perltrap(1)
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:
* A Perl program executes only once, not once for each input line.
You can do an implicit loop with "-n" or "-p".
* The English module, loaded via
use English;
allows you to refer to special variables (like $/) with names (like
$RS), as though they were in awk; see perlvar for details.
* Semicolons are required after all simple statements in Perl (except
at the end of a block). Newline is not a statement delimiter.
* Curly brackets are required on "if"s and "while"s.
* Variables begin with "$", "@" or "%" in Perl.
* Arrays index from 0. Likewise string positions in substr() and
index().
* You have to decide whether your array has numeric or string
indices.
* Hash values do not spring into existence upon mere reference.
* You have to decide whether you want to use string or numeric
comparisons.
* 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.
* 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.
* $<digit> does not refer to fields--it refers to substrings matched
by the last match pattern.
* 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.
* You must open your files before you print to them.
* The range operator is "..", not comma. The comma operator works as
in C.
* The match operator is "=~", not "~". ("~" is the one's complement
operator, as in C.)
* 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.)
* 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.)
* The "next", "exit", and "continue" keywords work differently.
* 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 $;
* You cannot set $RS to a pattern, only a string.
* When in doubt, run the awk construct through a2p and see what it
gives you.
C/C++ Traps
Cerebral C and C++ programmers should take note of the following:
* Curly brackets are required on "if"'s and "while"'s.
* You must use "elsif" rather than "else if".
* The "break" and "continue" keywords from C become in Perl "last"
and "next", respectively. Unlike in C, these do not work within a
"do { } while" construct. See "Loop Control" in perlsyn.
* The switch statement is called "given"/"when" and only available in
perl 5.10 or newer. See "Switch Statements" in perlsyn.
* Variables begin with "$", "@" or "%" in Perl.
* Comments begin with "#", not "/*" or "//". Perl may interpret
C/C++ comments as division operators, unterminated regular
expressions or the defined-or operator.
* You can't take the address of anything, although a similar operator
in Perl is the backslash, which creates a reference.
* "ARGV" must be capitalized. $ARGV[0] is C's "argv[1]", and
"argv[0]" ends up in $0.
* System calls such as link(), unlink(), rename(), etc. return
nonzero for success, not 0. (system(), however, returns zero for
success.)
* Signal handlers deal with signal names, not numbers. Use "kill -l"
to find their names on your system.
JavaScript Traps
Judicious JavaScript programmers should take note of the following:
* In Perl, binary "+" is always addition. "$string1 + $string2"
converts both strings to numbers and then adds them. To
concatenate two strings, use the "." operator.
* The "+" unary operator doesn't do anything in Perl. It exists to
avoid syntactic ambiguities.
* 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).
* To iterate over the indices of an array, use "foreach my $i (0 ..
$#array) {}". "foreach my $v (@array) {}" iterates over the
values.
* Perl requires braces following "if", "while", "foreach", etc.
* In Perl, "else if" is spelled "elsif".
* "? :" 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".
* Perl requires semicolons to separate statements.
* 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.
* "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.
* 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) {
$_->method;
}
* The object or class on which a method is called is passed as one of
the method's arguments, not as a separate "this" value.
Sed Traps
Seasoned sed programmers should take note of the following:
* A Perl program executes only once, not once for each input line.
You can do an implicit loop with "-n" or "-p".
* Backreferences in substitutions use "$" rather than "\".
* The pattern matching metacharacters "(", ")", and "|" do not have
backslashes in front.
* The range operator is "...", rather than comma.
Shell Traps
Sharp shell programmers should take note of the following:
* The backtick operator does variable interpolation without regard to
the presence of single quotes in the command.
* The backtick operator does no translation of the return value,
unlike csh.
* 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.
* 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).
* The arguments are available via @ARGV, not $1, $2, etc.
* The environment is not automatically made available as separate
scalar variables.
* 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:
* Remember that many operations behave differently in a list context
than they do in a scalar one. See perldata for details.
* 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.
* 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.
* 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.
* 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!
* Remember not to use "=" when you need "=~"; these two constructs
are quite different:
$x = /foo/;
$x =~ /foo/;
* The "do {}" construct isn't a real loop that you can use loop
control on.
* 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.
* 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.
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