Create list with looping - r

Question

I have a i times j (ixj) dummy-matrix for rating events of companies, with i dates and j different companies. On a day where a rating occurs rating [i,j]=1 and 0 otherwise.
I want to create a list, which contains 4 sublist (1 for each of the 4 companies). Each sublist states the row numbers of the rating event of the specific company.
This is my code:
r<-list(); i=1;j=2;
for(j in 1:4){
x<-list()
for(i in 100){
if(rating[i,j]!=0){
x<-c(x,i)
i=i+1
}
else{i=i+1}
}
r[[j]]<-x
j=j+1
}
It is somehow not working, and I really can not figure out where the bug is. The x sublists are always empty. Could somebody help?
Thanks a lot!
Here is an example rating matrix:
rating<-matrix(data = 0, nrow = (100), ncol = 4, dimnames=list(c(1:100), c(1:4)));
rating[3,1]=1;rating[7,1]=1;rating[20,1]=1;rating[75,1]=1;
rating[8,2]=1;rating[40,2]=1;rating[50,2]=1;rating[78,2]=1;
rating[1,3]=1;rating[4,3]=1;rating[17,3]=1;rating[99,3]=1;
rating[10,4]=1;rating[20,4]=1;rating[30,4]=1;rating[90,4]=1;

Answer 1

You may try this:
set.seed(123)
m <- matrix(data = sample(c(0, 1), 16, replace = TRUE), ncol = 4,
dimnames = list(date = 1:4, company = letters[1:4]))
m
# company
# date a b c d
# 1 0 1 1 1
# 2 1 0 0 1
# 3 0 1 1 0
# 4 1 1 0 1
lapply(as.data.frame(m), function(x){
which(x == 1)
})
# $a
# [1] 2 4
#
# $b
# [1] 1 3 4
#
# $c
# [1] 1 3
#
# $d
# [1] 1 2 4
Update
Or more compact (thanks to #flodel!):
lapply(as.data.frame(m == 1), which)

Answer 2

(Leave for-loops behind.) If ratings really is a matrix or even if its a dataframe, then why not use rowSums:
r <- rowSums(rating) # accomplished the stated task more effectively.
# simple example:
> rating <- matrix( rbinom(100, 1,prob=.5), 10)
> rating
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] 1 0 1 0 0 1 1 0 0 1
[2,] 1 0 0 0 0 0 0 1 1 1
[3,] 0 0 1 1 1 1 0 0 1 0
[4,] 1 0 0 0 1 1 0 0 0 1
[5,] 1 1 0 1 1 1 1 0 0 0
[6,] 1 1 1 0 1 1 1 0 1 0
[7,] 0 1 0 1 0 1 1 0 1 0
[8,] 0 1 0 0 1 1 0 1 1 0
[9,] 1 1 1 0 1 1 1 1 0 0
[10,] 0 1 0 0 1 0 0 1 0 1
> rowSums(rating)
[1] 5 4 5 4 6 7 5 5 7 4
> rowSums(as.data.frame(rating))
[1] 5 4 5 4 6 7 5 5 7 4
If it needs to be a list then just wrap as.list() around it.