## *7. R Data Structures
## 7.1 Vectors
## 7.1.1 Subsets of Vectors
c(Andreas=178, John=185, Jeff=183)[c("John","Jeff")]
## 7.1.2 Patterned Data
rep(c(2,3,5),4)
rep(c(2,3,5),c(4,4,4))    # An alternative is rep(c(2,3,5), each=4)

## 7.2 Missing Values
x <- c(1,6,2,NA)
is.na(x)  # TRUE for when NA appears, and otherwise FALSE
x==NA     # All elements are set to NA
NA==NA
y[x>2] <- x[x>2]

## 7.3 Data frames
## 7.3.1 Extraction of Component Parts of Data frames
names(barley)
levels(barley$site)
Duluth1932 <- barley[barley$year=="1932" & barley$site=="Duluth",
+ c("variety","yield")]
## 7.3.2 Data Sets that Accompany R Packages
data(package="datasets")

## 7.4 Data Entry Issues
## 7.4.1 Idiosyncrasies
## 7.4.2 Missing values when using      read.table()
## 7.4.3 Separators when using read.table()

## 7.5 Factors and Ordered Factors
as.character(islandcities$country)
unclass(islandcities$country)
table(islandcities$country)
lev <- levels(islandcities$country)
lev[c(7,4,6,2,5,3,1)]
country <- factor(islandcities$country, levels=lev[c(7,4,6,2,5,3,1)])
table(country)

## 7.6 Ordered Factors
stress.level<-rep(c("low","medium","high"),2)
ordf.stress<-ordered(stress.level, levels=c("low","medium","high"))
ordf.stress
ordf.stress<"medium"
ordf.stress>="medium"
class(ordf.stress)

## 7.7 Lists
elastic.lm <- lm(distance~stretch, data=elasticband)
names(elastic.lm)
elastic.lm$coefficients
elastic.lm[["coefficients"]]
elastic.lm[[1]]
elastic.lm[1]
options(digits=3)
elastic.lm$residuals
elastic.lm$call
mode(elastic.lm$call)

## *7.8 Matrices and Arrays
xx <- matrix(1:6,ncol=3)  # Equivalently, enter matrix(1:6,nrow=2)
xx
x <- as.vector(xx)
dim(xx)
[1] 2 3
x34 <- matrix(1:12,ncol=4)
x34
x34[2:3,c(1,4)]  # Extract rows 2 & 3 & columns 1 & 4
x34[2,]  # Extract the second row
x34[-2,]  # Extract all rows except the second
x34[-2,-3]  # Extract the matrix obtained by omitting row 2 & column 3
## 7.8.1 Arrays
x <- 1:24
dim(x) <- c(2,12)
x
dim(x) <-c(3,4,2)
x
## 7.8.2 Conversion of Numeric Data frames into Matrices

## 7.9 Exercises
## a)
answer <- c(2, 7, 1, 5, 12, 3, 4)
for (j in 2:length(answer)){ answer[j] <- max(answer[j],answer[j-1])}
## b)
answer <- c(2, 7, 1, 5, 12, 3, 4)
for (j in 2:length(answer)){ answer[j] <- sum(answer[j],answer[j-1])}