[R] partially sum variable of a dataframe

[markleeds at verizon.net]

There's probably a shorter way but below works
and doesn't require the starting points to be in
the a column.

startindex<-3
endindex<-5.25

start<-tail(which(c$a<=startindex),1)
end<-tail(which(c$a<=endindex),1)

sum(c$b[start:end])


>Hello,
>
>A stupid question:
>
>I have an array with two columns, the first "a" acting as my index in 0.25 steps, the second one "b" the column of interest. How can i sum up "b" only for a specified window in "a" (as the window command for time series)
>
>a=seq(0,10,0.25)
>b=runif(41)
>c=data.frame(a,b)
>
>Sum up c if 3<a<5.25
>
>How to do that? thanks
>
>marc
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