xpath can any one help me urjent

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biztalk

I have
Message1 of Scheema1(repeating records)
HeaderMessage of Scheema2
detailmessage of scheema3

My incoming message is Message1
1)After receving the mesage1 i asssaigned detail record xpath to some variable Details Counting
2) im constructing Headermessage.
headermessge=xpath(message1,"Xpath")
3)then i tookloop shape inside that im constructing detailmessage.
detailmessge=xpath(message1,"Xpath")
and incrementing the loop
Now i want to read the value of a particular field 'X'which is of decimal type
if it is null or "" i have to default its value to 0.

Can any one help me?

Regards

Raj