Mapping Issue:Assign src field val to target when src field not exists

Hi

Pls guide though the following issue in Message Mapping

I do have a segment of 0...n. where the target element is manipulated based on the three fields in source segment.

<Source>
Parent Segment (1..1)
	Segment	(0...n)
		Field_1  =  23k	
		Field_2  =	 01
		Field_3  =  (either 'X' or No field in IDOC segment)


<Target>
Segment (1...1)
	t_Field  =  ( if ( ( Field_1 = '23K') & (Field_2 = '01'))  then  assign Field_3  )

How can i check or handle this Field_3 issue when its not present in IDOC segment?

Note: I am using UDF, and getting errors while retrieving Field_3 data when the field itself is not present in segment inside loop statement.

Thanks,

Vinod V