How to give condition for internal table in if statement

Hi,

I have to use a condition in if statement inside a program but when i use this condition its showing error,So anybody  can help me for that.

my internal table declaration part is

  DATA: i_tabn TYPE STANDARD TABLE OF zsd_data.

  DATA: wa_tabn TYPE zsd_data.

i want to give condition before read statement.

if  i_tabn-field1 is initial.

i_tabn-field1 = '10'.

endif.

Thanks.