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<h1>perltrap</h1>
<!-- -->
<ul><li><a href="#NAME">NAME</a><li><a href="#DESCRIPTION">DESCRIPTION</a><ul><li><a href="#Awk-Traps">Awk Traps</a><li><a href="#C%2fC%2b%2b-Traps">C/C++ Traps</a><li><a href="#JavaScript-Traps">JavaScript Traps</a><li><a href="#Sed-Traps">Sed Traps</a><li><a href="#Shell-Traps">Shell Traps</a><li><a href="#Perl-Traps">Perl Traps</a></ul></ul><a name="NAME"></a><h1>NAME</h1>
<p>perltrap - Perl traps for the unwary</p>
<a name="DESCRIPTION"></a><h1>DESCRIPTION</h1>
<p>The biggest trap of all is forgetting to <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">warnings</span></code>
or use the <b>-w</b>
switch; see <a href="warnings.html">warnings</a> and <a href="perlrun.html">perlrun</a>. The second biggest trap is not
making your entire program runnable under <code class="inline"><a class="l_k" href="functions/use.html">use</a> <span class="w">strict</span></code>
. The third biggest
trap is not reading the list of changes in this version of Perl; see
<a href="perldelta.html">perldelta</a>.</p>
<a name="Awk-Traps"></a><h2>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 class="inline">-<span class="w">n</span></code>
or <code class="inline">-p</code>
.</p>
</li>
<li>
<p>The English module, loaded via</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/use.html">use</a> <span class="w">English</span><span class="sc">;</span></li></ol></pre><p>allows you to refer to special variables (like <code class="inline"><span class="i">$/</span></code>
) with names (like
$RS), as though they were in <b>awk</b>; see <a href="perlvar.html">perlvar</a> for details.</p>
</li>
<li>
<p>Semicolons are required after all simple statements in Perl (except
at the end of a block). Newline is not a statement delimiter.</p>
</li>
<li>
<p>Curly brackets are required on <code class="inline">if</code>
s and <code class="inline">while</code>
s.</p>
</li>
<li>
<p>Variables begin with "$", "@" 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. ($0 is the name of the program
executed.) See <a href="perlvar.html">perlvar</a>.</p>
</li>
<li>
<p>$<<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 class="inline"><span class="i">$,</span></code>
and <code class="inline"><span class="i">$\</span></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 class="inline"><span class="q">/pat/</span> /<span class="w">pat</span>/</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 class="inline"><a class="l_k" href="functions/next.html">next</a></code>, <code class="inline"><a class="l_k" href="functions/exit.html">exit</a></code>, and <code class="inline"><a class="l_k" href="functions/continue.html">continue</a></code> keywords work differently.</p>
</li>
<li>
<p>The following variables work differently:</p>
<pre class="verbatim"><ol><li> Awk Perl</li><li> ARGC scalar @ARGV (compare with $#ARGV)</li><li> ARGV[0] $0</li><li> FILENAME $ARGV</li><li> FNR $. - something</li><li> FS (whatever you like)</li><li> NF $#Fld, or some such</li><li> NR $.</li><li> OFMT $#</li><li> OFS $,</li><li> ORS $\</li><li> RLENGTH length($&)</li><li> RS $/</li><li> RSTART length($`)</li><li> SUBSEP $;</li></ol></pre></li>
<li>
<p>You cannot set $RS to a pattern, only a string.</p>
</li>
<li>
<p>When in doubt, run the <b>awk</b> construct through <b>a2p</b> and see what it
gives you.</p>
</li>
</ul>
<a name="C%2fC%2b%2b-Traps"></a><h2>C/C++ Traps</h2>
<p>Cerebral C and C++ programmers should take note of the following:</p>
<ul>
<li>
<p>Curly brackets are required on <code class="inline">if</code>
's and <code class="inline">while</code>
's.</p>
</li>
<li>
<p>You must use <code class="inline">elsif</code>
rather than <code class="inline">else if</code>
.</p>
</li>
<li>
<p>The <code class="inline"><span class="w">break</span></code>
and <code class="inline"><a class="l_k" href="functions/continue.html">continue</a></code> keywords from C become in Perl <code class="inline"><a class="l_k" href="functions/last.html">last</a></code>
and <code class="inline"><a class="l_k" href="functions/next.html">next</a></code>, respectively. Unlike in C, these do <i>not</i> work within a
<code class="inline"><a class="l_k" href="functions/do.html">do</a> <span class="s">{</span> <span class="s">}</span> while</code>
construct. See <a href="perlsyn.html#Loop-Control">Loop Control in perlsyn</a>.</p>
</li>
<li>
<p>The switch statement is called <code class="inline">given/when</code> and only available in
