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Splitting data reading code from data processing code - Ruby

An iterator (a.k.a. generator in Python) is a function which returns its results as it calculates them, rather than building them up into a larger structure to return all at once when the function is completed. So where you have a big flow of incoming data, you can handle it as it arrives rather than setting up massive arrays / lists.

Ruby makes very strong use of iterators. You define your iterator function to yield results as it gets them, and then you call your iterator and give it a block of code as an extra parameter. Here's a complete example:

def employees(filename)
   fh = File.new(filename)
   while staff = fh.gets
      yield staff
   end
end
 
def listskills(info)
   els = info.split(/\s+/)
   name = els.shift;
   print "#{name} knows #{els.join(", ")}\n"
end
 
employees("../data/requests.xyz") {|info| listskills(info)}


The exciting thing here is that you have split the code of your data reading phase (in the method "employees") from the code of your data processing phase (in the method "listskills") so that each function performs its own logical task, but they are co-existing / running almost in parallel. It would have been possible to write this code, easily, all as a single loop. But then the reader and processor elements would be forever intertwined, and there would be no easy way of reusing either element in another program.

My example above used a {} block to specify the operations to be performed on each value yielded by the iterator. I could also use a do ... end block. Example of the do-end [here], above example code in our course resources [here].
(written 2011-02-04, updated 2011-02-10)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
R104 - Ruby - Control Structures
  [4674] Alternating valuses / flip-flop / toggle - example in Ruby - (2016-05-17)
  [4504] Where does Ruby load modules from, and how to load from current directory - (2015-06-03)
  [4503] Separating your code for easier testing, understanding and re-use; example in Ruby - (2015-06-02)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [4370] Conditionals, loops and methods in Ruby - a primer with simple examples - (2014-12-29)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [3769] Muttable v immutable and implications - Ruby - (2012-06-20)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3619] Ruby v Perl - a comparison example - (2012-02-21)
  [3422] Assigning values to variables within other statements - Ruby - (2011-09-07)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3254] Multiple inputs, multiple out, ruby functions - (2011-04-19)
  [3253] Is this number between? Does this list include? - Ruby - (2011-04-18)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3158] Ruby training - some fresh examples for string handling applications - (2011-02-05)
  [2975] Why do I need brackets in Ruby ... or Perl, Python, C or Java - (2010-09-29)
  [2892] Alternative loops and conditionals in Ruby and Perl - (2010-07-28)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2619] Passing code to procedures and yield in Ruby - (2010-02-02)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2287] Learning to program in Ruby - examples of the programming basics - (2009-07-15)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [1891] Ruby to access web services - (2008-11-16)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1587] Some Ruby programming examples from our course - (2008-03-21)
  [1582] Ruby, C, Java and more - getting out of loops - (2008-03-19)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [995] Ruby's case - no break - (2006-12-17)
  [985] Equality in Ruby - == eql? and equal? - (2006-12-14)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [960] 1st, 2nd, 3rd revisited in Ruby - (2006-12-02)

R112 - Ruby - Parallelism
  [4680] Processing data line by line - iterator in Ruby with yield - (2016-05-19)
  [2979] Ruby - yield; parallel routines - (2010-10-01)


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