Question

What is an efficient method to check if a file has changed and only print the new data?

Originally, I leaned towards a readline approach comparing line counts but that seemed very inefficient.

My initial query was if there was a way to check file size or time stamp from Netlogo. The answer was ‘no’. However, Charles Staelin was kind enough to add the get-date-ms method to the pathdir extension. So, now the file changes (as per time stamp) can be checked for with set currenttimestamp pathdir:get-date-ms "somefile.nlist"
if currenttimestamp != filetimestamp

Using the added method:

to checkfile
  ifelse checktick mod 19 = 0
    [set currenttimestamp pathdir:get-date-ms "somefile.nlist" 
     if currenttimestamp != filetimestamp
     [file-open "somefile.nlist"
      let thiscount 0
      while [not file-at-end?][
        set in1 file-read-line
        set global-in1 in1
        set thiscount thiscount + 1
        if thiscount > global-filelength AND (not file-at-end?)[ ;it seems that this not-at-file-end is redundant - why not?
          print in1
          set global-filelength thiscount
          set hasfilechanged true

       ]

        ]
     file-close
      set filetimestamp currenttimestamp
      set checktick checktick + 1
      ]

     ]

    [ set checktick checktick + 1]

end

Answer 1

Charles Staelin has recently added this to his pathdir extension, which adds an assortment of file and path oriented primitives to NetLogo. see http://groups.yahoo.com/group/netlogo-users/message/15301 . The extension is listed at https://github.com/NetLogo/NetLogo/wiki/Extensions