Conditionals

• Usage:
  • Generate "logical":
    • TRUE if the condition is satisfied
    • FALSE if the condition is not satisfied
• Operators:
  • ==, !=
  • <, >
  • <=, >=

10 > 5
"aang" == "aang"
3 != 3

• Combine:
  • and, &
  • or, |

5 > 2 & 6 >=10
5 > 2 | 6 >=10

• Appearances:
  • subset()
  • dplyr::filter()
  • if(), else, while()

Do Exercise 1 - Choice Operators.

Discuss floating point with students.

• Did you notice anything weird while you were doing the exercise?
• Take a closer look at item 4.

Floating point

> x <- 1.3
> y <- 2.8
>  x * 2 + 0.2 == y
[1] FALSE
> 1.3 * 2 + 0.2
[1] 2.8

• What’s going on?
  • Unexpected result from computer arithmetic
  • Numbers combine to include very small trailing digit
    • See this more easily in Python

>>> 1.3 * 2 + 0.2 == 2.8
False
>>> 1.3 * 2 + 0.2
2.8000000000000003

• Avoid floating point problems.
  • round(x * 2 + 0.2, 1) == y
  • all.equal(x * 2 + 0.2, y)

if statements

• Conditional statements generate "logical" to filter inputs.
• if statements use conditional statements to control flow of program processing.
• if (the conditional statement is TRUE ) { do something }
  • ‘Who is the best superhero?’

superhero <- "Batman"
if (superhero == "Batman") {
  print("That is correct")
  }
print("Done")

superhero <- "Black Widow"

• } else { do a different something }

if (superhero == "Batman") {
  print("That is correct")
} else {
  print("Please try again")
}
print("Done")

• } else if ( a different conditional statement is TRUE ) {
  • do another different something

if (superhero == "Batman") {
  print("That is correct")
} else if (superhero == "Black Widow") {
  print("OK, you may have a point")
} else {
  print("Please try again")
}
print("Done")

superhero <- "Flash"

Convert to function

is_best_superhero <- function(superhero) {
  if (superhero == "Batman") {
	print("That is correct")
  } else if (superhero == "Black Widow") {
	print("OK, you may have a point")
  } else {
	print("Please try again")
  }
}

is_best_superhero("Hulk")

• OK. The Hulk is awesome!

is_best_superhero <- function(superhero) {
  if (superhero == "Batman") {
	print("That is correct")
  } else if (superhero == "Black Widow" | superhero == "Hulk") {
	print("OK, you may have a point")
  } else {
	print("Please try again")
  }
}

is_best_superhero("Hulk")

Do Exercise 2 - Complete the Code.