Write a function

Objective

Write a for loop and understand it focuses on the iteration and not actions

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Take 5 min to watch this video by Hadley Wickham, until 5 min.

Being lazy, instead of you doing the work, rely on the work of someone else | Hadley Wickham

Aim: write a function that takes a vector as an argument and returns if they are even numbers.

For example, passing the vector c(1, 3, 2, 4) to the function is_even() should output:

is_even(c(1, 3, 2, 4))

[1] FALSE FALSE  TRUE  TRUE

Write the pseudocode

The structure

• Here we would like to introduce the use of different programming statements you learned, such as if, else if, else or a for loop.
• Write, in human terms (or pseudocode), how the function could be constructed

Solution

1. Initialize a res vector with logicals
2. For each element of the argument
3. Test if the element is an even number
4. Return res

Do it using R

Version 0, not in a function

Start by writing a piece of code that does the job, following the steps of your pseudocode

Solution

# test vector named a
x <- c(1, 3, 2, 4)
# initialized a vector at 0
res <- vector(mode = "logical", length = length(x))
for (i in x) { # iterate all elements, R will do from 1 to length(x)
  res[i] <- x[i] %% 2 == 0  # %% is the modulo operator
}
res

[1] FALSE FALSE  TRUE  TRUE

Convert your functional piece of code as a function named is_even()

Tip

• Remember that function arguments are then local to the function’s environment
• Remember the modulo operation is a %% b in R and returns the remainder of the division of a/b. If b = 2 and the result is different from zero, then a is an odd number.
• Remember to test if a equals b using == and not =. a = b is an assignment leading a to take the value of b!
• Returning the expected value, should be placed outside the for loop

Solution

x <- c(1, 3, 2, 4)
is_even <- function(x) {
  res <- vector(mode = "logical", length = length(x))
  for (i in x) { # iterate all elements, R will do from 1 to length(x)
    res[i] <- x[i] %% 2 == 0  # %% is the modulo operator
  }
  res
}
is_even(x)

[1] FALSE FALSE  TRUE  TRUE

Defensive programming, sanity check.

What would happen if we provide some characters, or a matrix, as an input to is_even()? Is your function able to deal with such an input?

No, but it should. Add a test before doing anything else to check if numbers are provided to the function.

Tip

The R code:

stop("Warning! some conditions are not fulfilled to continue bla bla \n")

should help. It stops the execution and print the chosen message. To prevent a chunk to stop the rendering with a desired error, use the option

#| error: true

Solution

x <- c(1, 3, 2, 4)

is_even <- function(x) {
  # if the input is not numeric, we stop the code execution
  # of note, the call. = FALSE is to avoid printing the code line, makes the error more explicit
  if (!is.numeric(x)) stop("input not numeric!", call. = FALSE)
  if (length(x) == 0) stop("input cannot be empty!", call. = FALSE)
  res <- vector(mode = "logical", length = length(x))
  for (i in x) { # iterate all elements, R will do from 1 to length(x)
    res[i] <- x[i] %% 2 == 0  # %% is the modulo operator
  }
  res
}
is_even(x)

[1] FALSE FALSE  TRUE  TRUE

is_even("a")

Error: input not numeric!

is_even(double(0))

Error: input cannot be empty!

Vectorization

Now that the for loop was a pain to set-up, we can use what is good at: vectorization

Replace all the for loop code with one line of vectorization

Solution

is_even <- function(x) {
  if (!is.numeric(x)) stop("input not numeric!", call. = FALSE)
  # the vector is integer divided by 2, the iteration is transparent
  # then logical test: is different from 0
  # finally sum up the logical vector, TRUE is 1, FALSE is 0
  x %% 2 == 0
}

is_even(x)

[1] FALSE FALSE  TRUE  TRUE

Refinements

Create a new function that returns the number of even items

Such as:

count_even(c(1, 3, 3, 4))

[1] 1

count_even(c(1, 3, 3, 4, 4))

[1] 2

Modify again the function so it returns the sum of even items

Such as:

sum_even(c(1, 3, 3, 4))

[1] 4

sum_even(c(1, 3, 3, 4, 4))

[1] 8