df <- data.frame(x = c(1, 2, NA), y = c("a", NA, "b"), stringsAsFactors = FALSE)
df %>% replace_na(list(x = 0, y = "unknown"))
df %>% mutate(x = replace_na(x, 0))
df$x %>% replace_na(0)
df$y %>% replace_na("unknown")
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