Find difference between maximum value of group and current row with R - r

Question

Similar to this question here, I am trying to find the difference between the maximum value of a group and the value of the current row.
For instance, if I have the following dataset:
ID <- c(1,1,1,2,2,2,2,3,3)
Value <- c(2,3,5,2,5,8,17,3,5)
group <- data.frame(Subject=ID, pt=Value)
How would I go about creating a new column called "diff" that would be the difference between the value of the current row and the maximum value in that group?
Thank you for your help!

Answer 1

The OP has tried a data.table solution. Here, we benefit from grouping and updating by reference simultaneously.
library(data.table)
setDT(group)[, diff := max(pt) - pt, by = Subject][]
Subject pt diff
1: 1 2 3
2: 1 3 2
3: 1 5 0
4: 2 2 15
5: 2 5 12
6: 2 8 9
7: 2 17 0
8: 3 3 2
9: 3 5 0
Data
ID <- c(1,1,1,2,2,2,2,3,3)
Value <- c(2,3,5,2,5,8,17,3,5)
group <- data.frame(Subject=ID, pt=Value)
Benchmark
At the time of writing, 5 answers were posted, including Frank's comment on the efficiency of th data.table approach. So, I was wondering which of the five solutions were the fastest.
r2evans
mine
Frank
harelhan
JonMinton
Some solutions modify the data.frame in place. To ensure a fair comparison, In addition,
The OP has required to create a new column called "diff". For comparison, all results should return group with three columns. Some answers were modified accordingly. The answer of harelhan required substantial modifications to remove the errors.
As group is modified, all benchmark runs start with a fresh copy of group with two columns.
The benchmark is parameterized over the number of rows and the share of groups, i.e., the number of groups varies with the problem size in order to scale.
library(data.table)
library(dplyr)
library(bench)
bm <- press(
# n_row = c(1E2, 1E4, 1E5, 1E6),
n_row = c(1E2, 1E4, 1E5),
grp_share = c(0.01, 0.1, 0.5, 0.9),
{
n_grp <- grp_share * n_row
set.seed(1)
group0 <- data.frame(
Subject = sample(n_grp, n_row, TRUE),
pt = as.numeric(rpois(n_row, 100)))
mark(
r2Evans = {
group <- copy(group0)
group <- group %>%
group_by(Subject) %>%
mutate(diff = max(pt) - pt)
group
},
Uwe = {
group <- copy(group0)
setDT(group)[, diff := max(pt) - pt, by = Subject]
group
},
Frank = {
group <- copy(group0)
setDT(group)[, mx := max(pt), by=Subject][, diff := mx - pt][, mx := NULL]
group
},
harelhan = {
group <- copy(group0)
max_group <- group %>% group_by(Subject) %>% summarize(max_val = max(pt))
group <- left_join(group, max_group[, c("Subject", "max_val")], by = "Subject")
group$diff <- group$max_val - group$pt
group <- group %>% select(-max_val)
group
},
JonMinton = {
group <- copy(group0)
group <- group %>%
group_by(Subject) %>%
mutate(max_group_val = max(pt)) %>%
ungroup() %>%
mutate(diff = max_group_val - pt) %>%
select(-max_group_val)
group
}
)
}
)
ggplot2::autoplot(bm)

Answer 2

Using your example data and breaking the logic into smaller steps:
library(dplyr)
ID <- c(1,1,1,2,2,2,2,3,3)
Value <- c(2,3,5,2,5,8,17,3,5)
group <- data.frame(Subject=ID, pt=Value)
max_group <- group %>% group_by(ID) %>% summarize(max_val = max(Value))
group <- left_join(group, max_group[,c(ID, max_val)], by = ID)
group$diff <- group$max_val - group$Value
Hope this solves the problem.

Answer 3

Based on harelhan's answer, but with piping:
require(dplyr)
df <- data_frame(
id = c(1,1,1,2,2,2,2,3,3),
value = c(2,3,5,2,5,8,17,3,5)
)
df %>%
group_by(id) %>%
mutate(max_group_val = max(value)) %>%
ungroup() %>%
mutate(diff_frm_group_max = max_group_val - value)
A tibble: 9 x 4
id value max_group_val diff_frm_group_max
<dbl> <dbl> <dbl> <dbl>
1 1 2 5 3
2 1 3 5 2
3 1 5 5 0
4 2 2 17 15
5 2 5 17 12
6 2 8 17 9
7 2 17 17 0
8 3 3 5 2
9 3 5 5 0