df1 <- data.frame(a = LETTERS[1:2], b = LETTERS[3:4], c = 1:2)

I am looking for an idiom that swaps the elements of df$a and df$b  
when (e.g.) df$c == 2, resulting in

df2 <- data.frame(a = LETTERS[c(1, 4)], b = LETTERS[c(3, 2)], c = 1:2)
_____________________________
Professor Michael Kubovy
University of Virginia
Department of Psychology
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