Create a variable capturing the most frequent occurence by group

Question

Define:

df1 <-data.frame(
id=c(rep(1,3),rep(2,3)),
v1=as.character(c(\"a\",\"b\",\"b\",rep(\"c\",3)))
)

s.t.

> df1
           


        

Answer 1

mode <- function(x) names(table(x))[ which.max(table(x)) ]
df1$freq <- ave(df1$v1, df1$id, FUN=mode)
> df1
  id v1 freq
1  1  a    b
2  1  b    b
3  1  b    b
4  2  c    c
5  2  c    c
6  2  c    c

Answer 2

Another way consists of using tidyverse functions:

• grouping first, using group_by(), and counting the occurrence of the second variable using tally()
• arranging by the number of occurrences with arrange()
• summarizing and picking out the first row with summarize() and first()

Therefore:

df1 %>%
group_by(id, v1) %>%
tally() %>%
arrange(id, desc(n)) %>%
summarize(freq = first(v1))

This will give you just the mapping (which I find cleaner):

# A tibble: 2 x 2
     id   freq
  <dbl> <fctr>
1     1      b
2     2      c

You can then left_join your original data frame with that table.

Answer 3

You can do this using ddply and a custom function to pick out the most frequent value:

myFun <- function(x){
    tbl <- table(x$v1)
    x$freq <- rep(names(tbl)[which.max(tbl)],nrow(x))
    x
}

ddply(df1,.(id),.fun=myFun)

Note that which.max will return the first occurrence of the maximum value, in the case of ties. See ??which.is.max in the nnet package for an option that breaks ties randomly.