How do I use boolean variables in Perl

Question

I have tried    var   false   var   FALSE   var   False    None of these work  I get the error message  Bareword  false  not allowed while  strict subs  is in use

User · Answer

I came across a tutorial which have a well explaination about What values are true and false in Perl. It state that:

Following scalar values are considered false:

undef - the undefined value
0 the number 0, even if you write it as 000 or 0.0
'' the empty string.
'0' the string that contains a single 0 digit.

All other scalar values, including the following are true:

1 any non-0 number
' ' the string with a space in it
'00' two or more 0 characters in a string
"0\n" a 0 followed by a newline
'true'
'false' yes, even the string 'false' evaluates to true.

There is another good tutorial which explain about Perl true and false.

User · Answer

I recommend use boolean   You have to install the boolean module from cpan though

User · Answer

My favourites have always been  use constant FALSE   gt  1  0  use constant TRUE   gt  not FALSE    which is completely independent from the internal representation

User · Answer

The most complete  concise definition of false I ve come across is      Anything that stringifies to the empty string or the string 0 is false  Everything else is true    Therefore  the following values are false    The empty string Numerical value zero An undefined value An object with an overloaded boolean operator that evaluates one of the above  A magical variable that evaluates to one of the above on fetch    Keep in mind that an empty list literal evaluates to an undefined value in scalar context  so it evaluates to something false     A note on  true zeroes   While numbers that stringify to 0 are false  strings that numify to zero aren t necessarily  The only false strings are 0 and the empty string  Any other string  even if it numifies to zero  is true   The following are strings that are true as a boolean and zero as a number    Without a warning     0 0   0E0   00    0   -0    0   0 n    0   0    0 but true    t00    n0e1    0 e-9   With a warning    Any string for which Scalar  Util  looks like number returns false   e g   abc

User · Answer

use the following file prefix  this will add to your perl script eTRUE and eFALSE  it will actually be REAL    true and false  just like java      usr bin perl use strict  use warnings   use constant    real true false  compatible with encode json decode json for later  we don t want field false    will be field 0                     eTRUE    gt   bless  do   my  o   1     JSON  PP  Boolean                     eFALSE   gt   bless  do   my  o   0     JSON  PP  Boolean                      There are  actually  few reasons why you should use that   My reason is that working with JSON  I ve got 0 and 1 as values to keys  but this hack will make sure correct values are kept along your script

User · Answer

Perl doesn t have a native boolean type  but you can use comparison of integers or strings in order to get the same behavior  Alan s example is a nice way of doing that using comparison of integers  Here s an example  my  boolean   0  if    boolean         print   boolean evaluates to true n     else       print   boolean evaluates to false n       One thing that I ve done in some of my programs is added the same behavior using a constant      usr bin perl  use strict  use warnings   use constant false   gt  0  use constant true    gt  1   my  val1   true  my  val2   false   print  val1     amp  amp      val2  if    val1  amp  amp   val2         print   evaluates to true  n     else       print   evaluates to false  n      print  val1           val2  if    val1     val2         print   evaluates to true  n     else       print   evaluates to false  n       The lines marked in  use constant  define a constant named true that always evaluates to 1  and a constant named false that always evaluates by 0  Because of the way that constants are defined in Perl  the following lines of code fails as well   true   0  true   false    The error message should say something like  Can t modify constant in scalar assignment    I saw that in one of the comments you asked about comparing strings  You should know that because Perl combines strings and numeric types in scalar variables  you have different syntax for comparing strings and numbers   my  var1    5 0   my  var2    5    print  using operator eq n   if    var1 eq  var2         print   var1 and  var2 are equal  n     else       print   var1 and  var2 are not equal  n      print  using operator    n   if    var1     var2         print   var1 and  var2 are equal  n     else       print   var1 and  var2 are not equal  n       The difference between these operators is a very common source of confusion in Perl

User · Answer

Beautiful explanation given by bobf for Boolean values    True or False  A Quick Reference Guide  Truth tests for different values                             Result of the expression when  var is   Expression            1         0 0     a string   0       empty str   undef -------------------- -------- -------- ---------- ------- ----------- ------- if   var              true     true     true       false   false       false if  defined  var      true     true     true       true    true        false if   var eq           false    false    false      false   true        true if   var    0         false    true     true       true    true        true

User · Answer

In Perl  the following evaluate to false in conditionals   0  0  undef       Empty scalar       Empty list        The rest are true  There are no barewords for true or false

[perl] How do I use boolean variables in Perl?

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