R - enter basic formula - r

Question

I am new to R and struggling to understand its quirks. I'm trying to do something which should be really simple, but is turning out to be apparently very complicated.
I am used to Excel, SQL and Minitab, where you can enter a value in one column which includes references to other columns and parameters. However, R doesn't seem to be allowing me to do this.
I have a table with (currently) four columns:
Date Pallets Lt Tt
1 28/12/2011 491 NA NA
2 29/12/2011 385 NA 0.787890411
3 30/12/2011 662 NA NA
4 31/12/2011 28 NA NA
5 01/01/2012 46 NA NA
6 02/01/2012 403 NA NA
7 03/01/2012 282 NA NA
8 04/01/2012 315 NA NA
9 05/01/2012 327 NA NA
10 06/01/2012 458 NA NA
and have a parameter "beta", with a value which I have assigned as 0.0002.
All I want to do is assign a formula to rows 3:10 which is:
beta*(Pallets t - Pallets t-1)+(1-beta)*Tt t-1.
I thought that the appropriate code might be:
Table[3:10,4]<-beta*(Table[3:10,"Pallets"]-Table[2:9,"Pallets"])+(1-beta)*Table[2:9,"Tt"]
However, this doesn't work. The first time I enter this formula, it generates:
Date Pallets Lt Tt
1 28/12/2011 491 NA NA
2 29/12/2011 385 NA 0.7878904
3 30/12/2011 662 NA 0.8431328
4 31/12/2011 28 NA NA
5 01/01/2012 46 NA NA
6 02/01/2012 403 NA NA
7 03/01/2012 282 NA NA
8 04/01/2012 315 NA NA
9 05/01/2012 327 NA NA
10 06/01/2012 458 NA NA
So it's generated the correct answer for the second item in the series, but not for any of the subsequent values.
It seems as though R doesn't automatically update each row, and the relationship to each other row, when you enter a formula, as Excel does. Having said that, Excel actually would require me to enter the formula in cell [4,Tt], and then drag this down to all of the other cells. Perhaps R is the same, and there is an equivalent to "dragging down" which I need to do?
Finally, I also noticed that when I change the value of the beta parameter, through, e.g. beta<-0.5, and then print the Table values again, they are unchanged - so the table hasn't updated even though I have changed the value of the parameter.
Appreciate that these are basic questions, but I am very new to R.

Answer 1

In R, the computations are not made "cell by cell", but are vectorised - in your example, R takes the vectors Table[3:10,"Pallets"], Table[2:9,"Pallets"] and Table[2:9,"Tt"] as they are at the moment, computes the resulting vector, and finally assigns it to Table[3:10,4].
If you want to make some computations "cell by cell", you have to use the for loop:
beta <- 0.5
df <- data.frame(v1 = 1:12, v2 = 0)
for (i in 3:10) {
df[i, "v2"] <- beta * (df[i, "v1"] - df[i-1, "v1"]) + (1 - beta) * df[i-1, "v2"]
}
df
v1 v2
1 1 0.0000000
2 2 0.0000000
3 3 0.5000000
4 4 0.7500000
5 5 0.8750000
6 6 0.9375000
7 7 0.9687500
8 8 0.9843750
9 9 0.9921875
10 10 0.9960938
11 11 0.0000000
12 12 0.0000000
As it comes to your second question, R will never update any values on its own (imagine having set manual calculation in Excel). So you need to repeat the computations after changing beta.

Answer 2

Although it's generally a bad design, but you can iterate over rows in a loop:
Table$temp <- c(0,diff(Table$Palletes,1))
prevTt = 0
for (i in 1:10)
{
Table$Tt[i] = Table$temp * beta + (1-beta)*prevTt
prevTt = Table$Tt[i]
}
Table$temp <- NULL