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Editing: 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>. 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.
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