4.11 Combine words into a comma-separated phrase

When you want to output a character vector for humans to read (e.g., x <- c("apple", "banana", "cherry")), you probably do not want something like [1] "apple" "banana" "cherry", which is the normal way to print a vector in R. Instead, you may want a character string “apple, banana, and cherry”. There is a base R function, paste(), that you can use to concatenate a character vector into a single string, e.g., paste(x, collapse = ', '), and the output will be "apple, banana, cherry". The problems are (1) the conjunction “and” is missing, and (2) when the vector only contains two elements, we should not use commas (e.g., the output should be "apple and banana" instead of "apple, banana").

The function knitr::combine_words() can be used to concatenate words into a phrase regardless of the length of the character vector. Basically, for a single word, it will just return this word; for two words A and B, it returns "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 = '`'). Below are more examples with different arguments, and please see the help page ?knitr::combine_words if the meaning of any argument is not clear from the output here:

v <- c("apple", "banana", "cherry")
## apple, banana, and cherry
knitr::combine_words(v, before = "`", after = "'")
## `apple', `banana', and `cherry'
knitr::combine_words(v, and = "")
## apple, banana, cherry
knitr::combine_words(v, sep = " / ", and = "")
## apple / banana / cherry
knitr::combine_words(v[1])  # a single word
## apple
knitr::combine_words(v[1:2])  # two words
## apple and banana
## A, B, C, D, and E

This function can be particularly handy when it is used in an inline R expression, e.g.,

This morning we had `r knitr::combine_words(v)` for breakfast.