for loop & if function in R

Question

I was writing a loop with if function in R. The table is like below:

ID  category
1   a
1   b
1   c
2   a
2   b
3   a
3   b
4   a
5   a

I want

Answer 1

looping solution will be painfully slow for bigger data. Here is one line solution using data.table:

require(data.table)
a<-data.table(ID=c(1,1,1,2,2,3,3,4,5),category=c('a','b','c','a','b','a','b','a','a'))
a[,':='(category_count = 1:.N),by=.(ID)]