The wildcard package (CRAN, GitHub) is a templating mechanism for data frames in R. Wildcards are placeholders for text, and you can evaluate them to generate new data frames from templates. The technique is similar to expand.grid(), but more flexible: rather than deal with all possible combinations of your variables, you can choose which rows to expand.

The tutorial vignette on CRAN demonstrates the full functionality, and I have included a taste below.

library(wildcard)
myths <- data.frame(myth = c("Bigfoot", "UFO", "Loch Ness Monster"), 
                    claim = c("various", "day", "day"), 
                    note = c("various", "pictures", "reported day"))
myths

##                myth   claim         note
## 1           Bigfoot various      various
## 2               UFO     day     pictures
## 3 Loch Ness Monster     day reported day

wildcard(myths, wildcard = "day", values = c("today", "yesterday"))

##                myth     claim               note
## 1           Bigfoot   various            various
## 2               UFO     today           pictures
## 3               UFO yesterday           pictures
## 4 Loch Ness Monster     today     reported today
## 5 Loch Ness Monster yesterday reported yesterday

wildcard(myths, wildcard = "day", values = c("today", "yesterday"), expand = FALSE)

##                myth     claim           note
## 1           Bigfoot   various        various
## 2               UFO     today       pictures
## 3 Loch Ness Monster yesterday reported today