Control Flow

Choices and Iteration

Last Time

1. A list is a non-atomic vector: types can be mixed.
2. [] subsets a list and returns a list.
3. [[ ]] and $ return the object inside the list.
4. Lists can nest, so subsetting can be chained.
5. $ allows subsetting by name.

Key Ideas for Today

1. Use comparison operators to compare two values.
2. if-then runs code only when a condition is TRUE.
3. if-else chooses between two blocks of code based on condition.
4. A for loop repeats code for each element of a vector.
5. ifelse() is a vectorized if-else.

Concept Acquisition

1. If-then executes code only when its condition evaluates to TRUE; otherwise nothing happens.
2. If-else guarantees that exactly one of two blocks runs.
3. Chaining else if clauses lets a decision tree with many branches be expressed in R.
4. A for loop repeats an action once per element, with a loop variable taking each value (or index) in turn.
5. Wrapping an if-then inside a for loop applies conditional logic element-by-element; ifelse() does the same thing vectorized, without an explicit loop.

Tool Acquisition

1. if (condition) { ... }
2. if (condition) { ... } else { ... }
3. if (condition) { ... } else if (condition) { ... } else { ... }
4. for (item in vector) { ... }
5. ifelse(test, yes, no)

Concept Application

1. Translate a decision tree (e.g. how to get to campus) into nested if-then statements.
2. Predict the output of a for loop paired with an if statement, run over a vector of random values.
3. Rewrite an explicit for + if loop as a single vectorized ifelse() call.

Comparisons

Comparison operators

Comparison Operator

A symbol that compares two values and returns a logical: TRUE if the comparison holds, FALSE otherwise.

<
>
<=
>=

==
!=
%in%

Live code simple numerical examples like a <- 3; a < 5

Examples

c("apple", "orange", "apple", "kiwi") == "apple"

[1]  TRUE FALSE  TRUE FALSE

"apple" == c("apple", "orange", "apple", "kiwi")

[1]  TRUE FALSE  TRUE FALSE

"apple" %in% c("apple", "orange", "apple", "kiwi")

[1] TRUE

c("apple", "orange", "apple", "kiwi") %in% "apple"

[1]  TRUE FALSE  TRUE FALSE

Examples

c("apple", "kiwi") %in% c("apple", "orange", "apple", "kiwi")

[1] TRUE TRUE

c("apple", "orange", "apple", "kiwi") %in% c("apple", "kiwi")

[1]  TRUE FALSE  TRUE  TRUE

• Don’t forget logicals are 1s and 0s.

Examples

m <- matrix(1:4, nrow = 2)
m

     [,1] [,2]
[1,]    1    3
[2,]    2    4

m > 2

      [,1] [,2]
[1,] FALSE TRUE
[2,] FALSE TRUE

sum(m > 2)

[1] 2

sum(m[m > 2])

[1] 7

Choices

Ask: raise your hand if you walked here today. Took a scooter? Drove? Took the bus?

Do you always do that routine or do you decide how to get here based on conditions of the moment?

Getting to Campus

If my bike is working
     then I ride my bike to campus.

If my bike is not working and I have enough time
     then I ride the bus to campus.

If my bike is not working and I don’t have enough time
     then I take a cab.

A random condition in R

sample(x = -10:10, size = 1)

[1] -4

sample() will randomly sample 1 element from the vector x.

if-then Statement

Executes a block of code only when a condition evaluates to TRUE; otherwise the block is skipped.

x <- sample(x = -10:10, size = 1)

if (x > 0) {
  print("x is positive :)")
}

Drop:

• Run this multiple times to see how it’s random.
• Could set x <- 1 to make it deterministic or even just TRUE.
• Annotate code with different colors: if-then syntax, logical condition, and what to do.

if-else Statement

Executes one block of code if a condition is met and a different block if it is not — exactly one branch always runs.

x <- sample(x = -10:10, size = 1)

if (x > 0) {
  print("x is positive :)")
} else {
  print("x is negative :(")
}

Similarly, run it multiple times and annotate code with different colors: if-then syntax, logical condition, and what to do.

Nested ifs

x <- sample(x = -10:10, size = 1)

if (x > 0) {
  print("x is positive :)")
} else if (x < 0) {
  print("x is negative :(")
} else {
  print("x is zero :|")
}

Run it multiple times, then get the zero to toggle using x <- 0

Getting to Campus

If my bike is working
     then I ride my bike to campus.

If my bike is not working and I have enough time
     then I ride the bus to campus.

If my bike is not working and I don’t have enough time
     then I take a cab.

Your Turn

Using the R objects bike_working and enough_time, both logical vectors of length 1, write this decision tree in R using nested if-then statements and print().

02:30

if (bike_working) {
  print("ride bike")
} else if (enough_time) {
  print("ride bus")
} else {
  print("call cab")
}

If time you can set the values of bike_working and enough_time to verify that it works.

Iteration

For Loop

Repeats a block of code once for each element of a vector, with a loop variable taking each value in turn.

Prof. Sanchez Slides

for (item in vector) {
    perform_action
}

Example

Goal: Build the first 10 elements of the Fibonacci Sequence.

n <- 10
fib <- rep(1, n) # pre-allocate a double vector

for (i in 3:n) {
  fib[i] <- fib[i - 1] + fib[i - 2]
}

fib

 [1]  1  1  2  3  5  8 13 21 34 55

Iterated Choices

What if x has multiple elements?

x <- rnorm(3)
x

[1] -0.2155415 -1.0925180  0.7512622

      if (x > 0) {
        print("x is positive :)")
      }

Annotate this if-then by wrapping it in a for loop.

Option 1

for (i in 1:length(x)) {
  if (x[i] > 0) {
    print("x is positive :)")
  }
}

[1] "x is positive :)"

Option 2

for (i in 1:length(x)) {
  if (x[i] > 0) {
    print(paste("element", i, "is positive :)"))
  }
}

[1] "element 3 is positive :)"

Option 3

for (x_elem in x) {
  if (x_elem > 0) {
    print(paste(x_elem, "is positive :)"))
  }
}

[1] "0.751262154884219 is positive :)"

ifelse()

ifelse()

A vectorized if-else: applies a condition to every element of a vector at once, without an explicit loop.

x < 0 

[1]  TRUE  TRUE FALSE

ifelse(test = x < 0, yes = "is positive :)", no = "is negative :(")

[1] "is positive :)" "is positive :)" "is negative :("

Guess the metaphor

An if-then statement is like a fork in the road: the computer will take one of two paths based on a condition.

Key Ideas for Today

1. Use comparison operators to compare two values.
2. if-then runs code only when a condition is TRUE.
3. if-else chooses between two blocks of code based on condition.
4. A for loop repeats code for each element of a vector.
5. ifelse() is a vectorized if-else.

For Next Time

1. Practice with Control Flow
2. Finish Project and prepare for Project Studio 1!