#############
# Section 8
#############


#Logical vectors

data(mtcars)

guzzlers <- (mtcars$mpg < 20)

guzzlers

names(guzzlers) <- row.names(mtcars)

guzzlers

small_engines=(mtcars$cyl==4)

names(small_engines)=row.names(mtcars)

small_engines


mtcars$mpg

(mtcars$mpg == 21)

(mtcars$mpg != 21)

(mtcars$mpg >= 21)

(mtcars$mpg <= 21)

(mtcars$mpg <= 1:32)

(mtcars$mpg <= 21:22)


eq21 <- (mtcars$mpg == 21)

eq21

!eq21

#Logical operators

eq21 & guzzlers

eq21 | guzzlers

all(eq21)

any(eq21)

eq21

sum(eq21)

mean(eq21)

# Note there are '&&' and '||" logical operators which are not as generally useful
# From the help file: The longer form evaluates left to right examining only the first element of each vector. 
# Evaluation proceeds only until the result is determined. 
# The longer form is appropriate for programming control-flow and typically preferred in if clauses. 


# Handling missing values (NAs)

x=c(10,NA,20)
x<15
x>15
sum(x>15)
sum(x>15,na.rm=T)

guzzlers&small_engines

guzzlers|small_engines

guzzlers&!small_engines

any(mtcars$mpg>40)


a <- sqrt(2)

a * a == 2

a * a - 2

all.equal(a*a, 2)

#The if statement
x=rnorm(1)
y=rnorm(1)
x>y
if(x>y) {z=x^2; w=30*x} else {z=30*x; w=x^2}

if(x>y) {
z=x^2
w=30*x
} else {
z=30*x
w=x^2
}

#Look at z and w closely--they're based only on x[1] vs. y[1]
x=rnorm(5)
y=rnorm(5)
x>y
if(x>y) {z=x^2; w=30*x} else {z=30*x; w=x^2}
cbind(x,y,z,w)

# use ifelse for elementwise conditional logic:

x=rnorm(5)
y=rnorm(5)
x>y
z = ifelse(test = x>y, yes = x^2, no = 30*x) #explicitly including argument names
w = ifelse(x>y, 30*x, x^2)

cbind(x,y,z,w)


# look at "conditional.txt" file

#The ifelse statement

matwithNAs <- matrix(c(3,6,NA,7,1,5,NA,NA,10), nrow=3,ncol=3, byrow=T)

mat.replace <- ifelse(is.na(matwithNAs), 0, matwithNAs)

mmp.truncated <-ifelse(mtcars$mpg>20,20,mtcars$mpg)