Most frequent value (mode) by group [duplicate]

Question

I am trying to find the most frequent value by group. In the following example dataframe:

df<-data.frame(a=c(1,1,1,1,2,2,2,3,3),b=c(2,2,1,2,3,3,1,1,2))  
> df  
  a b  
1 1 2  
2 1 2  
3 1 1  
4 1 2  
5 2 3  
6 2 3  
7 2 1  
8 3 1  
9 3 2  

I would like to add a column 'c' which has the most occurring value in 'b' when its values are grouped by 'a'. I would like the following output:

> df  
  a b c  
1 1 2 2    
2 1 2 2    
3 1 1 2    
4 1 2 2    
5 2 3 3    
6 2 3 3    
7 2 1 3    
8 3 1 1   
9 3 2 1    

I tried using table and tapply but didn't get it right. Is there a fast way to do that?
Thanks!

Answer 1

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.