There's often a requirement in a program to see if one value is between two others, and in most languages you'll write that as as double condition:
  if (n >= 5 && n <= 8) printf("Yesssss!\n");
in C or C++, for example.

Some languages give you further options / methods you can use to test for values between two others - usually they're the object oriented languages where additional operators can be defined as methods. In other words - when the authors of he language ran out of special symbols, they started to use words! Here is a use of the between? method in Ruby:
  if k.between?(5,8)

Another requirement is to see if a value is present in a list / array / collection. And in languages such as C you'll write that as a loop, checking each item of the list in turn and using a break go get out of the loop if and when you find the item you're looking for.

Looking for an item in a list is such a common requirement that many of the object oriented languages have defined methods which let you do the test in a single line, hiding (encapsulating) the loop that's actually still going to be run within the class code. So in Ruby you cab simply write:
  if [2,3,5,6,7].include?(k)

There's a new demonstration of each of these, in the context of a complete program, [here] on our web site. I'm running a private Ruby course this week. Public courses - see [here].
(written 2011-04-18, updated 2011-04-19)

 R107 - Collections (Arrays and Hashes) in Ruby
  [991] Adding a member to a Hash in Ruby - (2006-12-16)
  [2291] Collection objects (array and hash) in Ruby - (2009-07-16)
  [2606] Sorting arrays and hashes in Ruby - (2010-01-30)
  [2618] What are Ruby Symbols? - (2010-02-02)
  [2621] Ruby collections and strings - some new examples - (2010-02-03)
  [2976] Creating, extending, traversing and combining Ruby arrays - (2010-09-30)
  [3255] Process every member of an array, and sort an array - Ruby - (2011-04-21)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3435] Sorta sorting a hash, and what if an exception is NOT thrown - Ruby - (2011-09-12)
  [3757] Ruby - a teaching example showing many of the language features in short but useful program - (2012-06-09)
  [4368] Shuffling a list - Ruby and Python - (2014-12-28)
  [4499] Significant work - beyond helloworld in Ruby - (2015-05-27)
  [4502] Reading and parsing a JSON object in Ruby - (2015-06-01)

R104 - Ruby - Control Structures
  [960] 1st, 2nd, 3rd revisited in Ruby - (2006-12-02)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [985] Equality in Ruby - == eql? and equal? - (2006-12-14)
  [995] Ruby's case - no break - (2006-12-17)
  [1163] A better alternative to cutting and pasting code - (2007-04-26)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1582] Ruby, C, Java and more - getting out of loops - (2008-03-19)
  [1587] Some Ruby programming examples from our course - (2008-03-21)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [1870] What to do with a huge crop of apples - (2008-11-04)
  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
  [1891] Ruby to access web services - (2008-11-16)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2287] Learning to program in Ruby - examples of the programming basics - (2009-07-15)
  [2471] A short form of if ... then ... else - (2009-10-23)
  [2619] Passing code to procedures and yield in Ruby - (2010-02-02)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2892] Alternative loops and conditionals in Ruby and Perl - (2010-07-28)
  [2975] Why do I need brackets in Ruby ... or Perl, Python, C or Java - (2010-09-29)
  [3156] Splitting data reading code from data processing code - Ruby - (2011-02-04)
  [3158] Ruby training - some fresh examples for string handling applications - (2011-02-05)
  [3159] Returning multiple values from a function call in various languages - a comparison - (2011-02-06)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3254] Multiple inputs, multiple out, ruby functions - (2011-04-19)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3422] Assigning values to variables within other statements - Ruby - (2011-09-07)
  [3619] Ruby v Perl - a comparison example - (2012-02-21)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3769] Muttable v immutable and implications - Ruby - (2012-06-20)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4370] Conditionals, loops and methods in Ruby - a primer with simple examples - (2014-12-29)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [4503] Separating your code for easier testing, understanding and re-use; example in Ruby - (2015-06-02)
  [4504] Where does Ruby load modules from, and how to load from current directory - (2015-06-03)
  [4674] Alternating valuses / flip-flop / toggle - example in Ruby - (2016-05-17)

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