perltrap - Perl traps for the unwary
The biggest trap of all is forgetting to use warnings or use the -w
switch; see the perllexwarn manpage and the perlrun manpage. 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
the perldelta manpage.
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
$RS), as though they were in awk; see the perlvar manpage for details.
-
Semicolons are required after all simple statements in Perl (except
at the end of a block). Newline is not a statement delimiter.
-
Curly brackets are required on ifs and whiles.
-
Variables begin with "$", "@" 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 the perlvar manpage.
-
$<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 $RS to a pattern, only a string.
-
When in doubt, run the awk construct through a2p and see what it
gives you.
Cerebral C and C++ programmers should take note of the following:
-
Curly brackets are required on if's and while's.
-
You must use elsif rather than else if.
-
The break and continue keywords from C become in Perl last
and next, respectively. Unlike in C, these do not work within a
do { } while construct. See Loop Control in the perlsyn manpage.
-
There's no switch statement. (But it's easy to build one on the fly,
see Basic BLOCKs and Switch Statements in the perlsyn manpage)
-
Variables begin with "$", "@" 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 $0.
-
System calls such as link(), unlink(), rename(), etc. return nonzero for
success, not 0. (system(), however, returns zero for success.)
-
Signal handlers deal with signal names, not numbers. Use kill -l
to find their names on your system.
Seasoned sed programmers should take note of the following:
-
A Perl program executes only once, not once for each input line. You can
do an implicit loop with -n or -p.
-
Backreferences in substitutions use "$" rather than "\".
-
The pattern matching metacharacters "(", ")", and "|" do not have backslashes
in front.
-
The range operator is ..., rather than comma.
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.
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 the perldata manpage 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 the perlop manpage and the perlsub manpage.
-
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>;
-
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 the perlform manpage 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.
Practicing Perl4 Programmers should take note of the following
Perl4-to-Perl5 specific traps.
They're crudely ordered according to the following list:
- Discontinuance, Deprecation, and BugFix traps
-
Anything that's been fixed as a perl4 bug, removed as a perl4 feature
or deprecated as a perl4 feature with the intent to encourage usage of
some other perl5 feature.
- Parsing Traps
-
Traps that appear to stem from the new parser.
- Numerical Traps
-
Traps having to do with numerical or mathematical operators.
- General data type traps
-
Traps involving perl standard data types.
- Context Traps - scalar, list contexts
-
Traps related to context within lists, scalar statements/declarations.
- Precedence Traps
-
Traps related to the precedence of parsing, evaluation, and execution of
code.
- General Regular Expression Traps using s///, etc.
-
Traps related to the use of pattern matching.
- Subroutine, Signal, Sorting Traps
-
Traps related to the use of signals and signal handlers, general subroutines,
and sorting, along with sorting subroutines.
- OS Traps
-
OS-specific traps.
- DBM Traps
-
Traps specific to the use of dbmopen(), and specific dbm implementations.
- Unclassified Traps
-
Everything else.
If you find an example of a conversion trap that is not listed here,
please submit it to <perlbug@perl.org> for inclusion.
Also note that at least some of these can be caught with the
use warnings pragma or the -w switch.
Anything that has been discontinued, deprecated, or fixed as
a bug from perl4.
- Symbols starting with "_" no longer forced into main
Symbols starting with "_" are no longer forced into package main, except
for $_ itself (and @_, etc.).
package test;
$_legacy = 1;
package main;
print "\$_legacy is ",$_legacy,"\n";
- Double-colon valid package separator in variable name
Double-colon is now a valid package separator in a variable name. Thus these
behave differently in perl4 vs. perl5, because the packages don't exist.
$a=1;$b=2;$c=3;$var=4;
print "$a::$b::$c ";
print "$var::abc::xyz\n";
Given that :: is now the preferred package delimiter, it is debatable
whether this should be classed as a bug or not.
(The older package delimiter, ' ,is used here)
$x = 10;
print "x=${'x}\n";
You can avoid this problem, and remain compatible with perl4, if you
always explicitly include the package name:
$x = 10;
print "x=${main'x}\n";
Also see precedence traps, for parsing $:.
- 2nd and 3rd args to
splice() are now in scalar context
The second and third arguments of splice() are now evaluated in scalar
context (as the Camel says) rather than list context.
sub sub1{return(0,2) }
sub sub2{ return(1,2,3)}
@a1 = ("a","b","c","d","e");
@a2 = splice(@a1,&sub1,&sub2);
print join(' ',@a2),"\n";
- Can't do
goto into a block that is optimized away
You can't do a goto into a block that is optimized away. Darn.
goto marker1;
for(1){
marker1:
print "Here I is!\n";
}
- Can't use whitespace as variable name or quote delimiter
It is no longer syntactically legal to use whitespace as the name
of a variable, or as a delimiter for any kind of quote construct.
Double darn.
$a = ("foo bar");
$b = q baz;
print "a is $a, b is $b\n";
while/if BLOCK BLOCK gone
The archaic while/if BLOCK BLOCK syntax is no longer supported.
if { 1 } {
print "True!";
}
else {
print "False!";
}
** binds tighter than unary minus
The ** operator now binds more tightly than unary minus.
