Group rows based one column and sum up the rest of the columns

I try to group the rows of a data.frame by the ID column and sum up all the numerical columns and drop the string column. For that, I created a small data.frame df1:

   ID string 2018 2019 2020 2021
1: a1     x2    3    3    0    4
2: a2     g3    5    5    4    0
3: a2     n2   11    6   13    3
4: a1     m3    3   21    9    8
5: a2     2w    9    1   16    5
6: a1    ps2   22    4    7    4
7: a1    kg2    6    0    9    6

and I try to get the sum of the years like in df2:

   ID 2018 2019 2020 2021
1: a1   34   28   25   22
2: a2   25   12   33    8

I was trying it with the group_by and summarize function of dplyr, but I wasn’t able to get what I want.

library(dplyr)
df1 %>%
  group_by(ID) %>%
  summarize(df$2018 = sum(2018))

Thanks a lot

>Solution :

You could use

library(data.table)

df1[, lapply(.SD[, `2018`:`2021`], sum), by = ID]

This returns

   ID 2018 2019 2020 2021
1: a1   34   28   25   22
2: a2   25   12   33    8