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perltrap - Perl traps for the unwary - Perldoc Browser
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This is a development version of Perl. </div> <h1><a id="toc">CONTENTS</a></h1> <ul> <li> <a class="text-decoration-none" href="#NAME">NAME</a> </li> <li> <a class="text-decoration-none" href="#DESCRIPTION">DESCRIPTION</a> <ul> <li> <a class="text-decoration-none" href="#Awk-Traps">Awk Traps</a> </li> <li> <a class="text-decoration-none" href="#C/C++-Traps">C/C++ Traps</a> </li> <li> <a class="text-decoration-none" href="#JavaScript-Traps">JavaScript Traps</a> </li> <li> <a class="text-decoration-none" href="#Sed-Traps">Sed Traps</a> </li> <li> <a class="text-decoration-none" href="#Shell-Traps">Shell Traps</a> </li> <li> <a class="text-decoration-none" href="#Perl-Traps">Perl Traps</a> </li> </ul> </li> </ul> <h1 id="NAME"><a class="permalink" href="#NAME">#</a>NAME</h1> <p>perltrap - Perl traps for the unwary</p> <h1 id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">#</a>DESCRIPTION</h1> <p>The biggest trap of all is forgetting to <code>use warnings</code> or use the <b>-w</b> switch; see <a href="/5.41.1/warnings">warnings</a> and <a href="/5.41.1/perlrun#-w">"-w" in perlrun</a>. The second biggest trap is not making your entire program runnable under <code>use strict</code>. The third biggest trap is not reading the list of changes in this version of Perl; see <a href="/5.41.1/perldelta">perldelta</a>.</p> <h2 id="Awk-Traps"><a class="permalink" href="#Awk-Traps">#</a><a id="Awk"></a>Awk Traps</h2> <p>Accustomed <b>awk</b> users should take special note of the following:</p> <ul> <li><p>A Perl program executes only once, not once for each input line. You can do an implicit loop with <code>-n</code> or <code>-p</code>.</p> </li> <li><p>The English module, loaded via</p> <pre><code>use English;</code></pre> <p>allows you to refer to special variables (like <code>$/</code>) with names (like $RS), as though they were in <b>awk</b>; see <a href="/5.41.1/perlvar">perlvar</a> for details.</p> </li> <li><p>Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.</p> </li> <li><p>Curly brackets are required on <code>if</code>s and <code>while</code>s.</p> </li> <li><p>Variables begin with "$", "@" or "%" in Perl.</p> </li> <li><p>Arrays index from 0. Likewise string positions in substr() and index().</p> </li> <li><p>You have to decide whether your array has numeric or string indices.</p> </li> <li><p>Hash values do not spring into existence upon mere reference.</p> </li> <li><p>You have to decide whether you want to use string or numeric comparisons.</p> </li> <li><p>Reading an input line does not split it for you. You get to split it to an array yourself. And the split() operator has different arguments than <b>awk</b>'s.</p> </li> <li><p>The current input line is normally in $_, not $0. It generally does not have the newline stripped. ($0 is the name of the program executed.) See <a href="/5.41.1/perlvar">perlvar</a>.</p> </li> <li><p>$<<i>digit</i>> does not refer to fields--it refers to substrings matched by the last match pattern.</p> </li> <li><p>The print() statement does not add field and record separators unless you set <code>$,</code> and <code>$\</code>. You can set $OFS and $ORS if you're using the English module.</p> </li> <li><p>You must open your files before you print to them.</p> </li> <li><p>The range operator is "..", not comma. The comma operator works as in C.</p> </li> <li><p>The match operator is "=~", not "~". ("~" is the one's complement operator, as in C.)</p> </li> <li><p>The exponentiation operator is "**", not "^". "^" is the XOR operator, as in C. (You know, one could get the feeling that <b>awk</b> is basically incompatible with C.)</p> </li> <li><p>The concatenation operator is ".", not the null string. (Using the null string would render <code>/pat/ /pat/</code> unparsable, because the third slash would be interpreted as a division operator--the tokenizer is in fact slightly context sensitive for operators like "/", "?", and ">". And in fact, "." itself can be the beginning of a number.)</p> </li> <li><p>The <code>next</code>, <code>exit</code>, and <code>continue</code> keywords work differently.</p> </li> <li><p>The following variables work differently:</p> <pre><code>Awk Perl ARGC scalar @ARGV (compare with $#ARGV) ARGV[0] $0 FILENAME $ARGV FNR $. - something FS (whatever you like) NF $#Fld, or some such NR $. OFMT $# OFS $, ORS $\ RLENGTH length($&) RS $/ RSTART length($`) SUBSEP $;</code></pre> </li> <li><p>You cannot set $RS to a pattern, only a string.</p> </li> <li><p>When in doubt, run the <b>awk</b> construct through <b>a2p</b> and see what it gives you.</p> </li> </ul> <h2 id="C/C++-Traps"><a class="permalink" href="#C/C++-Traps">#</a><a id="C"></a><a id="C-C-Traps"></a>C/C++ Traps</h2> <p>Cerebral C and C++ programmers should take note of the following:</p> <ul> <li><p>Curly brackets are required on <code>if</code>'s and <code>while</code>'s.