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Control structures (loops/ conditional executions)

ifelse: vectorization with ifelse

ifelse(condition, TRUE condition output, FALSE condition output)

Example

test_even_odd_v2 <- function(x){
  ifelse(x %% 2 == 0, "even number", "odd number")
}

test_even_odd_v2(5)

FALSE [1] "odd number"

test_even_odd_v2(c(1,6))

FALSE [1] "odd number"  "even number"

Difference between if, else and ifelse

test_even_odd <- function(x){
  if (x %% 2 == 0) {
    print("even number")
  } else {
    print("odd number")
  }
}

test_even_odd(5)

FALSE [1] "odd number"

test_even_odd(c(1,6))
# returns an error

test_even_odd_v2 <- function(x){
  ifelse (x %% 2 == 0, "even number", "odd number")
}
test_even_odd_v2(5)

FALSE [1] "odd number"

test_even_odd_v2(c(1,6))

FALSE [1] "odd number"  "even number"

Nested if-else

• Multiple conditions

grade_marks <- function(marks){
  if (marks < 20) {
    "D"
  } else if (marks <= 50) {
    "C"
  } else if (marks <= 60) {
    "B"
  } else {
    "A"
  }
}
grade_marks(75)

[1] "A"

R for Data Science-Exercises 19.4.4 - Q2

Help:

lubridate::now() and lubridate::hour()

if

check.negative <- function(x){
  if (x < 0 ) {
  print("X is negative")
  }
}

check.negative(-10)

[1] "X is negative"

check.negative(10)

for loop

• execute a block of code a specific number of times or until the end of a sequence.

for (i in 1:5) {
  print(i*100)
}

[1] 100
[1] 200
[1] 300
[1] 400
[1] 500

continents <- c("Asia", "EU", "AUS", "NA", "SA", "Africa")

for (i in 1:4) {
  print(continents[i])
}

[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"

for (i in 1:4)  print(continents[i])

[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"

for (i in seq(continents)) {
  print(continents[i])
}

[1] "Asia"
[1] "EU"
[1] "AUS"
[1] "NA"
[1] "SA"
[1] "Africa"

Write a for loop that iterates over the numbers 1 to 4 and prints the squared of each number using print().

Nested loops

a <- 1:10
testfun <- function(x){
for (x in 1:5) {
   if (x %% 2 == 0) {
    print("even number")
  } else {
    print("odd number")
  }
}}
testfun(a)

[1] "odd number"
[1] "even number"
[1] "odd number"
[1] "even number"
[1] "odd number"

Nested loops

mat <- matrix(1:6, ncol=2)
mat

     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

for (i in 1:3) {
  for (j in 1:2) {
    print(mat[i, j])
  }
}

[1] 1
[1] 4
[1] 2
[1] 5
[1] 3
[1] 6

Write a function to count the number of even numbers in a vector.

while

i <- 1 # initial value
while (i != 10) {
  print(i)
  i <- i + 1 # increment
}

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9

i <- 1 # initial value
while (i < 10) {
  print(i)
  i <- i + 1 # increment
}

[1] 1
[1] 2
[1] 3
[1] 4
[1] 5
[1] 6
[1] 7
[1] 8
[1] 9

Can you guess the output?

i <- 1 # initial value
while (i == 10) {
  print(i)
  i <- i + 1 # increment
}

Generate the first n numbers of the Fibonacci Sequence using while loop.

0 1 1 2 3 5 ...

repeat and break

• Iterate over a block of code multiple number of times.

• No condition check in repeat loop to exit the loop.

• The only way to exit a repeat loop is to call break.

repeat and break

x <- 5
repeat {
  print(x)
  x = x+1
  if (x == 10){
    break
  }
}

[1] 5
[1] 6
[1] 7
[1] 8
[1] 9

set.seed(1)
repeat {
 x<-runif(1, 5, 10)
 print(x)
 if(x < 6.1){
   break
 }
}

[1] 6.327543
[1] 6.860619
[1] 7.864267
[1] 9.541039
[1] 6.00841

Your turn

1. Write a function to print random numbers from the standard normal distribution but stops (breaks) if you get a number bigger than 1.

2. Using next adapt the loop from last exercise so that doesn’t print negative numbers.

next

for(i in 1:10) {
  if(i <= 5) {
  next # Skip the first 5 iterations
  }
print(i)
}

[1] 6
[1] 7
[1] 8
[1] 9
[1] 10

switch

When you want a function to do different things in different circumstances, then the switch function can be useful.

feelings <- c("sad", "afraid")
for (i in feelings){
  print(
  switch(i,
  happy = "I am glad you are happy",
  afraid = "There is nothing to fear",
  sad = "Cheer up",
  angry = "Calm down now"
  ))
}

[1] "Cheer up"
[1] "There is nothing to fear"