perltrap
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 $RS), as though they were in awk; see perlvar for details. -
Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.
-
Variables begin with "$", "@" 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 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 $RS to a pattern, only a string.
-
When in doubt, run the awk construct through a2p and see what it gives you.
C/C++ Traps
Cerebral C and C++ programmers should take note of the following:
-
The
break
andcontinue
keywords from C become in Perllast
andnext
, respectively. Unlike in C, these do not work within ado { } while
construct. See Loop Control in perlsyn. -
The switch statement is called
given/when
and only available in perl 5.10 or newer. See Switch Statements in perlsyn. -
Variables begin with "$", "@" 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'sargv[1]
, andargv[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'sfor
(also spelledforeach
) does not allow the left-hand side to be an arbitrary expression. It must be a variable:Furthermore, don't forget the
keys
in there, asforeach 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. -
? :
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() }
isfor
, 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 useseq
,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:
-
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). Usinglocal()
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.