R: create new matrix with outcomes from mathematical operations within another matrix through loops

Question

I am trying to generate a function that conducts various mathematical operations within a matrix and stores the outcomes of these operations in a new matrix with similar dimensions. Here's an example matrix (a lot of silly computations in it to get sufficient variability in the data)

test<-matrix(1:290,nrow=10,ncol=29) ; colnames(test)<-1979+seq(1,29)
rownames(test)<-c("a","b","c","d","e","f","g","h","i","j")
test[,4]<-rep(8)
test[7,]<-seq(1,29)
test[c(3,5,9),]<-test[c(3,5,9),] * 1/2
test[,c(4,6,8,9,10,15,16,18)]<-test[,c(4,6,8,9,10,15,16,18)]*1/3

I want for instance to be able to calculate the difference between the value in (a,1999) and the average of the 3 values before (a, 1999). This needs to be flexible and for every rowname (firm) and every column (year).

The code I am trying to build looks something like this (I guess):

for(year in 1:29)
  for (k in 1:10)
      qw<-matrix((test[k, year] + 1/3*(- test[k, year-1] - test[k,year -2] - test[k, year-3])), nrow=10, ncol=29)

When I run it, this code generates a matrix but the value in that matrix is always the one for the last calculation (i.e. 20 in my example) while every matrix value should be stored in qw.

Any suggestions on how I can achieve this (maybe via an apply function)?

Thanks in advance

Answer 1

You are creating a matrix qw in every iteration. Each new matrix overwrites the previous one. Here's how to do what I think you would like to do, altough I didn't know how you want to handle the first 3 years.

qw <- matrix(nrow=10, ncol=29)
colnames(qw)<-1979+seq(1,29)
rownames(qw)<-c("a","b","c","d","e","f","g","h","i","j")
for(year in 4:29){
  for (k in 1:10){
    qw[k, year] <- (test[k, year] + 1/3*(- test[k, year-1] - test[k,year -2] - test[k, year-3]))
  }
}
qw

In R, it is usually a bad idea to use loops, since there are much more efficient functions. Here is the R way of doing this, using the package zoo.

require(zoo)
qw <- matrix(nrow=10, ncol=29)
colnames(qw)<-1979+seq(1,29)
rownames(qw)<-c("a","b","c","d","e","f","g","h","i","j")
qw[,4:29] <- test[,4:29]-t(head(rollmean(t(test), 3),-1))
qw