Find Most Frequent Combination within a Vector by

问题:

I have a table with two columns namely id and item:

df <- data.frame(id=c(1,1,2,2,2,2,3,3,3,4,4,4,4,4),item=c(1,2,3,1,2,3,4,1,2,3,1,2,1,2))

I want to find the most frequent combination (order doesn't matter) of 3 items per id. So basically, n choose r where n = number of items within id and r = 3. The number of items per id varies - some have more than 3, some have less.

I am new to R and read about combn and expand.grid, but I don't know how to use them in my case (to work within each id).

"Find most frequent combination of values in a data.frame" is the closest question I found.

EDIT: The expected answer based on the example is the combination "1, 2, 3", which appears in id 2 and 4.

回答1:

Here is one idea using dplyr

df1 <- df %>% 
        group_by(id) %>% 
        arrange(item) %>% 
        summarise(new = paste(unique(combn(item, length(unique(item)), toString)), collapse = '/'))
df1
# A tibble: 4 × 2
#     id                                             new
#  <dbl>                                           <chr>
#1     1                                            1, 2
#2     2                     1, 2, 3 / 1, 3, 3 / 2, 3, 3
#3     3                                         1, 2, 4
#4     4 1, 1, 2 / 1, 1, 3 / 1, 2, 2 / 1, 2, 3 / 2, 2, 3

names(sort(table(unlist(strsplit(df1$new, '/'))), decreasing = TRUE)[1])
#[1] "1, 2, 3"

回答2:

library(dplyr)
grouped <- df %>% group_by(id,item) %>% summarize(count = n()) %>% arrange(desc(count))

Voila. The highest counts sorted from highest to lowest.

EDIT: Just realized I didn't fully answer your question. I hope I gave you a good start.

回答3:

I think this is what you want using base R (no package needed):

a <- aggregate(item~id, df, unique)
a <- lapply(a$item, 'length<-', max(lengths(a$item)))
m <- matrix(unlist(a), ncol=3, byrow = T)
m <- t(apply(m,1,function(x) sort(x,na.last = T)))

#     [,1] [,2] [,3]
#[1,]    1    2   NA
#[2,]    1    2    3
#[3,]    1    2    4
#[4,]    1    2    3

Once we get matrix m, the most frequent row of the matrix is what you want:

t <- table(apply(m, 1, paste, collapse = "/")) 
as.numeric(strsplit(names(which.max(t)), "/")[[1]]) 

#[1] 1 2 3