Loop At Assigning syntax question

I have a table that I want to reverse the values of each of the numeric fields. I want to use a Loop At I_TABLE assigning <fs>. Then within that structure I want to calculate the inverse of each of the fields in the internal table containing the appropriate field names. Here is my sample code:

*Loop at each record in internal table...

LOOP AT DATA_PACKAGE assigning <DP>.

• Loop at table with numeric field names

LOOP AT I_DD03L INTO WA_DD03L.

MOVE WA_DD03L-FIELDNAME TO FNAME.

ASSIGN (FNAME) TO <F>.

• Reverse each numeric field in <DP>

<DP>-<F> = <DP>-<F> * (-1).

ENDLOOP.

ENDLOOP.

I get an error that states <DP>-<F> does not exist.

Any ideas?