path: root/pod/perltrap.pod

=head1 NAME

perltrap - Perl traps for the unwary

=head1 DESCRIPTION

The biggest trap of all is forgetting to C<use warnings> or use the B<-w>
switch; see L<warnings> and L<perlrun/-w>. The second biggest trap is not
making your entire program runnable under C<use strict>.  The third biggest
trap is not reading the list of changes in this version of Perl; see
L<perldelta>.

=head2 Awk Traps

Accustomed B<awk> users should take special note of the following:

=over 4

=item *

A Perl program executes only once, not once for each input line.  You can
do an implicit loop with C<-n> or C<-p>.

=item *

The English module, loaded via

    use English;

allows you to refer to special variables (like C<$/>) with names (like
$RS), as though they were in B<awk>; see L<perlvar> for details.

=item *

Semicolons are required after all simple statements in Perl (except
at the end of a block).  Newline is not a statement delimiter.

=item *

Curly brackets are required on C<if>s and C<while>s.

=item *

Variables begin with "$", "@" or "%" in Perl.

=item *

Arrays index from 0.  Likewise string positions in substr() and
index().

=item *

You have to decide whether your array has numeric or string indices.

=item *

Hash values do not spring into existence upon mere reference.

=item *

You have to decide whether you want to use string or numeric
comparisons.

=item *

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>'s.

=item *

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 L<perlvar>.

=item *

$<I<digit>> does not refer to fields--it refers to substrings matched
by the last match pattern.

=item *

The print() statement does not add field and record separators unless
you set C<$,> and C<$\>.  You can set $OFS and $ORS if you're using
the English module.

=item *

You must open your files before you print to them.

=item *

The range operator is "..", not comma.  The comma operator works as in
C.

=item *

The match operator is "=~", not "~".  ("~" is the one's complement
operator, as in C.)

=item *

The exponentiation operator is "**", not "^".  "^" is the XOR
operator, as in C.  (You know, one could get the feeling that B<awk> is
basically incompatible with C.)

=item *

The concatenation operator is ".", not the null string.  (Using the
null string would render C</pat/ /pat/> 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.)

=item *

The C<next>, C<exit>, and C<continue> keywords work differently.

=item *


The following variables work differently:

      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	$;

=item *

You cannot set $RS to a pattern, only a string.

=item *

When in doubt, run the B<awk> construct through B<a2p> and see what it
gives you.

=back

=head2 C/C++ Traps

Cerebral C and C++ programmers should take note of the following:

=over 4

=item *

Curly brackets are required on C<if>'s and C<while>'s.

=item *

You must use C<elsif> rather than C<else if>.

=item *

The C<break> and C<continue> keywords from C become in Perl C<last>
and C<next>, respectively.  Unlike in C, these do I<not> work within a
C<do { } while> construct.  See L<perlsyn/"Loop Control">.

=item *

The switch statement is called C<given>/C<when> and only available in
perl 5.10 or newer.  See L<perlsyn/"Switch Statements">.

=item *

Variables begin with "$", "@" or "%" in Perl.

=item *

Comments begin with "#", not "/*" or "//".  Perl may interpret C/C++
comments as division operators, unterminated regular expressions or
the defined-or operator.

=item *

You can't take the address of anything, although a similar operator
in Perl is the backslash, which creates a reference.

=item *

C<ARGV> must be capitalized.  C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]>
ends up in C<$0>.

=item *

System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0. (system(), however, returns zero for success.)

=item *

Signal handlers deal with signal names, not numbers.  Use C<kill -l>
to find their names on your system.

=back

=head2 JavaScript Traps

Judicious JavaScript programmers should take note of the following:

=over 4

=item *

In Perl, binary C<+> is always addition.  C<$string1 + $string2> converts
both strings to numbers and then adds them.  To concatenate two strings,
use the C<.> operator.

=item *

The C<+> unary operator doesn't do anything in Perl.  It exists to avoid
syntactic ambiguities.

