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

what you want is actually a column of factor level. do this

df$count=as.numeric(df$category)

this will give out put as

  ID category count
1  1        a     1
2  1        b     2
3  1        c     3
4  2        a     1
5  2        b     2
6  3        a     1
7  3        b     2
8  4        a     1
9  5        a     1

provided your category is already a factor. if not first convert to factor

df$category=as.factor(df$category)
df$count=as.numeric(df$category)