[R] Count factor if

[mathijsdevaan]

Hi,

I have a data frame containing two columns:   

x<-as.factor(c('a','a','a','a','a','b','b','b','c','d','d','d')) 
y<-c(1,3,6,8,12,3,4,7,5,6,7,10) 
X<-data.frame(x,y) 
X
   x  y
1  a  1
2  a  3
3  a  6
4  a  8
5  a 12
6  b  3
7  b  4
8  b  7
9  c  5
10 d  6
11 d  7
12 d 10

I would like to add a column that counts the number of occurences of x for
all y < focal y. So,

Xnew
   x  y  z
1  a  1  0
2  a  12  4
3  a  6  2
4  a  8  3
5  a 3  1
6  b  3  1
7  b  4  2
8  b  7  3
9  c  5  1
10 d  6  1
11 d  7  2
12 d 10  3

I guess this should be easy, but I have no clue on how to do it. Thanks very
much!
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