</p> </li> <li><p>You must use <code>elsif</code> rather than <code>else if</code>.</p> </li> <li><p>The <code>break</code> and <code>continue</code> keywords from C become in Perl <code>last</code> and <code>next</code>, respectively. Unlike in C, these do <i>not</i> work within a <code>do { } while</code> construct. See <a href="/5.41.1/perlsyn#Loop-Control">"Loop Control" in perlsyn</a>.</p> </li> <li><p>The switch statement is called <code>given</code>/<code>when</code> and only available in perl 5.10 or newer. See <a href="/5.41.1/perlsyn#Switch-Statements">"Switch Statements" in perlsyn</a>.</p> </li> <li><p>Variables begin with "$", "@" or "%" in Perl.</p> </li> <li><p>Comments begin with "#", not "/*" or "//". Perl may interpret C/C++ comments as division operators, unterminated regular expressions or the defined-or operator.</p> </li> <li><p>You can't take the address of anything, although a similar operator in Perl is the backslash, which creates a reference.</p> </li> <li><p><code>ARGV</code> must be capitalized. <code>$ARGV[0]</code> is C's <code>argv[1]</code>, and <code>argv[0]</code> ends up in <code>$0</code>.</p> </li> <li><p>System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)</p> </li> <li><p>Signal handlers deal with signal names, not numbers. Use <code>kill -l</code> to find their names on your system.</p> </li> </ul> <h2 id="JavaScript-Traps"><a class="permalink" href="#JavaScript-Traps">#</a><a id="JavaScript"></a>JavaScript Traps</h2> <p>Judicious JavaScript programmers should take note of the following:</p> <ul> <li><p>In Perl, binary <code>+</code> is always addition. <code>$string1 + $string2</code> converts both strings to numbers and then adds them. To concatenate two strings, use the <code>.</code> operator.</p> </li> <li><p>The <code>+</code> unary operator doesn't do anything in Perl. It exists to avoid syntactic ambiguities.</p> </li> <li><p>Unlike <code>for...in</code>, Perl's <code>for</code> (also spelled <code>foreach</code>) does not allow the left-hand side to be an arbitrary expression. It must be a variable:</p> <pre><code> for my $variable (keys %hash) { ... }</code></pre> <p>Furthermore, don't forget the <code>keys</code> in there, as <code>foreach my $kv (%hash) {}</code> iterates over the keys and values, and is generally not useful ($kv would be a key, then a value, and so on).</p> </li> <li><p>To iterate over the indices of an array, use <code>foreach my $i (0 .. $#array) {}</code>. <code>foreach my $v (@array) {}</code> iterates over the values.</p> </li> <li><p>Perl requires braces following <code>if</code>, <code>while</code>, <code>foreach</code>, etc.</p> </li> <li><p>In Perl, <code>else if</code> is spelled <code>elsif</code>.</p> </li> <li><p><code>? :</code> has higher precedence than assignment. In JavaScript, one can write:</p> <pre><code class="plaintext">condition ? do_something() : variable = 3</code></pre> <p>and the variable is only assigned if the condition is false. In Perl, you need parentheses:</p> <pre><code>$condition ? do_something() : ($variable = 3);</code></pre> <p>Or just use <code>if</code>.</p> </li> <li><p>Perl requires semicolons to separate statements.</p> </li> <li><p>Variables declared with <code>my</code> only affect code <i>after</i> the declaration. You cannot write <code>$x = 1; my $x;</code> and expect the first assignment to affect the same variable. It will instead assign to an <code>$x</code> declared previously in an outer scope, or to a global variable.</p> <p>Note also that the variable is not visible until the following <i>statement</i>. This means that in <code>my $x = 1 + $x</code> the second $x refers to one declared previously.</p> </li> <li><p><code>my</code> variables are scoped to the current block, not to the current function. If you write <code>{my $x;} $x;</code>, the second <code>$x</code> does not refer to the one declared inside the block.</p> </li> <li><p>An object's members cannot be made accessible as variables. The closest Perl equivalent to <code>with(object) { method() }</code> is <code>for</code>, which can alias <code>$_</code> to the object:</p> <pre><code> for ($object) { $_->method; }</code></pre> </li> <li><p>The object or class on which a method is called is passed as one of the method's arguments, not as a separate <code>this</code> value.</p> </li> </ul> <h2 id="Sed-Traps"><a class="permalink" href="#Sed-Traps">#</a><a id="Sed"></a>Sed Traps</h2> <p>Seasoned <b>sed</b> programmers should take note of the following:</p> <ul> <li><p>A Perl program executes only once, not once for each input line. You can do an implicit loop with <code>-n</code> or <code>-p</code>.</p> </li> <li><p>Backreferences in substitutions use "$" rather than "\".</p> </li> <li><p>The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.</p> </li> <li><p>The range operator is <code>...</code>, rather than comma.