R Group by with conditional and sum other columns using data.table

Question

I want to sum all columns except one specific column based on the condition by groups.

For example:

Col1    Col2   Condition   Name    P1    P2    P3    P4 
1990    1      0           APPLE   10    20    20    30   
1990    1      1           BAN     30    40    50    50   
1990    1      1           CAR     40    40    30    40   
1990    2      0           DOG     100   20    30    40   
1990    2      1           APPLE   10    20    20    30   
1990    2      1           APPLE   50    20    20    30   

I want to SUM P2 , P3 , P4 and then APPEND P1 when Condition equals to "0" by Col1 and Col2 .

So the result will be:

Col1    Col2   Condition     P1    P2     P3    P4 
1990    1      0             10    100    100   120     
1990    2      0             100   60     70    100   

I know how to add in data.table but have no idea with this.

DT[, .(lapply(.SD, sum, na.rm=TRUE), by=.(Col1, Col2), .SDcols=c("P2", "P3", "P4")] 

It seems that DT[, setdiff(names(DT), c("P2", "P3", "P4")), with = FALSE] is a key but still have no idea.

Answer 1

We can group by 'Col1', 'Col2', then summarise_at the columns that starts_with 'P' (changed to range of columns as per OP's request) to get the sum of values where 'Condition' is 0 and then create a 'Condition' column of 0

library(dplyr)
df1 %>% 
  group_by(Col1, Col2) %>%
  group_by(P1 = sum(P1[Condition == 0]), add = TRUE) %>% 
  summarise_at(vars(P2:P4), sum) %>%
  mutate(Condition = 0) %>%
  ungroup 
# A tibble: 2 x 7
#   Col1  Col2    P1    P2    P3    P4 Condition
#  <int> <int> <int> <int> <int> <int>     <dbl>
#1  1990     1    10   100   100   120         0
#2  1990     2   100    60    70   100         0

---

Or using data.table

library(data.table)
setDT(df1)[,  P1 := sum(P1[Condition == 0]), .(Col1, Col2)][, 
     lapply(.SD, sum) , .(Col1, Col2, P1), .SDcols = P2:P4]