Most frequent value (mode) by group [duplicate]

by

Building on Davids comments your solution is the following:

Mode <- function(x) {
  ux <- unique(x)
  ux[which.max(tabulate(match(x, ux)))]
}

library(dplyr)
df %>% group_by(a) %>% mutate(c=Mode(b))

Notice though that for the tie when df$a is 3 then the mode for b is 1.

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