[R] Summing using a boolean index vector (repost in plain text).

Dan Stanger DStanger at EatonVance.Com
Fri Jun 18 17:39:33 CEST 2010

Hello all:
I have a dataframe f of weekdays and value, and a Boolean vector with Fridays set to true, and other days set to false, created by fridays<-(diff(f$weekdays) < -1).
I would like to create a vector of sums, for each week.  That is, start summing on the first false value in the vector, and when I get to true, produce the sum, and start summing again.
Is there a vector operation which can do this, without writing an explicit loop?
Thank you,
Dan Stanger
P.S. Sorry about the repost.
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