[1:10],ordered=T)

Factor con1nued

Some1mes it is useful to convert a numerical vector into factor.

The func1on cut can be useful in doing so. data=stack.x[,Water.Temp] f3=cut(data,breaks=3,labels=c(Low,Medium,High))

#we can add the ordering f3=cut(data,breaks=3, labels=c(Low,Medium,High),ordered=T)

List

Lists are collec1ons, like vectors. But unlike vectors, the same list can hold dierent types of objects.

Lists also have the a]ribute names that you can set and use to access the components.

You can also access the components using the operator [[ ]] not [ ] as for vectors and matrices.

Why use lists: it allows you to put together objects of dierent types. Vectors dont.

List con1nued

V=c(A,A,C,B,B); X=1:5; Y=rnorm(5); L=list(X,Y,V) names(L) names(L)=c(V1,V2,V3) L$V1 L[[3]]

Data frame

Roughly a data frame is a rectangular table with rows and columns. The columns represent variables and can be of dierent types (numeric, integer, factor, character); a major dierence from matrices. The rows are records or cases.

Typically we create data frames by reading data from les. We can also create data frames from vectors with the func1on data.frame.

You can convert a data frame to a matrix with data.matrix.

Data frame con1nued

V=factor(c(A,A,C,B,B)); X=1:5; Y=rnorm(5); dt1=data.frame(X,Y,V) names(dt1)=c(meas1,meas2,meas3)

Data frame con1nued

We think of data frames both as a matrix (a rectangular table with rows and columns) and as a list (a list of sta1s1cal variables on which we have observa1ons).

#Con1nuing with the previous example: dt1[1,1] dt1$meas1 dt1[[1]]

Data frame con1nued Working with data frames:

dt2=airquality dim(dt2); names(dt2) edit(dt2) #Compare and understand the following mean(dt2$Ozone); mean(dt2$Ozone,na.rm=T) #Be]er yet:complete.cases Ind=complete.cases(dt2) dt=dt2[Ind,] mean(dt$Ozone) #or mean(dt[,1]) or mean(dt[[1]]) plot(dt$Ozone,type=l)

Next Lecture

Data Input/output Graphics More func1ons