[R] Conditional sum

[mathijsdevaan]

Hi,

I have a DF like this:

DF = data.frame(read.table(textConnection("    A  B  C 
1 b1  1999  0.25
2 c1  1999  0.25
3 d1  1999  0.25
4 a2  1999  0.25
5 c2  1999  0.25
6 d2  1999  0.25
7 a3  1999  0.25
8 b3  1999  0.25
9 d3  1999  0.25
10 a4  1999  0.25
11 b4  1999  0.25
12 c4  1999  0.25
13 b1  2001  0.5
14 a2  2001  0.5
15 b1  2004  0.33
16 c1  2004  0.33
17 a2  2004  0.33
18 c2  2004  0.33
19 a3  2004  0.33
20 b3  2004  0.33
21 d2  1980  0.4
22 a3  1980  0.4
23 b4  1981  0.4
24 c1  1981  0.4"),head=TRUE,stringsAsFactors=FALSE))

For each factor in A I want to sum the values of C for all years(Bn) prior
to the current year(Bi):

1 b1  1999  0.25  0
2 c1  1999  0.25  0.4
3 d1  1999  0.25  0
4 a2  1999  0.25  0
5 c2  1999  0.25  0
6 d2  1999  0.25  0.4
7 a3  1999  0.25  0.4
8 b3  1999  0.25  0
9 d3  1999  0.25  0
10 a4 1999  0.25  0
11 b4 1999  0.25  0.4
12 c4 1999  0.25  0
13 b1 2001  0.50  0.25
14 a2 2001  0.50  0.25
15 b1 2004  0.33  0.75
16 c1 2004  0.33  0.65
17 a2 2004  0.33  0.75
18 c2 2004  0.33  0.25
19 a3 2004  0.33  0.65
20 b3 2004  0.33  0.25
21 d2 1980  0.40  0
22 a3 1980  0.40  0
23 b4 1981  0.40  0
24 c1 1981  0.40  0

If I do this - DF$D<-ave(DF$C,DF$A,FUN = function(x) sum(x)) - it returns
for each factor the sum of C for all years. How do I build the (B) condition
in this function? Thanks very much! 

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