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PERLTRAP(1pm)          Perl Programmers Reference Guide          PERLTRAP(1pm)




NAME

       perltrap - Perl traps for the unwary


DESCRIPTION

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

   Awk Traps
       Accustomed awk users should take special note of the following:

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

       o   The English module, loaded via

               use English;

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

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

       o   Curly brackets are required on "if"s and "while"s.

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

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

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

       o   Hash values do not spring into existence upon mere reference.

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

       o   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 awk's.

       o   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 perlvar.

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

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

       o   You must open your files before you print to them.

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

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

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

       o   The concatenation operator is ".", not the null string.  (Using the
           null string would render "/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.)

       o   The "next", "exit", and "continue" keywords work differently.

       o   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    $;

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

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

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

       o   Curly brackets are required on "if"'s and "while"'s.

       o   You must use "elsif" rather than "else if".

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

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

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

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

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

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

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

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

   JavaScript Traps
       Judicious JavaScript programmers should take note of the following:

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

       o   The "+" unary operator doesn't do anything in Perl.  It exists to
           avoid syntactic ambiguities.

       o   Unlike "for...in", Perl's "for" (also spelled "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 "keys" in there, as "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).

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

       o   Perl requires braces following "if", "while", "foreach", etc.

       o   In Perl, "else if" is spelled "elsif".

       o   "? :" 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 "if".

       o   Perl requires semicolons to separate statements.

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

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

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

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

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

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

   Sed Traps
       Seasoned sed programmers should take note of the following:

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

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

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

       o   The range operator is "...", rather than comma.

   Shell Traps
       Sharp shell programmers should take note of the following:

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

       o   The backtick operator does no translation of the return value,
           unlike csh.

       o   Shells (especially 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.

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

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

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

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

   Perl Traps
       Practicing Perl Programmers should take note of the following:

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

       o   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.

       o   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 only be list operators, never unary ones.)  See
           perlop and perlsub.

       o   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.

       o   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!

       o   Remember not to use "=" when you need "=~"; these two constructs
           are quite different:

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

       o   The "do {}" construct isn't a real loop that you can use loop
           control on.

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

       o   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.

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



perl v5.24.0                      2016-03-01                     PERLTRAP(1pm)

perl 5.24 - Generated Sun Dec 4 19:29:30 CST 2016
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