[R] running balance down a dataframe referring back to previous row

[seanpor]

Good morning, I've searched high and low and I've tried many different ways
of doing this, but I can't seem to get it to work.

I'm looking for a way of vectorising a "running balance"; i.e. the value in
the first row of the dataframe is zero, and following rows add to this
running balance.  This is easy to write in a loop, but I can't seem to get
it working in vectorised code.  Hopefully the example below will explain
what I'm trying to do...

#####
# create a dummy dataframe
txns <- data.frame(LETTERS)
set.seed(123)
# randomly specify debit / credit columns
txns$drcr <- sample(c('d','c'), nrow(txns), replace=T)
txns$dr <- 0
txns$cr <- 0
# give values to the debits / credits...
txns[txns$drcr == 'd', 'dr'] <-  runif(nrow(txns[txns$drcr == 'd',]), min=0,
max=10000)
txns[txns$drcr == 'c', 'cr'] <-  runif(nrow(txns[txns$drcr == 'c',]), min=0,
max=10000)

# reset the initial dr/cr value to zero...
txns[1,'dr'] <- 0
txns[1,'cr'] <- 0

# intialize the entire running balance column to zero
txns$rbal <- 0

# setup a row index starting at row 2 so that we only operate on these
rows...
r0 <- c(2:nrow(txns))

# setup a row index offset by 1 so that we can access the running balance
# from the previous line...
r1 <- c(2:nrow(txns)) - 1

# calculate the running balance using vectorized code unfortunately this
doesn't work...
txns[r0,'rbal'] <- txns[r1,'rbal'] +  txns[r0,'dr'] - txns[r0,'cr']

# calculate the running balance using a loop
txns$running.bal <- 0
for (i in (2:nrow(txns))) {
	txns[i,'running.bal'] <- txns[(i-1), 'running.bal'] + txns[i, 'dr'] -
txns[i, 'cr']
}
txns
#####

I was hoping that rbal and running.bal would be the same... evidently not...
I've even tried --vanilla...

Is there a specified order in which vectorized dataframe calculations are
carried out? Top to bottom or unspecified?  Does it work off a copy and then
replace the old column?  Do I just have to use a loop for this?

                           
platform       i386-pc-mingw32             
arch           i386                        
os             mingw32                     
system         i386, mingw32               
status                                     
major          2                           
minor          6.2                         
year           2008                        
month          02                          
day            08                          
svn rev        44383                       
language       R                           
version.string R version 2.6.2 (2008-02-08)


Many thanks in advance,

Best regards,
Sean O'Riordain

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