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POPerl booleans, negation (and how to explain it)?
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copied!<p>I'm new here. After reading through how to ask and format, I hope this will be an OK question. I'm not very skilled in perl, but it is the programming language what I known most.</p> <p>I trying apply Perl to real life but I didn't get an great understanding - especially not from my wife. I tell her that:</p> <blockquote> <p>if she didn't bring to me <strong>3</strong> beers in the evening, that means I got <em>zero (or nothing)</em> beers. </p> </blockquote> <p>As you probably guessed, without much success. :(</p> <p>Now factually. From <a href="http://perldoc.perl.org/perlop.html#Symbolic-Unary-Operators">perlop</a>:</p> <blockquote> <p>Unary "!" performs logical negation, that is, "not".</p> </blockquote> <p>Languages, what have <code>boolean</code> types (what can have only two "values") is OK:</p> <blockquote> <p>if it is not the one value -> <em>must be</em> the another one.</p> </blockquote> <p>so naturally:</p> <pre><code>!true -> false !false -> true </code></pre> <p>But perl doesn't have <code>boolean</code> variables - have only a <code>truth system</code>, whrere everything is not <code>0</code>, <code>'0'</code> <code>undef</code>, <code>''</code> is TRUE. Problem comes, when applying <em>logical negation</em> to an <em>not logical value</em> e.g. numbers.</p> <p>E.g. If some number <em>IS NOT 3</em>, thats mean it <em>IS ZERO</em> or empty, instead of the <em>real life</em> meaning, where if something is NOT 3, mean it can be <em>anything but 3</em> (e.g. zero too).</p> <p>So the next code:</p> <pre><code>use 5.014; use Strictures; my $not_3beers = !3; say defined($not_3beers) ? "defined, value>$not_3beers<" : "undefined"; say $not_3beers ? "TRUE" : "FALSE"; my $not_4beers = !4; printf qq{What is not 3 nor 4 mean: They're same value: %d!\n}, $not_3beers if( $not_3beers == $not_4beers ); say qq(What is not 3 nor 4 mean: @{[ $not_3beers ? "some bears" : "no bears" ]}!) if( $not_3beers eq $not_4beers ); say ' $not_3beers>', $not_3beers, "<"; say '-$not_3beers>', -$not_3beers, "<"; say '+$not_3beers>', -$not_3beers, "<"; </code></pre> <p>prints:</p> <pre><code>defined, value>< FALSE What is not 3 nor 4 mean: They're same value: 0! What is not 3 nor 4 mean: no bears! $not_3beers>< -$not_3beers>0< +$not_3beers>0< </code></pre> <p>Moreover:</p> <pre><code>perl -E 'say !!4' </code></pre> <p>what is <em>not not 4</em> IS <strong>1</strong>, instead of 4!</p> <p>The above statements with wife are "false" (mean 0) :), but <strong>really trying teach my son</strong> Perl and he, after a while, asked my wife: <em>why, if something is not 3 mean it is 0 ?</em> .</p> <p>So the questions are:</p> <ul> <li>how to explain this to my son</li> <li>why perl has this design, so why <code>!0</code> is everytime 1 </li> <li>Is here something "behind" what requires than <code>!0</code> is not any <em>random number</em>, but 0.</li> <li>as I already said, I don't know well other languages - in every language is <code>!3 == 0</code>?</li> </ul>

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