Depends on where you are:
if it is in the itemchanged event then the data argument contains the person_id:
ll_row = ldwc.Find ( 'person_id=' + data , 1 , ldwc.RowCount())
ItemChanged event happens before the value hits the data buffer.
if you are in a function or event and the value has hit the buffer, just use GetItemNumber
long ll_id
ll_id = dw_1.GetItemNumber ( 1 , 'person_id' )
ldwc.Find ( 'person_id=' + string ( ll_id ) , 1 , ldwc.RowCount())