Arijit - please download the attached zipfile, decompress it, and import the SLP into your org to view the example pipeline I've given. Since SnapLogic is a streaming platform, it does not inherently support access to previous documents, so this example uses the Script snap to save previous non-null values for the "Value" field and continue to reuse it until a new value is present in the current document being processed.
Note that I have marked my changes to the default script that is generated with "// ***"
Hope this helps!
