How to get percent for value for keys in R using data.table?

Question

I have data.tableone that looks like this:

x, y, sum(count)
1, 1, 3
1, 2, 7
1, 3, 8
2, 1, 4
2, 2, 3
2, 3, 10

Etc. He was attached to xand y, and I did sumin the column count. I would like to divide it into percentages by x values, so that it becomes:

x, y, percentage(counts)
1, 1, 16.7
1, 2, 38.9
1, 3, 44.4
2, 1, 23.5
2, 2, 17.6
2, 3, 58.8

Thus, the total percentage for xis 100%. I do this with a package data.table. Thank you in advance for your help.

Answer 1

data.table, , ( , , (!) ):

dt[ , `percentage(counts)` := `sum(count)` / sum( `sum(count)` ) * 100 , by = "x" ]
#   x y sum(count) percentage(counts)
#1: 1 1          3           16.66667
#2: 1 2          7           38.88889
#3: 1 3          8           44.44444
#4: 2 1          4           23.52941
#5: 2 2          3           17.64706
#6: 2 3         10           58.82353

Answer 2

( ) :

allsums <- sum(data[, counts])
data[, percentage = 100*sum(counts)/allsums, by="x,y"]

:

allsums <- sum(data[, sums])
data[, percentage = 100*sums/allsums, by="x,y"]