[R] how to add row index based a categorical column

[xin wei]

hello, I have the following data manipulation issue. the following is the
sample data:

value level
4       A
5       A
2       A
10     B
9       B
34     B
100   C
34     C
101    C.

I hope to get the following result:
value level  rank
2       A       1
4       A       2
5       A       3
9       B       1
10      B      2
34      B      3
34      C      1
100    C       2
101    C.      3

as you may see, I need the data sorted and indexed within each level of
"level". what do I need to do calculate the rank variable?

thanks,

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