r - How can you anticipate the correct way to quote a variable? -

there must pattern/rationale here, can't see it. how quote variables right first time?

require(ggplot2) require(reshape2) require(plyr)  # reshape2  # these work. dcast(mpg, manufacturer ~ class, mean, value.var = "cty") dcast(mpg, "manufacturer ~ class", mean, value.var = "cty")  # these don't. dcast(mpg, .(manufacturer ~ class), mean, value.var = "cty") dcast(mpg, manufacturer ~ class, mean, value.var = cty) dcast(mpg, manufacturer ~ class, mean, value.var = .(cty))   # plyr  # these work. ddply(mpg, .(manufacturer), summarize, mean = mean(cty)) ddply(mpg, "manufacturer", summarize, mean = mean(cty)) ddply(mpg, manufacturer ~ class, summarize, mean = mean(cty))  # these don't. ddply(mpg, manufacturer, summarize, mean = mean(cty)) ddply(mpg, .(manufacturer), summarize, mean = mean(.(cty))) ddply(mpg, .(manufacturer), summarize, mean = mean("cty")) ddply(mpg, .(manufacturer ~ class), summarize, mean = mean(cty)) ddply(mpg, "manufacturer ~ class", summarize, mean = mean(cty))   # ggplot  # works qplot(displ, hwy, data = mpg)  # these don't qplot(.(displ), .(hwy), data = mpg) qplot("displ", "hwy", data = mpg)  p <- qplot(displ, hwy, data = mpg)  # these work p + facet_wrap(~ cyl) p + facet_wrap(.(cyl)) p + facet_wrap("cyl")  # doesn't p + facet_wrap(cyl) 

feel free add missing permutations , combinations.

an argument format entirely depends on function implementation. function page describes way have provide them.

take ddply function plyr. second argument called .variables , described :

.variables: variables split data frame by, ‘as.quoted’ variables, formula or character vector

that's why can input them .(manufacturer) (as.quoted variables), "manufacturer" (character vector), or manufacturer~class (formula).

you can judge them non-consistent, know how quote arguments have understand way function works , expecting input...