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Saying NOT in Perl, PHP, Python, Lua ...

"Isn't there one standard way to say NOT?" asked one of my delegates on today's course - and it's an excellent question. But the answer to a question about a negative subject is itself in the negative - no, there isn't just a single way!

In fact .. I can think of no fewer that 12 ways!

• 1. ! The ! is the unary not operator in a wide range of languages.

• 2. != Using != means "does not equal". In languages such as C and PHP, it checks the contents of the variables, and in Perl it checks whether two values are numerically the same. In many OO languages, the operator is (or can be) overridden to mean something of your own choice.

• 3. <> This operator in some languages Perl is an alternative to the the != operator.

• 4. ~= In Lua, instead of using != to mean not equals, you used ~=.

• 5. ^ The caret character is used to indicate NOT on of in a regular expression group, and it has the same meaning in Lua's pattern matching too. For example ... [^aeiou] means match any one character which is not a lower case vowel.

• 6. not The word not operates in the same way as the ! operator in many languages. However - it has a different place in the operator precedence tree which means that you'll sometimes need to use round brackets in different places with not and with !.

• 7. capitalisation In a regular expression in Perl, and in Perl-like regular expression such as PHP's preg functions and Python, you can invert a charactered group b y capitalising the letter. So \s means a space character, but \S means a none-space. Capitalisation also applies in this way in Lua's pattern matching - so for example %a matches an alphabetic character, but %A matches a none-alpha.

• 8. unless In Perl, an unless statement may be used to cause a block of code to be performed only if a condition is NOT true.

• 9. until You'll find until loops in Perl and Lua - a block of code is repeated while a condition is NOT true (i.e. until it is true).

• 10. ne Perl's ne operator checks whether strings are equal. And the -ne operator in shell programming checks whether numbers are equal, using the == operator that Perl uses for numbers for strings instead. (This sort of difference really helps to keep me on my toes when I'm training!)

• 11. !~ "Does not match ..." a regular expression. Perl.

• 12. !== "Is not identical to" in PHP. The difference between !== and != is that the latter operator will return false (indicating two values are equal) if you compare the number zero to either the string 0, or an empty string, or a variable that does not exist. The former will ONLY return false if you compare two values of the same type that have the same value.
(written 2008-07-04, updated 2008-07-05)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C203 - C and C based languages - Conditionals and Loops
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  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3384] Loops - a comparison of goto, while and for - (2011-08-10)
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  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
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  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [2570] Function Prototypes in C - (2010-01-11)
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  [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)
  [1191] Smart English Output - via PHP and Perl ? : operator - (2007-05-18)
  [962] Breaking a loop - Ruby and other languages - (2006-12-03)
  [353] Wimbledon Neck - (2005-06-20)

H104 - PHP - Control Statements
  [3914] While, for, foreach or something else to loop. - (2012-11-06)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [2912] Predictions for the seagull population - (2010-08-09)
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  [1825] Question Mark - Colon operator (Perl and PHP) - (2008-10-08)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1199] Testing for one of a list of values. - (2007-05-22)
  [863] Double and Triple equals operator in PHP - (2006-09-12)
  [657] The ternary operator in Python - (2006-03-25)
  [421] Don't repeat code - use loops or functions - (2005-08-21)
  [406] Assignment, equality and identity in PHP - (2005-08-08)
  [340] Code and code maintainance efficiency - (2005-06-08)

J704 - Java - Loops and Conditional Statements
  [4427] Java example - for loop and conditionals from course exercise - (2015-02-16)
  [4346] A behaviour driven example of writing a Java program - (2014-12-09)
  [3038] Setting up individual variables, and arrays, in Java - some commented examples - (2010-11-09)
  [2734] for and foreach in Java - (2010-04-22)

P204 - Perl - Conditionals and Loops
  [4031] Showing what programming errors look like - web site pitfall - (2013-03-06)
  [2832] Are you learning Perl? Some more examples for you! - (2010-06-27)
  [2711] For loop - checked once, or evety time? Ruby v Perl comparison and contrast - (2010-04-07)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
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  [1607] Learning to program in Perl - (2008-04-11)
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P206 - Perl - More Loops and Conditionals
  [3619] Ruby v Perl - a comparison example - (2012-02-21)
  [3398] Perl - making best use of the flexibility, but also using good coding standards - (2011-08-19)
  [2972] Some more advanced Perl examples from a recent course - (2010-09-27)
  [2967] Multiway branches in Perl - the given and when syntax - (2010-09-22)
  [2892] Alternative loops and conditionals in Ruby and Perl - (2010-07-28)
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  [2817] Setting a safety net or fallback value in Perl - (2010-06-19)
  [2815] switch and case, or given and when in Perl - (2010-06-17)
  [299] What - no switch or case statement? - (2005-05-03)
  [138] Perl - redo and last without a loop - (2004-12-02)

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)
  [3769] Muttable v immutable and implications - Ruby - (2012-06-20)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3422] Assigning values to variables within other statements - Ruby - (2011-09-07)
  [3254] Multiple inputs, multiple out, ruby functions - (2011-04-19)
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  [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)
  [3156] Splitting data reading code from data processing code - Ruby - (2011-02-04)
  [2975] Why do I need brackets in Ruby ... or Perl, Python, C or Java - (2010-09-29)
  [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)
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  [1887] Ruby Programming Course - Saturday and Sunday - (2008-11-16)
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  [1738] Clean code, jump free (Example in Lua) - (2008-08-06)
  [1587] Some Ruby programming examples from our course - (2008-03-21)
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  [995] Ruby's case - no break - (2006-12-17)
  [985] Equality in Ruby - == eql? and equal? - (2006-12-14)
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T203 - Tcl/Tk - Conditionals and Loops
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)
  [3571] Comparing loop commands in Tcl - (2012-01-06)
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U103 - Lua - Conditionals and loops
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Y103 - Python - Conditionals and Loops
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  [4713] Equality (in Python) - (2016-10-30)
  [4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
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  [4210] If elif elif elif - multiway selection in Python - (2013-11-16)
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  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
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  [835] Python - when to use the in operator - (2006-08-16)
  [788] New - Conditional expressions in Python 2.5 - (2006-07-01)
  [668] Python - block insets help with documentation - (2006-04-04)


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