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 "-w" in 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.