"for" loop in R and checking previous value from a column - r

Question

I'm working on a data frame which looks like this
Here's how it looks like:
shape id day hour week id footfall category area name
22496 22/3/14 3 12 634 Work cluster CBD area 1
22670 22/3/14 3 12 220 Shopping cluster Orchard Road 1
23287 22/3/14 3 12 723 Airport Changi Airport 2
16430 22/3/14 4 12 947 Work cluster CBD area 2
4697 22/3/14 3 12 220 Residential area Ang Mo Kio 2
4911 22/3/14 3 12 1001 Shopping cluster Orchard Rd 3
11126 22/3/14 3 12 220 Residential area Ang Mo Kio 2
and so on... until 635 rows return.
and the other dataset that I want to compare with can be found here
Here's how it looks like:
category Foreigners Locals
Work cluster 1600000 3623900
Shopping cluster 1800000 3646666.667
Airport 15095152 8902705
Residential area 527700 280000
They both share the same attribute, i.e. category
I want to check if I can compare the previous hour from the column hour in the first dataset so I can compare it with the value from the second dataset.
Here's, what I ideally want to find in R:
#for n in 1: number of rows{
# check the previous hour from IDA dataset !!!!
# calculate hourSum - previousHour = newHourSum and store it as newHourSum
# calculate hour/(newHourSum-previousHour) * Foreigners and store it as footfallHour
# add to the empty dataframe }
I'm not sure how to do that and here's what i tried:
tbl1 <- secondDataset
tbl2 <- firstDataset
mergetbl <- function(tbl1, tbl2)
{
newtbl = data.frame(hour=numeric(),forgHour=numeric(),locHour=numeric())
ntbl1rows<-nrow(tbl1) # get the number of rows
for(n in 1:ntbl1rows)
{
#get the previousHour
newHourSum <- tbl1$hour - previousHour
footfallHour <- (tbl1$hour/(newHourSum-previousHour)) * tbl2$Foreigners
#add to newtbl
}
}
This would what i expected:
shape id day hour week id footfall category area name forgHour locHour
22496 22/3/14 3 12 634 Work cluster CBD area 1 1 12
22670 22/3/14 3 12 220 Shopping cluster Orchard Road 1 21 25
23287 22/3/14 3 12 723 Airport Changi Airport 2 31 34
16430 22/3/14 4 12 947 Work cluster CBD area 2 41 23
4697 22/3/14 3 12 220 Residential area Ang Mo Kio 2 51 23
4911 22/3/14 3 12 1001 Shopping cluster Orchard Rd 3 61 45
11126 22/3/14 3 12 220 Residential area Ang Mo Kio 2 72 54