Python dictionaries - reaching to new uses

For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))

If it sounds easy, it should be easy.

"Go through a file and echo the lines ... but if a line's duplicated, echo it only once". This was a sample application requested on today's Python course ... but one which has been troubling the folks for a while as they tried to filter the data in other languages.

The solution? Set up a dictionary, and as each line is read pretend that the whole line is a key ... and see if the key exists (i.e. if the line occurred previously). If it didn't, then output the line and add a member to the dictionary to indicate that you've done so - keyed on the line, and with a value of "1" which we're not really interested in - we're just looking at the presence or absence of a member. Source code [here] and it's very short!

Such a use of dictionaries has provided a big leap forward for this application. What was previously a nighmare task is now just a few lines of code, and the hashing algorithm used by the dictionary means that it's going to be efficient even as the data set grows.

But there are lots of other ways that dictionaries can be used in this "unique line" application - just as there are lots of ways to get across the water once you find the narrowest and easiest crossing point ...

[here] is the application expanded to give a count of the number of times each line occurs, sorted by the number of occurrences.

[here] is an exended report, telling you all the line numbers on which a particular string occurs (makes up the line)

[here] is the same report, but sorted by the number of occurrences of each line - so you have all the unique lines together, for example.

Oh - the sample data is [here].

Further pictures at St Budeaux / the Saltash ferry ...

(written 2010-10-05)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

Y107 - Python - Dictionaries
  [103] Can't resist writing about Python - (2004-10-29)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [1144] Python dictionary for quick look ups - (2007-04-12)
  [1145] Using a list of keys and a list of values to make a dictionary in Python - zip - (2007-04-13)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3488] Python sets and frozensets - what are they? - (2011-10-20)
  [3554] Learning more about our web site - and learning how to learn about yours - (2011-12-17)
  [3555] Football league tables - under old and new point system. Python program. - (2011-12-18)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [3934] Multiple identical keys in a Python dict - yes, you can! - (2012-11-24)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4409] Setting up and using a dict in Python - simple first example - (2015-01-30)
  [4469] Sorting in Python 3 - and how it differs from Python 2 sorting - (2015-04-20)
  [4661] Unique word locator - Python dict example - (2016-03-06)
  [4668] Sorting a dict in Python - (2016-04-01)

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Python dictionaries - reaching to new uses
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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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