#
# Unit 7 - Vectorization 
#
# Filtering vectors 
#


# Problem 
# Find all names of students 
# whose grade is under 60. 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 41)
myNames <- c() 
for (ind in 1:length(grades)) {
  if (grades[ind] < 60) {
    myNames <- c(myNames, names[ind]) 
  }
}
print(myNames)


# Here to R's vectorization provides
# a more efficient code. 
# Note the new context of using 
# the [ ] operator 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 41)
under60 <- names[grades < 60]
print(under60)



# A deeper look on the above code. 

# We know what this does. 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
result <- names[c(1, 4)]
print(result)


# Now look at these. 
names <- c(      "Yasmin", "Bella", "Adam", "Ruth")
result <- names[c(FALSE,    TRUE,    TRUE,   FALSE)]
print(result)


names <- c("Yasmin", "Bella", "Adam", "Ruth")
result <- names[grades < 60]
print(result)


# Now the condition we use within
# the brackets, produces a boolean 
# vector. 
grades <- c(52, 93, 82, 41)
vec <- grades < 60 
print(vec)


# So actually what we are doing here: 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 41)
under60 <- names[grades < 60]

# is this 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
under60 <- names[c(TRUE, FALSE, FALSE,  TRUE)]
print(under60)

# Problem:
# Find all names of students whose 
# grade is above 50 and under 90. 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 41)
myNames <- names[grades > 50 & grades < 90]
print(myNames)

vec1 <- grades > 50 
print(vec1)
vec2 <- grades < 90
print(vec2)
print(vec1 & vec2)
print(vec1 | vec2)

# Problem:
# Find all names of students whose 
# grade is NOT (above 50 and under 90). 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 41)
myNames <- names[!(grades > 50 & grades < 90)]
print(myNames)


# Now look at this code: 
# What happens if the vectorization 
# is carried out on vectors of different 
# lengths? 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82)
under60 <- names[grades < 60]
print(under60)


# Problem: 
# Find all names of students who received
# a grade in the following group of grades. 
names <- c("Yasmin", "Bella", "Adam", "Ruth")
grades <- c(52, 93, 82, 58)
gradeGroup <- c(52, 82, 93)
myNames <- names[grades %in% gradeGroup]


print(grades %in% gradeGroup)



# Finally: replacement
# Problem:
# Set as 100 the grades of all students 
# in third year.  
year <- c(1, 2, 3, 3)
grades <- c(52, 93, 82, 58)
grades[year == 3] <- 100
print(grades)

year <- c(1, 2, 3, 3)
grades <- c(52, 93, 82, 58)
grades[year == 3] <- c(100, 100) 


year <- c(1, 2, 3, 3)
grades <- c(52, 93, 82, 58)
grades[year == 3] <- c(100, 100) 


# Increase by 5 the grades of all students 
# in third year.  
year <- c(1, 2, 3, 3)
grades <- c(52, 93, 82, 58)
grades[year == 3] <- grades[year == 3] + 5


# Last problem: 
# Replace with 100 each grade > 90, 
# and with 0 each grade <= 90 
# 1
grades <- c(52, 93, 82, 58)
grades[grades > 90] <- 100
grades[grades <= 90] <- 0
print(grades)

# 2
grades <- c(52, 93, 82, 58)
grades <- ifelse(grades > 90, 100, 0) 
print(grades)

print(ifelse(grades > 90, 100, 0) )