Apply a function to all pairs in the same group - r

Question

I want to apply a function to all pairs of items in the same group e.g.
Example input:
Group Item Value
A 1 89
A 2 76
A 3 2
B 4 21
B 5 10
The desired output is a vector of the function output for all items in the same group.
e.g. for arguments sake if the function was:
addnums=function(x,y){
x+y
}
Then the desired output would be:
165, 91, 78, 31
I have tried to do this using summarize in the dplyr package but this can only be used if the output is a single value.

Answer 1

We can split Value for each Group and then use combn to calculate sum for each pair.
sapply(split(df$Value, df$Group), combn, 2, sum)
#$A
#[1] 165 91 78
#$B
#[1] 31
If needed as one vector we can use unlist.
unlist(sapply(split(df$Value, df$Group), combn, 2, sum), use.names = FALSE)
#[1] 165 91 78 31
If you are interested in tidyverse solution using the same logic we can do
library(dplyr)
library(purrr)
df %>%
group_split(Group) %>%
map(~combn(.x %>% pull(Value), 2, sum)) %>% flatten_dbl
#[1] 165 91 78 31

Answer 2

We can use a group by option with data.table
library(data.table)
setDT(df1)[, combn(Value, 2, FUN = sum), Group]
# Group V1
#1: A 165
#2: A 91
#3: A 78
#4: B 31
If we want to use addnums from the OP's post
setDT(df1)[, combn(Value, 2, FUN = function(x) addnums(x[1], x[2])), Group]
# Group V1
#1: A 165
#2: A 91
#3: A 78
#4: B 31
Or using tidyverse
library(dplyr)
library(tidyr)
df1 %>%
group_by(Group) %>%
summarise(Sum = list(combn(Value, 2, FUN = sum))) %>%
unnest
# A tibble: 4 x 2
# Group Sum
# <chr> <int>
#1 A 165
#2 A 91
#3 A 78
#4 B 31
Using addnums
df1 %>%
group_by(Group) %>%
summarise(Sum = list(combn(Value, 2, FUN =
function(x) addnums(x[1], x[2])))) %>%
unnest
Or using base R with aggregate
aggregate(Value ~ Group, df1, FUN = function(x) combn(x, 2, FUN = sum))
data
df1 <- structure(list(Group = c("A", "A", "A", "B", "B"), Item = 1:5,
Value = c(89L, 76L, 2L, 21L, 10L)), class = "data.frame", row.names = c(NA,
-5L))