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Lab 6: Simple Conditionals

Gaston Sanchez

Learning Objectives

  • Get into the habit of writing simple functions
  • Making sure your functions work
  • Understand the concept of vectorization

If Conditionals

Write R code that will "squish" a number into the interval [0, 100], so that a number less than 0 is replaced by 0 and a number greater than 100 is replaced by 100.

z <- 100*pi
# Fill in the following if-else statements. You may (or may not) 
# have to add or subtract else if or else statements.
if (TRUE) { # Replace TRUE with a condition.
  
} else if (TRUE) { # Replace TRUE with a condition.
  
} else {
  
}
## NULL

Multiple If's

A common situation involves working with multiple conditions at the same time. You can chain multiple if-else statements:

y <- 1 # Change this value!

if (y > 0) {
  print("positive")
} else if (y < 0) {
  print("negative")
} else {
  print("zero?")
}
## [1] "positive"

Even number

Write a function is_even() that determines whether a number is even (i.e. multiple of 2). If the input number is even, the output should be TRUE. If the input number is odd, the output should be FALSE. If the input is not a number, the output should be NA

For example:

# even number
is_even(10)
## [1] TRUE

# odd number
is_even(33)
## [1] FALSE

# not a number
is_even('a')
## [1] NA

Odd number

Use your function is_even() to write a function is_odd() that determines if a number is odd (i.e. not a multiple of 2). If a number is odd, the output should be TRUE; if a number is even the output should be FALSE; if the input is not a number the output should be NA

For example:

# odd number
is_odd(1)
## [1] TRUE

# even number
is_odd(4)
## [1] FALSE

# not a number
is_odd('a')
## [1] NA

Switch

Working with multiple chained if's becomes cumbersome. Consider the following example that uses several if's to convert a day of the week into a number:

# Convert the day of the week into a number.
day <- "Tuesday" # Change this value!

if (day == 'Sunday') {
  num_day <- 1
} else {
  if (day == "Monday") {
    num_day <- 2
  } else {
    if (day == "Tuesday") {
      num_day <- 3
    } else {
      if (day == "Wednesday") {
        num_day <- 4
      } else {
        if (day == "Thursday") {
          num_day <- 5
        } else {
          if (day == "Friday") {
            num_day <- 6
          } else {
            if (day == "Saturday") {
              num_day <- 7
            }
          }
        }
      }
    }
  }
}


num_day
## [1] 3

Working with several nested if's like in the example above can be a nigthmare.

In R, you can get rid of many of the braces like this:

# Convert the day of the week into a number.
day <- "Tuesday" # Change this value!

if (day == 'Sunday') {
  num_day <- 1
} else if (day == "Monday") {
  num_day <- 2
} else if (day == "Tuesday") {
  num_day <- 3
} else if (day == "Wednesday") {
  num_day <- 4
} else if (day == "Thursday") {
  num_day <- 5
} else if (day == "Friday") {
  num_day <- 6
} else if (day == "Saturday") {
  num_day <- 7
}

num_day
## [1] 3

But still we have too many if's, and there's a lot of repetition in the code. If you find yourself using many if-else statements with identical structure for slightly different cases, you may want to consider a switch statement instead:

# Convert the day of the week into a number.
day <- "Tuesday" # Change this value!

switch(day, # The expression to be evaluated.
  Sunday = 1,
  Monday = 2,
  Tuesday = 3,
  Wednesday = 4,
  Thursday = 5,
  Friday = 6,
  Saturday = 7,
  NA) # an (optional) default value if there are no matches
## [1] 3

Switch statements can also accept integer arguments, which will act as indices to choose a corresponding element:

# Convert a number into a day of the week.
day_num <- 3 # Change this value!

switch(day_num,
  "Sunday",
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
  "Saturday")
## [1] "Tuesday"

Converting Miles to other units

The table below shows the different formulas for converting miles (mi) into other scales:

Units Formula
Inches mi x 63360
Feet mi x 5280
Yards mi x 1760
Meters mi / 0.00062137
Kms mi / 0.62137

Write the following five functions for each type of conversion. Each function must take one argument x with default value: x = 1.

  • miles2inches()
  • miles2feet()
  • miles2yards()
  • miles2meters()
  • miles2kms()

For example:

miles2inches(2)
## [1] 126720

miles2feet(2)
## [1] 10560

miles2yards(2)
## [1] 3520

miles2meters(2)
## [1] 3218.694

miles2kms(2)
## [1] 3.218694

Using switch()

Create a function convert() that converts miles into the specified units. Use switch() and the previously defined functions---miles2inches(), miles2feet(), ..., miles2kms---to define convert(). Use two arguments: x and to, like this:

convert(40, to = "in")

By default, to = "km", but it can take values such as "in", "ft", "yd", or "m".

For instance:

convert(3, "in")
convert(3, "ft")
convert(3, "yd")
convert(3, "m")
convert(3, "km")

One more swtich

Write a switch statement to determine if a given lowercase letter is a vowel. Assume that "y" is not a vowel.

letter <- "e" # Change this value!

switch(letter,
  a = "vowel") # Modify as necessary.