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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Identifying the first and last records in a sequence

When programming to analyse event or log files, you'll often find it fairly easy to identify the initial or opening record of a linked series, but harder to spot the closing one until your program has gone well past it. Take this data, for example:

  16:15:47 +00 INF: ===================Move Location===================
  16:15:47 +00 INF: Location: Set Initial Position: 6033998 encoder counts
  16:15:47 +00 INF: Location: Current Position: 6817024
  16:15:48 +00 INF: Location: Current Position: 6155537
  16:15:48 +00 INF: Location: Current Position: 6033996
  16:15:48 +00 INF: Location: Current Position: 6033993
  16:16:01 +00 INF: ===================Move Location===================
  16:16:01 +00 INF: Location: Set Initial Position: 6033998 encoder counts
  16:16:01 +00 INF: Location: Current Position: 6033996
  16:16:01 +00 INF: Location: Current Position: 6034021
  16:16:08 +00 INF: ===================Move Location===================
  16:16:09 +00 INF: Location: Set Initial Position: -5966001 encoder counts
  16:16:09 +00 INF: Location: Current Position: -5949212
  16:16:09 +00 INF: Location: Current Position: -5239635


You can spot the start point (liens with "Initial Position") straight away as you parser the file, but the end points (the last "Current Position" before a new "Initial Position") can only be confirmed as end points once you'v emoved beyond and found either a different record to confirm that the earlier sequence is done, or have reached the end of file. Programming-wise, this means making a note of each potentially final record and replacing it or finalaising a sequence at each subsequent read, with an additional "finalising" check after the file read has been completed. All of which sounds very complex, and if you don't get the code exactly right is prone to error!

There is an alternative - which is to keep note of each intermediate record as you read then in, overwriting them if they turn out not to be the final record. I have illustarted this using a movement object, in Python (version 3):

  starter = re.compile('Initial Position: (-?\d+)')
  increment = re.compile('Current Position: (-?\d+)')
  
  moves = []
  for record in open("steps.txt"):
    start = starter.findall(record)
    if start:
      moves.append(movement(start))
    else:
      inc = increment.findall(record)
      if inc:
        moves[-1].setStepPoint(inc)


And once I've parsed all the data, I can simply print out my objects:

  for move in moves:
    print(move)


Here's the class defintion:

  class movement:
    def __init__(self,startpoint):
      self.startpoint = int(startpoint[0])
    def setStepPoint(self,inc):
      self.nowAt = int(inc[0])
      self.soFar = self.startpoint - self.nowAt
    def __str__(self):
      return "From {} to {} movement is {}".format(
        self.startpoint, self.nowAt,self.soFar)


Complete source code [here]

Sample output:

  WomanWithCat:feb16 grahamellis$ python3 mlog
  From 6033998 to 6033993 movement is 5
  From 6033998 to 6034021 movement is -23
  From -5966001 to -5239635 movement is -726366
  WomanWithCat:feb16 grahamellis$

(written 2016-02-26)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Q110 - Object Orientation and General technical topics - Programming Algorithms
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  [227] Bellringing and Programming and Objects and Perl - (2005-02-25)
  [642] How similar are two words - (2006-03-11)
  [1157] Speed Networking - a great evening and how we arranged it - (2007-04-21)
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  [1391] Ordnance Survey Grid Reference to Latitude / Longitude - (2007-10-14)
  [1840] Validating Credit Card Numbers - (2008-10-14)
  [1949] Nuclear Physics comes to our web site - (2008-12-17)
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  [2509] A life lesson from the accuracy of numbers in Excel and Lua - (2009-11-21)
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Y050 - Python - General
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  [2017] Python - a truly dynamic language - (2009-01-30)
  [2020] Learning Python - many new example programs - (2009-01-31)
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  [2285] Great new diagrams for our notes ... Python releases - (2009-07-13)
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  [4712] A reminder of the key issues to consider in moving from Python 2 to Python 3 - (2016-10-30)


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Some other Articles
What is happening on the 231 bus? What are you going to do about it?
Prining a pound sign from Python AND running from the command line at the same time
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Rumours of bus changes by First in Wiltshire - what we know and suspect
Identifying the first and last records in a sequence
Image indexer / thumbnail display scripts in PHP
Getting to the Royal United Hospital - the Hopper and the alternatives
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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