=item *

Unlike C<for...in>, Perl's C<for> (also spelled C<foreach>) does not allow
the left-hand side to be an arbitrary expression.  It must be a variable:

   for my $variable (keys %hash) {
	...
   }

Furthermore, don't forget the C<keys> in there, as
C<foreach my $kv (%hash) {}> iterates over the keys and values, and is
generally not useful ($kv would be a key, then a value, and so on).

=item *

To iterate over the indices of an array, use C<foreach my $i (0 .. $#array)
{}>.  C<foreach my $v (@array) {}> iterates over the values.

=item *

Perl requires braces following C<if>, C<while>, C<foreach>, etc.

=item *

In Perl, C<else if> is spelled C<elsif>.

=item *

C<? :> has higher precedence than assignment.  In JavaScript, one can
write:

    condition ? do_something() : variable = 3

and the variable is only assigned if the condition is false.  In Perl, you
need parentheses:

    $condition ? do_something() : ($variable = 3);

Or just use C<if>.

=item *

Perl requires semicolons to separate statements.

=item *

Variables declared with C<my> only affect code I<after> the declaration.
You cannot write C<$x = 1; my $x;> and expect the first assignment to
affect the same variable.  It will instead assign to an C<$x> declared
previously in an outer scope, or to a global variable.

Note also that the variable is not visible until the following
I<statement>.  This means that in C<my $x = 1 + $x> the second $x refers
to one declared previously.

=item *

C<my> variables are scoped to the current block, not to the current
function.  If you write C<{my $x;} $x;>, the second C<$x> does not refer to
the one declared inside the block.

=item *

An object's members cannot be made accessible as variables.  The closest
Perl equivalent to C<with(object) { method() }> is C<for>, which can alias
C<$_> to the object:

    for ($object) {
	$_->method;
    }

=item *

The object or class on which a method is called is passed as one of the
method's arguments, not as a separate C<this> value.

=back

=head2 Sed Traps

Seasoned B<sed> programmers should take note of the following:

=over 4

=item *

A Perl program executes only once, not once for each input line.  You can
do an implicit loop with C<-n> or C<-p>.

=item *

Backreferences in substitutions use "$" rather than "\".

=item *

The pattern matching metacharacters "(", ")", and "|" do not have backslashes
in front.

=item *

The range operator is C<...>, rather than comma.

=back

=head2 Shell Traps

Sharp shell programmers should take note of the following:

=over 4

=item *

The backtick operator does variable interpolation without regard to
the presence of single quotes in the command.

=item *

The backtick operator does no translation of the return value, unlike B<csh>.

=item *

Shells (especially B<csh>) 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.

=item *

Shells interpret scripts a little bit at a time.  Perl compiles the
entire program before executing it (except for C<BEGIN> blocks, which
execute at compile time).

=item *

The arguments are available via @ARGV, not $1, $2, etc.

=item *

The environment is not automatically made available as separate scalar
variables.

=item *

The shell's C<test> uses "=", "!=", "<" etc for string comparisons and "-eq",
"-ne", "-lt" etc for numeric comparisons. This is the reverse of Perl, which
uses C<eq>, C<ne>, C<lt> for string comparisons, and C<==>, C<!=> C<< < >> etc
for numeric comparisons.

=back

=head2 Perl Traps

Practicing Perl Programmers should take note of the following:

=over 4

=item *

Remember that many operations behave differently in a list
context than they do in a scalar one.  See L<perldata> for details.

=item *

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.

=item *

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> be list
operators, never unary ones.)  See L<perlop> and L<perlsub>.

=item *

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.

=item *

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:

    while (<FH>)      { }
    while (defined($_ = <FH>)) { }..
    <FH>;  # data discarded!

=item *

Remember not to use C<=> when you need C<=~>;
these two constructs are quite different:

    $x =  /foo/;
    $x =~ /foo/;

=item *

The C<do {}> construct isn't a real loop that you can use
loop control on.

=item *

Use C<my()> for local variables whenever you can get away with
it (but see L<perlform> for where you can't).
Using C<local()> actually gives a local value to a global
variable, which leaves you open to unforeseen side-effects
of dynamic scoping.

=item *

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.

=back

As always, if any of these are ever officially declared as bugs,
they'll be fixed and removed.