[R] rowsum

[xtracto]

Hello,

I am trying to achieve something which I *think* is possible using rowsum,
but a little help should be useful:

Consider the following dataframe DF0: 
A	B	C
89	1	140
89	06	20
89	29	137
89	52	13
89	57	10
89	97	23
89	1	37
89	1	12
89	1	3
52	1	11
52	1	31
52	1	16
52	1	6
52	1	10
52	1	13
52	1	10
52	1	25
52	1	2
52	59	38
52	97	75
57	1	14
57	1	13
57	1	14
57	114	12
57	1	23
57	06	26


I need create a new dataframe containing the sums of all the rows where B =
1 for the different values of A, keeping the rows with other B values the
same. That is, for this data sample, the result I expect is something like
this (the order of the rows does not matter):

A	B	C
89	1	192    #From adding up: [140 + 37 + 12 + 3]
89	06	20
89	29	137
89	52	13
89	57	10
89	97	23
52	1	124    # From adding up: [11 + 31 + 16 + 6 + 10 + 13 + 10 + 25 + 2]
52	59	38
52	97	75
57	1	64     #From adding up: [14 +13 +14 +23]
57	114	12
57	06	26


Now, I now it should be possible to first separate the data in two sets,
where 
DF1 <- DF0[DF0$B != 1,] 
DF2 <- DF0[DF0$B == 1,] 

Then I should apply sumrow to DF2 with some "group" vector, but I do not
know where to go from here. 

Can anyone help? 
Thanks in advance!


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