perl 5.10 or newer. See <a href="perlsyn.html#Switch-Statements">Switch Statements in perlsyn</a>.</p>
</li>
<li>
<p>Variables begin with "$", "@" 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 class="inline"><span class="w">ARGV</span></code>
must be capitalized. <code class="inline"><span class="i">$ARGV</span>[<span class="n">0</span>]</code>
is C's <code class="inline"><span class="w">argv</span><span class="s">[</span><span class="n">1</span><span class="s">]</span></code>
, and <code class="inline"><span class="w">argv</span><span class="s">[</span><span class="n">0</span><span class="s">]</span></code>
ends up in <code class="inline"><span class="i">$0</span></code>
.</p>
</li>
<li>
<p>System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0. (system(), however, returns zero for success.)</p>
</li>
<li>
<p>Signal handlers deal with signal names, not numbers. Use <code class="inline"><a class="l_k" href="functions/kill.html">kill</a> -l</code>
to find their names on your system.</p>
</li>
</ul>
<a name="JavaScript-Traps"></a><h2>JavaScript Traps</h2>
<p>Judicious JavaScript programmers should take note of the following:</p>
<ul>
<li>
<p>In Perl, binary <code class="inline">+</code>
is always addition. <code class="inline"><span class="i">$string1</span> + <span class="i">$string2</span></code>
converts
both strings to numbers and then adds them. To concatenate two strings,
use the <code class="inline">.</code> operator.</p>
</li>
<li>
<p>The <code class="inline">+</code>
unary operator doesn't do anything in Perl. It exists to avoid
syntactic ambiguities.</p>
</li>
<li>
<p>Unlike <code class="inline"><a class="l_k" href="functions/for.html">for...in</a></code>, Perl's <code class="inline">for</code>
(also spelled <code class="inline">foreach</code>
) does not allow
the left-hand side to be an arbitrary expression. It must be a variable:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/for.html">for</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$variable</span> <span class="s">(</span><a class="l_k" href="functions/keys.html">keys</a> <span class="i">%hash</span><span class="s">)</span> <span class="s">{</span></li><li> ...</li><li> <span class="s">}</span></li></ol></pre><p>Furthermore, don't forget the <code class="inline"><a class="l_k" href="functions/keys.html">keys</a></code> in there, as
<code class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$kv</span> <span class="s">(</span><span class="i">%hash</span><span class="s">)</span> <span class="s">{</span><span class="s">}</span></code>
iterates over the keys and values, and is
generally not useful ($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 class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$i</span> <span class="s">(</span><span class="n">0</span> .. <span class="i">$#array</span><span class="s">)</span>
<span class="s">{</span><span class="s">}</span></code>
. <code class="inline"><a class="l_k" href="functions/foreach.html">foreach</a> <a class="l_k" href="functions/my.html">my</a> <span class="i">$v</span> <span class="s">(</span><span class="i">@array</span><span class="s">)</span> <span class="s">{</span><span class="s">}</span></code>
iterates over the values.</p>
</li>
<li>
<p>Perl requires braces following <code class="inline">if</code>
, <code class="inline">while</code>
, <code class="inline">foreach</code>
, etc.</p>
</li>
<li>
<p>In Perl, <code class="inline">else if</code>
is spelled <code class="inline">elsif</code>
.</p>
</li>
<li>
<p><code class="inline"><span class="q">? :</span></code>
has higher precedence than assignment. In JavaScript, one can
write:</p>
<pre class="verbatim"><ol><li> <span class="w">condition</span> ? <span class="i">do_something</span><span class="s">(</span><span class="s">)</span> <span class="co">:</span> <span class="w">variable</span> = <span class="n">3</span></li></ol></pre><p>and the variable is only assigned if the condition is false. In Perl, you
need parentheses:</p>
<pre class="verbatim"><ol><li> <span class="i">$condition</span> ? <span class="i">do_something</span><span class="s">(</span><span class="s">)</span> <span class="co">:</span> <span class="s">(</span><span class="i">$variable</span> = <span class="n">3</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>Or just use <code class="inline">if</code>
.</p>
</li>
<li>
<p>Perl requires semicolons to separate statements.</p>
</li>
<li>
<p>Variables declared with <code class="inline"><a class="l_k" href="functions/my.html">my</a></code> only affect code <i>after</i> the declaration.
