[R] Avoiding loops

[dolar]

Would like some tips on how to avoid loops as I know they are slow in R

i've got a data frame :

a  b  c
1  5  2
4  6  9
5  2  3
8  3  2

What i'd like is to sum for each value of a, the sum of b and the sum of c
where a equal to or less than (with a distance of 5) 

i.e. for row three
we have a=5
i'd like to sum up b and sum up c with the above rule
since 5, 4 and 1 are less than (within a distance of 5) or equal to 5, then
we should get the following result:

a  b   c
5  13  14

the overall result should be
a   b   c
1   5   2
4   11  11
5   13  14
8   11  14

how can i do this without a loop?
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