</p> </li> </ul> <h2 id="Shell-Traps"><a class="permalink" href="#Shell-Traps">#</a><a id="Shell"></a>Shell Traps</h2> <p>Sharp shell programmers should take note of the following:</p> <ul> <li><p>The backtick operator does variable interpolation without regard to the presence of single quotes in the command.</p> </li> <li><p>The backtick operator does no translation of the return value, unlike <b>csh</b>.</p> </li> <li><p>Shells (especially <b>csh</b>) do several levels of substitution on each command line. Perl does substitution in only certain constructs such as double quotes, backticks, angle brackets, and search patterns.</p> </li> <li><p>Shells interpret scripts a little bit at a time. Perl compiles the entire program before executing it (except for <code>BEGIN</code> blocks, which execute at compile time).</p> </li> <li><p>The arguments are available via @ARGV, not $1, $2, etc.</p> </li> <li><p>The environment is not automatically made available as separate scalar variables.</p> </li> <li><p>The shell's <code>test</code> uses "=", "!=", "<" etc for string comparisons and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which uses <code>eq</code>, <code>ne</code>, <code>lt</code> for string comparisons, and <code>==</code>, <code>!=</code> <code><</code> etc for numeric comparisons.</p> </li> </ul> <h2 id="Perl-Traps"><a class="permalink" href="#Perl-Traps">#</a><a id="Perl"></a>Perl Traps</h2> <p>Practicing Perl Programmers should take note of the following:</p> <ul> <li><p>Remember that many operations behave differently in a list context than they do in a scalar one. See <a href="/5.41.1/perldata">perldata</a> for details.</p> </li> <li><p>Avoid barewords if you can, especially all lowercase ones. You can't tell by just looking at it whether a bareword is a function or a string. By using quotes on strings and parentheses on function calls, you won't ever get them confused.</p> </li> <li><p>You cannot discern from mere inspection which builtins are unary operators (like chop() and chdir()) and which are list operators (like print() and unlink()). (Unless prototyped, user-defined subroutines can <b>only</b> be list operators, never unary ones.) See <a href="/5.41.1/perlop">perlop</a> and <a href="/5.41.1/perlsub">perlsub</a>.</p> </li> <li><p>People have a hard time remembering that some functions default to $_, or @ARGV, or whatever, but that others which you might expect to do not.</p> </li> <li><p>The <FH> construct is not the name of the filehandle, it is a readline operation on that handle. The data read is assigned to $_ only if the file read is the sole condition in a while loop:</p> <pre><code>while (<FH>) { } while (defined($_ = <FH>)) { }.. <FH>; # data discarded!</code></pre> </li> <li><p>Remember not to use <code>=</code> when you need <code>=~</code>; these two constructs are quite different:</p> <pre><code>$x = /foo/; $x =~ /foo/;</code></pre> </li> <li><p>The <code>do {}</code> construct isn't a real loop that you can use loop control on.</p> </li> <li><p>Use <code>my()</code> for local variables whenever you can get away with it (but see <a href="/5.41.1/perlform">perlform</a> for where you can't). Using <code>local()</code> actually gives a local value to a global variable, which leaves you open to unforeseen side-effects of dynamic scoping.</p> </li> <li><p>If you localize an exported variable in a module, its exported value will not change. The local name becomes an alias to a new value but the external name is still an alias for the original.</p> </li> </ul> <p>As always, if any of these are ever officially declared as bugs, they'll be fixed and removed.</p> </div> <div id="footer"> <p>Perldoc Browser is maintained by Dan Book (<a href="https://metacpan.org/author/DBOOK">DBOOK</a>). Please contact him via the <a href="https://github.com/Grinnz/perldoc-browser/issues">GitHub issue tracker</a> or <a href="mailto:dbook@cpan.org">email</a> regarding any issues with the site itself, search, or rendering of documentation.</p> <p>The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the <a href="https://github.com/Perl/perl5/issues">Perl issue tracker</a>, the <a href="https://lists.perl.org/list/perl5-porters.html">mailing list</a>, or <a href="https://kiwiirc.com/client/irc.perl.org/p5p">IRC</a> to report any issues with the contents or format of the documentation.</p> </div> </div> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.slim.min.js" integrity="sha512-/DXTXr6nQodMUiq+IUJYCt2PPOUjrHJ9wFrqpJ3XkgPNOZVfMok7cRw6CSxyCQxXn6ozlESsSh1/sMCTF1rL/g==" crossorigin="anonymous"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.1/umd/popper.min.js" integrity="sha512-ubuT8Z88WxezgSqf3RLuNi5lmjstiJcyezx34yIU2gAHonIi27Na7atqzUZCOoY4CExaoFumzOsFQ2Ch+I/HCw==" crossorigin="anonymous"></script> <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.5.2/js/bootstrap.min.js" integrity="sha384-B4gt1jrGC7Jh4AgTPSdUtOBvfO8shuf57BaghqFfPlYxofvL8/KUEfYiJOMMV+rV" crossorigin="anonymous"></script> <script src="/js/highlight.pack.js"></script> <script>hljs.highlightAll();</script> </body> </html>