perltrap - Perl traps for the unwary
The biggest trap of all is forgetting touse warnings or use the-w switch; seewarnings andperlrun. The second biggest trap is not making your entire program runnable underuse strict. The third biggest trap is not reading the list of changes in this version of Perl; seeperldelta.
Accustomedawk 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 $RS), as though they were inawk; seeperlvar for details.
Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.
Curly brackets are required onifs andwhiles.
Variables begin with "$", "@" 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 thanawk'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.) Seeperlvar.
$<digit> does not refer to fields--it refers to substrings matched by the last match pattern.
The print() statement does not add field and record separators unless you set$, 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 thatawk 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.)
Thenext,exit, andcontinue keywords work differently.
The following variables work differently:
AwkPerlARGCscalar @ARGV (compare with $#ARGV)ARGV[0]$0FILENAME$ARGVFNR$. - somethingFS(whatever you like)NF$#Fld, or some suchNR$.OFMT$#OFS$,ORS$\RLENGTHlength($&)RS$/RSTARTlength($`)SUBSEP$;You cannot set $RS to a pattern, only a string.
When in doubt, run theawk construct througha2p and see what it gives you.
Cerebral C and C++ programmers should take note of the following:
Curly brackets are required onif's andwhile's.
You must useelsif rather thanelse if.
Thebreak andcontinue keywords from C become in Perllast andnext, respectively. Unlike in C, these donot work within ado { } while construct. See"Loop Control" in perlsyn.
The switch statement is calledgiven/when and only available in perl 5.10 or newer. See"Switch Statements" in perlsyn.
Variables begin with "$", "@" 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$0.
System calls such as link(), unlink(), rename(), etc. return nonzero for success, not 0. (system(), however, returns zero for success.)
Signal handlers deal with signal names, not numbers. Usekill -l to find their names on your system.
Judicious JavaScript programmers should take note of the following:
In Perl, binary+ is always addition.$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.
Unlikefor...in, Perl'sfor (also spelledforeach) 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 thekeys 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, useforeach my $i (0 .. $#array) {}.foreach my $v (@array) {} iterates over the values.
Perl requires braces followingif,while,foreach, etc.
In Perl,else if is spelledelsif.
? : has higher precedence than assignment. In JavaScript, one can write:
condition ? do_something() : variable = 3and the variable is only assigned if the condition is false. In Perl, you need parentheses:
$condition ? do_something() : ($variable = 3);Or just useif.
Perl requires semicolons to separate statements.
Variables declared withmy only affect codeafter 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 followingstatement. This means that inmy $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 towith(object) { method() } isfor, which can alias$_ to the object:
for ($object) {$_->method; }The object or class on which a method is called is passed as one of the method's arguments, not as a separatethis value.
Seasonedsed programmers should take note of the following:
A Perl program executes only once, not once for each input line. You can do an implicit loop with-n or-p.
Backreferences in substitutions use "$" rather than "\".
The pattern matching metacharacters "(", ")", and "|" do not have backslashes in front.
The range operator is..., rather than comma.
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, unlikecsh.
Shells (especiallycsh) 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 forBEGIN 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'stest 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.
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. Seeperldata 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 canonly be list operators, never unary ones.) Seeperlop andperlsub.
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/;Thedo {} construct isn't a real loop that you can use loop control on.
Usemy() for local variables whenever you can get away with it (but seeperlform 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.
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