You cannot write <code class="inline"><span class="i">$x</span> = <span class="n">1</span><span class="sc">;</span> <a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span><span class="sc">;</span></code>
and expect the first assignment to
affect the same variable. It will instead assign to an <code class="inline"><span class="i">$x</span></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 class="inline"><a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span> = <span class="n">1</span> + <span class="i">$x</span></code>
the second $x refers
to one declared previously.</p>
</li>
<li>
<p><code class="inline"><a class="l_k" href="functions/my.html">my</a></code> variables are scoped to the current block, not to the current
function. If you write <code class="inline"><span class="s">{</span><a class="l_k" href="functions/my.html">my</a> <span class="i">$x</span><span class="sc">;</span><span class="s">}</span> <span class="i">$x</span><span class="sc">;</span></code>
, the second <code class="inline"><span class="i">$x</span></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 class="inline"><span class="i">with</span><span class="s">(</span><span class="w">object</span><span class="s">)</span> <span class="s">{</span> <span class="i">method</span><span class="s">(</span><span class="s">)</span> <span class="s">}</span></code>
is <code class="inline">for</code>
, which can alias
<code class="inline"><span class="i">$_</span></code>
to the object:</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="functions/for.html">for</a> <span class="s">(</span><span class="i">$object</span><span class="s">)</span> <span class="s">{</span></li><li> <span class="i">$_</span><span class="i">->method</span><span class="sc">;</span></li><li> <span class="s">}</span></li></ol></pre></li>
<li>
<p>The object or class on which a method is called is passed as one of the
method's arguments, not as a separate <code class="inline"><span class="w">this</span></code>
value.</p>
</li>
</ul>
<a name="Sed-Traps"></a><h2>Sed Traps</h2>
<p>Seasoned <b>sed</b> programmers should take note of the following:</p>
<ul>
<li>
<p>A Perl program executes only once, not once for each input line. You can
do an implicit loop with <code class="inline">-<span class="w">n</span></code>
or <code class="inline">-p</code>
.</p>
</li>
<li>
<p>Backreferences in substitutions use "$" 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 class="inline">...</code>
, rather than comma.</p>
</li>
</ul>
<a name="Shell-Traps"></a><h2>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 class="inline">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 class="inline"><span class="w">test</span></code>
uses "=", "!=", "<" etc for string comparisons and "-eq",
"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
uses <code class="inline">eq</code>
, <code class="inline">ne</code>
, <code class="inline">lt</code>
for string comparisons, and <code class="inline">==</code>
, <code class="inline">!=</code>
<code class="inline"><</code>
etc
for numeric comparisons.</p>
</li>
</ul>
<a name="Perl-Traps"></a><h2>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="perldata.html">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="perlop.html">perlop</a> and <a href="perlsub.html">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 $_ only if the
file read is the sole condition in a while loop:</p>
<pre class="verbatim"><ol><li> while <span class="s">(</span><span class="q"><FH></span><span class="s">)</span> <span class="s">{</span> <span class="s">}</span></li><li> while <span class="s">(</span><a class="l_k" href="functions/defined.html">defined</a><span class="s">(</span><span class="i">$_</span> = <span class="q"><FH></span><span class="s">)</span><span class="s">)</span> <span class="s">{</span> <span class="s">}</span>..</li><li> <span class="q"><FH></span><span class="sc">;</span> <span class="c"># data discarded!</span></li></ol></pre></li>
<li>
<p>Remember not to use <code class="inline">=</code>
when you need <code class="inline">=~</code>
;
these two constructs are quite different:</p>
<pre class="verbatim"><ol><li> <span class="i">$x</span> = <span class="q">/foo/</span><span class="sc">;</span></li><li> <span class="i">$x</span> =~ <span class="q">/foo/</span><span class="sc">;</span></li></ol></pre></li>
<li>
<p>The <code class="inline"><a class="l_k" href="functions/do.html">do</a> <span class="s">{</span><span class="s">}</span></code>
construct isn't a real loop that you can use
loop control on.</p>
</li>
<li>
<p>Use <code class="inline"><a class="l_k" href="functions/my.html">my()</a></code> for local variables whenever you can get away with
it (but see <a href="perlform.html">perlform</a> for where you can't).
Using <code class="inline"><a class="l_k" href="functions/local.html">local()</a></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>
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