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Lazy operators in Lua - what they mean, and examples

If you use the word or in an expression in Lua, the expression to the right of the word will only be evaluated and returned of the value to the left is nil. So a very common idiom is:
  stuff = stuff or {}
which means "if stuff already has a value, leave it alone - otherwise make it an empty table"

If you use the word and on the other hand, the expression to the right of the word will only be evaluated nd returned if the value to the left is not nil. This is a much less common requirement:
  stuff = stuff and {}
which means "if stuff has a value, replace it with an empty table. But if it's nil, leave it like that"

Example from the Lua course completed at the end of last week - [here] and another example from earlier on [here].

(written 2014-05-05, updated 2014-05-06)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
U103 - Lua - Conditionals and loops
  [4574] repeat until in Lua - a one or more rather than a zero or more loop - (2015-11-05)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [3686] The goto statement in Lua - (2012-04-06)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2454] For loops in Lua - (2009-10-14)
  [2351] Ternary operators alternatives - Perl and Lua lazy operators - (2009-08-12)
  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1685] Short circuit evaluation (lazy operator) in Lua - (2008-06-22)


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Dot or Colon separator between table name and member in Lua - what is the difference?
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