Average neighbours inside a vector - r

Question

My data :
data <- c(1,5,11,15,24,31,32,65)
There are 2 neighbours: 31 and 32. I wish to remove them and keep only the mean value (e.g. 31.5), in such a way data would be :
data <- c(1,5,11,15,24,31.5,65)
It seems simple, but I wish to do it automatically, and sometimes with vectors containing more neighbours. For instance :
data_2 <- c(1,5,11,15,24,31,32,65,99,100,101,140)

Answer 1

Here is another idea that creates an id via cumsum(c(TRUE, diff(a) > 1)), where 1 shows the gap threshold, i.e.
#our group variable
grp <- cumsum(c(TRUE, diff(a) > 1))
#keep only groups with length 1 (i.e. with no neighbor)
i1 <- a[!!!ave(a, grp, FUN = function(i) length(i) > 1)]
#Find the mean of the groups with more than 1 rows,
i2 <- unname(tapply(a, grp, function(i)mean(i[length(i) > 1])))
#Concatenate the above 2 (eliminating NAs from i2) to get final result
c(i1, i2[!is.na(i2)])
#[1] 1.0 5.0 11.0 15.0 24.0 65.0 31.5
You can also wrap it in a function. I left the gap as a parameter so you can adjust,
get_vec <- function(x, gap) {
grp <- cumsum(c(TRUE, diff(x) > gap))
i1 <- x[!!!ave(x, grp, FUN = function(i) length(i) > 1)]
i2 <- unname(tapply(x, grp, function(i) mean(i[length(i) > 1])))
return(c(i1, i2[!is.na(i2)]))
}
get_vec(a, 1)
#[1] 1.0 5.0 11.0 15.0 24.0 65.0 31.5
get_vec(a_2, 1)
#[1] 1.0 5.0 11.0 15.0 24.0 65.0 140.0 31.5 100.0
DATA:
a <- c(1,5,11,15,24,31,32,65)
a_2 <- c(1, 5, 11, 15, 24, 31, 32, 65, 99, 100, 101, 140)

Answer 2

Here is my solution, which uses run-length encoding to identify groups:
foo <- function(x) {
y <- x - seq_along(x) #normalize to zero differences in groups
ind <- rle(y) #run-length encoding
ind$values <- ind$lengths != 1 #to find groups
ind$values[ind$values] <- cumsum(ind$values[ind$values]) #group ids
ind <- inverse.rle(ind)
xnew <- x
xnew[ind != 0] <- ave(x, ind, FUN = mean)[ind != 0] #calculate means
xnew[!(duplicated(ind) & ind != 0)] #remove duplicates from groups
}
foo(data)
#[1] 1.0 5.0 11.0 15.0 24.0 31.5 65.0
foo(data_2)
#[1] 1.0 5.0 11.0 15.0 24.0 31.5 65.0 100.0 140.0
data_3 <- c(1, 2, 4, 1, 2)
foo(data_3)
#[1] 1.5 4.0 1.5
I assume that you don't need an extremely efficient solution. If you do, I'd recommend a simple C++ for loop in Rcpp.

Answer 3

I have a data.table based solution, same could be translated into dplyr I guess:
library(data.table)
df <- data.table(data2 = c(1,5,11,15,24,31,32,65,99,100,101,140))
df[,neighbours := ifelse(c(0,diff(data_2)) == 1,1,0)]
df[,neighbours := c(neighbours[1:(.N-1)],1),by = rleid(neighbours)]
df[,neigh_seq := rleid(neighbours)]
unique(df[,ifelse(neighbours == 1,mean(data2),data2),by = neigh_seq])
neigh_seq V1
1: 1 1.0
2: 1 5.0
3: 1 11.0
4: 1 15.0
5: 1 24.0
6: 2 31.5
7: 3 65.0
8: 4 100.0
9: 5 140.0
What it does :
first line set neigbours to 1 if the difference with following number is 1
1: 1 0
2: 5 0
3: 11 0
4: 15 0
5: 24 0
6: 31 0
7: 32 1
8: 65 0
9: 99 0
10: 100 1
11: 101 1
12: 140 0
I wanr to group so that neighbour variable is 1 for all neigbours. I need to add 1 to each end of each groups:
df[,neighbours := c(neighbours[1:(.N-1)],1),by = rleid(neighbours)]
data2 neighbours
1: 1 0
2: 5 0
3: 11 0
4: 15 0
5: 24 0
6: 31 1
7: 32 1
8: 65 0
9: 99 1
10: 100 1
11: 101 1
12: 140 0
Then after I just do a grouping on changing neighbour value, and set the value to mean if they are neihbours
df[,ifelse(neighbours == 1,mean(data2),data2),by = rleid(neighbours)]
rleid V1
1: 1 1.0
2: 1 5.0
3: 1 11.0
4: 1 15.0
5: 1 24.0
6: 2 31.5
7: 2 31.5
8: 3 65.0
9: 4 100.0
10: 4 100.0
11: 4 100.0
12: 5 140.0
and take the unique values. And voila.

Answer 4

This is a dplyr version, also using as a grouping variable cumsum(c(1,diff(x)!=1)):
library(dplyr)
data_2 %>% data.frame(x = .) %>%
group_by(id = cumsum(c(1,diff(x)!=1))) %>%
summarise(res = mean(x)) %>%
select(res)
# A tibble: 9 x 1
res
<dbl>
1 1.0
2 5.0
3 11.0
4 15.0
5 24.0
6 31.5
7 65.0
8 100.0
9 140.0