function by: order within subsets

When the 'by' function forms subsets, are the rows in the same order as they
are in the original data frame?

For example, I want to use 'by' to calculate cumulative sums of a value 'v'
by date 'd' for different levels of a factor 'f':

df<-data.frame(f=c("A","A","B"),d=as.Date(c("2010-1-1","2010-2-1","2010-1-1")),v=c(100,200,150))

df

?f ? ? ? ? ?d ? v
1 A 2010-01-01 100
2 A 2010-02-01 200
3 B 2010-01-01 150

do.call(rbind,by(df,df$f,FUN=function(x)

data.frame(x[1],x[2],cumsum(x[3]))))
? ?f ? ? ? ? ?d ? v
A.1 A 2010-01-01 100
A.2 A 2010-02-01 300
B ? B 2010-01-01 150

This is exactly what I want, namely, cumulative sums by date.

Can I be sure that the rows within subset A will be arranged in date order
as they are in the original data frame? I would not want 'by' to randomly
switch the order and create, for example,
? ?f ? ? ? ? ?d ? v
A.1 A 2010-02-01 200
A.2 A 2010-01-01 300
B ? B 2010-01-01 150

I could force the order of each subset within the FUN of by, adding to the
execution time. Would that be advised?

Thanks,

Dan

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