[R] summing up and cut off with looping

[oslo]

Hi all;
I am quite new in R. I have tried write a loop to sum up a column and cut off when summation reach certain point. Here are a small example and my R codes.
Your helps are truly appreciated,
Oslo
file=AposA posB1     92     75     124     79     13.    . .    .   
File=Bpos  a  b   c    4   .4  7  .82   .1  5  .47  .5  8  .321   .4  1  .113  .1  6  .1312  .2  11 .019   .3  12 .23.    .   .  ..    .   .  .
I would like to sum up the B$a column and cut off at 0.7 for the each row of intervals giving in file=A.For example the interval  at the first row in A$posA and A$posB is 1 and 9. So, I need adding up the B$a and cut off B$a>.7 from the 1 to 9 in B$pos. And then I need to the same using the intervals in the second, third..... rows in A. Obviously my loop is wrong and  does not work properly. Please help for my this first experience.  Thanks.
Here are my codes
#sorting B$possort=B[order(B$pos),]
#Running loop
for(i in 1:nrow(A)) {if(sum(B[a$B, i:A[1:2])>0.7) {print(A[1:i,]) } }

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