Field symbols

Hello Friends,

I have an internal table, some of the fields are empty. How can i found the empty fields in the structure and i should fill that with Zero.

Eg:

ITAB:

f1 f2 f3 f4 f5

1 1 3 2

2 1 6

5 2 6 2

1 2 6 6

Here itab is the internal table and f1,f2,f3,f4 and f5 are fileds in the internal table.

Thanks in advance

Siva Kumar