Invalid values in an array on executing for loop in R - r

Question

I am new to R and stuck up in a very naive thing. I am getting 'NA' values in count array after executing following code:
i=1
j=2
l=1
count=0
while(j<length(positions)){
a=positions[i]
b=positions[j]
for(k in a:b){
if(y$feature[k]==x$feature[l]){
count[l]=count[l]+1
}
}
i=i+2
j=j+2
l=l+1
}
For reference, y and x data frames are as follows:
y data frame
positions id feature
1 1 45128
2 1 28901
3 1 48902
. .
. .
. .
. .
2344 1 45579
2345 2 37689
2346 2 45547
. .
. .
5677 2 12339
5678 3 98034
5679
.
.
x dataframe :
id feature
1 28901
2 23498
3 98906
. .
. .
. .
I have inserted the positions in the position array, at the point where new id starts and where it ends
positions is an array consisting of [1,2344,2345,5677,5678,7390,7391,...]. I am incrementing the for loop as elements in position array, i being 1,3,5... j being 2,4,6... If y$feature and x$feature match I increment count[l]
So first feature of x is compared with all features in y with id=1, second feature in x is compared with all features in y with id=2 and so on. When they match, count[l] is incremented. i and j are incremented twice, to make them start with correct positions. *But I just get a valid answer for count[1], rest all values are NA.
Please tell a reason why this happens and a valid way to do this using the loops.

Answer 1

It's because you are trying to add a nonexistent value count[l] to 1. You start out with count<-0, so count is of length one. There is no count[2], so a reference to count[2] returns NA. Then (assuming l = 2 in your loop), NA + l returns NA.
If you initialize count<-rep(0,length(positions)) this particular problem will go away.
Meanwhile, you can vectorize your operations quite a lot. I believe you can replace the k-loop with
count[l] <- sum(y$feature[a:b]==x$feature[l])
for one example.