When you want to output a character vector for humans to read, you probably don’t want something like
 a b c, which is the normal way to print a vector in R. Instead, you may want a character string
"a, b, and c" (Oxford comma FTW!). In 2014, I gave a guest lecture in a course at Iowa State. I prepared an example in which I wanted to output the names of some genes dynamically with an inline R expression in a knitr document. In the beginning, I didn’t want to spend too much time on this task, so I just used
`r paste(x, collapse = ', ')`, which would output a character string like
"a, b, c". This did not feel natural in the context (e.g., “We identified genes a, b, c.”).
Driven by perfectionism, I decided to write a function, which ended up in the knitr package as
knitr::combine_words(). The implementation was actually quite simple. Basically, for a single word, it will just return this word; for two words A and B, it return
"A and B"; for three or more words, it returns
"A, B, C, ..., Y, and Z". The function also has a few arguments that can customize the output. For example, if you want to output the words in pairs of backticks, you may use
knitr::combine_words(x, before = '`'):
knitr::combine_words(LETTERS[1:5], before = '`') # output: `A`, `B`, `C`, `D`, and `E`
Since then, I have used this function by myself several times. Today I saw a Stack Overflow question that asked about the same thing, so I thought it might be a good idea to reveal this little function in knitr, although I’m relatively sure someone else must have also implemented it independently in other packages.
P.S. On an only marginally related note, here is a cat trying to demonstrate the importance of the “Oxford comma” (the Oxford cat?):