R: Ratio of the values with specific labels in the data.frame --- the filling issue

R: Ratio of the values with specific labels in the data.frame --- the
filling issue

I have a data.frame and I need to add additional column that is a ratio
between labels == 1 and labels == 2 for same cID. I have the code that can
do that but the results is the reduced form according to the number of
unique "l". But I need a full list with duplicates. Any suggestions?
Thank's in advance!
x y l cID
0.03588851 0.081635056 1 1
0.952514891 0.82677373 1 1
0.722920691 0.687278396 1 1
0.772207687 0.743329599 2 1
0.682710551 0.946685728 1 2
0.795816439 0.024320077 2 2
0.50788885 0.106910923 2 2
0.145871035 0.802771467 2 2
0.092942384 0.335054397 1 3
0.439765866 0.199329139 1 4
to reproduce
x =
c(0.03588851,0.952514891,0.722920691,0.772207687,0.682710551,0.795816439,0.50788885,0.145871035,0.092942384,0.439765866)
y =
c(0.081635056,0.82677373,0.687278396,0.743329599,0.946685728,0.024320077,0.106910923,0.802771467,0.335054397,0.199329139)
l = c(1,1,1,2,1,2,2,2,1,1)
cID = c(1,1,1,1,2,2,2,2,3,4)
dt <- data.table(x,y,l,cID)
dt[,sum(l == 1)/sum(l == 2), by = cID]
I need to obtain the ratio column that looks like this
x y l cID ratio
0.03588851 0.081635056 1 1 3
0.952514891 0.82677373 1 1 3
0.722920691 0.687278396 1 1 3
0.772207687 0.743329599 2 1 3
0.682710551 0.946685728 1 2 0.333333333
0.795816439 0.024320077 2 2 0.333333333
0.50788885 0.106910923 2 2 0.333333333
0.145871035 0.802771467 2 2 0.333333333
0.092942384 0.335054397 1 3 Inf
0.439765866 0.199329139 1 4 Inf