TD programming (homework)

Writing a function

Objective: write a for loop and understand it focuses on the iteration and not actions

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

write a R function that takes a vector as an argument and returns the number of odd numbers. For example, passing the vector c(1, 2, 3, 4) to the function odd_count should output:

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

## [1] 2

1. 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

2. Do it using R

Version 0, not a function

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

Version 1

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

Tip

remenber that function arguments are then local to the function’s environmen
remember the modulo operation is a %% b in R and returns the reminder 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

Version 2: Making a better function, sanity check.

What would happen if we provide some characters, or a matrix, as an input to odd_count()? 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 stop("Warning! some conditions are not fulfilled to continue bla bla \n") should help. It stops the execution and print the chosen message

Version 3: Customize the function, multiple arguments.

Now we would like to count only unique numbers that are odd. Change the function so that it takes a second argument uniq changing this behaviour: a boolean (TRUE/FALSE).

When set to TRUE, only unique numbers should be considered (see the function unique()). When set to FALSE (which we would like to be the default value), the result should be equal to version 2.

Chart of outcome

Please check that your 3 versions have the expected behaviour using the following table:

input	version 1	version 2	version 3 (unique = TRUE)
c(0, 1)	1	1	1
c(0, 1, 2, ‘A’)	nasty error	input not numeric!	input not numeric!
c(0, 1, 2, 3, 5)	3	3	3
c(0, 1, 2, 3, 5, 3)	4	4	3

Version 4, vectorized

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

replace all the for loop code that one line of code using vectorization

Tip

remenber that TRUE is 1 and FALSE is 0, so summing a logical vector truly helps here.