It was documented to work this way before, but didn't.
print -4**2,"\n";
foreach changed when iterating over a list
The meaning of foreach{} has changed slightly when it is iterating over a
list which is not an array. This used to assign the list to a
temporary array, but no longer does so (for efficiency). This means
that you'll now be iterating over the actual values, not over copies of
the values. Modifications to the loop variable can change the original
values.
@list = ('ab','abc','bcd','def');
foreach $var (grep(/ab/,@list)){
$var = 1;
}
print (join(':',@list));
To retain Perl4 semantics you need to assign your list
explicitly to a temporary array and then iterate over that. For
example, you might need to change
foreach $var (grep(/ab/,@list)){
to
foreach $var (@tmp = grep(/ab/,@list)){
Otherwise changing $var will clobber the values of @list. (This most often
happens when you use $_ for the loop variable, and call subroutines in
the loop that don't properly localize $_.)
split with no args behavior changed
split with no arguments now behaves like split ' ' (which doesn't
return an initial null field if $_ starts with whitespace), it used to
behave like split /\s+/ (which does).
$_ = ' hi mom';
print join(':', split);
- -e behavior fixed
Perl 4 would ignore any text which was attached to an -e switch,
always taking the code snippet from the following arg. Additionally, it
would silently accept an -e switch without a following arg. Both of
these behaviors have been fixed.
perl -e'print "attached to -e"' 'print "separate arg"'
perl -e
push returns number of elements in resulting list
In Perl 4 the return value of push was undocumented, but it was
actually the last value being pushed onto the target list. In Perl 5
the return value of push is documented, but has changed, it is the
number of elements in the resulting list.
@x = ('existing');
print push(@x, 'first new', 'second new');
- Some error messages differ
Some error messages will be different.
split() honors subroutine args
In Perl 4, if in list context the delimiters to the first argument of
split() were ??, the result would be placed in @_ as well as
being returned. Perl 5 has more respect for your subroutine arguments.
- Bugs removed
Some bugs may have been inadvertently removed. :-)
Perl4-to-Perl5 traps from having to do with parsing.
- Space between . and = triggers syntax error
Note the space between . and =
$string . = "more string";
print $string;
- Better parsing in perl 5
Better parsing in perl 5
sub foo {}
&foo
print("hello, world\n");
- Function parsing
"if it looks like a function, it is a function" rule.
print
($foo == 1) ? "is one\n" : "is zero\n";
- String interpolation of
$#array differs
String interpolation of the $#array construct differs when braces
are to used around the name.
@a = (1..3);
print "${#a}";
@ = (1..3);
print "$#{a}";
- Perl guesses on
map, grep followed by { if it starts BLOCK or hash ref
When perl sees map { (or grep {), it has to guess whether the {
starts a BLOCK or a hash reference. If it guesses wrong, it will report
a syntax error near the } and the missing (or unexpected) comma.
Use unary + before { on a hash reference, and unary + applied
to the first thing in a BLOCK (after {), for perl to guess right all
the time. (See map in the perlfunc manpage.)
Perl4-to-Perl5 traps having to do with numerical operators,
operands, or output from same.
- Formatted output and significant digits
Formatted output and significant digits. In general, Perl 5
tries to be more precise. For example, on a Solaris Sparc:
print 7.373504 - 0, "\n";
printf "%20.18f\n", 7.373504 - 0;
# Perl4 prints:
7.3750399999999996141
7.375039999999999614
# Perl5 prints:
7.373504
7.375039999999999614
Notice how the first result looks better in Perl 5.
Your results may vary, since your floating point formatting routines
and even floating point format may be slightly different.
- Auto-increment operator over signed int limit deleted
This specific item has been deleted. It demonstrated how the auto-increment
operator would not catch when a number went over the signed int limit. Fixed
in version 5.003_04. But always be wary when using large integers.
If in doubt:
use Math::BigInt;
- Assignment of return values from numeric equality tests doesn't work
Assignment of return values from numeric equality tests
does not work in perl5 when the test evaluates to false (0).
Logical tests now return a null, instead of 0
$p = ($test == 1);
print $p,"\n";
Also see General Regular Expression Traps using s///, etc.
for another example of this new feature...
- Bitwise string ops
When bitwise operators which can operate upon either numbers or
strings (& | ^ ~) are given only strings as arguments, perl4 would
treat the operands as bitstrings so long as the program contained a call
to the vec() function. perl5 treats the string operands as bitstrings.
(See Bitwise String Operators in the perlop manpage for more details.)
$fred = "10";
$barney = "12";
$betty = $fred & $barney;
print "$betty\n";
# Perl4 prints:
8
# Perl5 prints:
10
# If vec() is used anywhere in the program, both print:
10
Perl4-to-Perl5 traps involving most data-types, and their usage
within certain expressions and/or context.
- Negative array subscripts now count from the end of array
Negative array subscripts now count from the end of the array.
@a = (1, 2, 3, 4, 5);
print "The third element of the array is $a[3] also expressed as $a[-2] \n";
- Setting
$#array lower now discards array elements
Setting $#array lower now discards array elements, and makes them
impossible to recover.
@a = (a,b,c,d,e);
print "Before: ",join('',@a);
$#a =1;
print ", After: ",join('',@a);
$#a =3;
print ", Recovered: ",join('',@a),"\n";
- Hashes get defined before use
Hashes get defined before use
local($s,@a,%h);
die "scalar \$s defined" if defined($s);
die "array \@a defined" if defined(@a);
die "hash \%h defined" if defined(%h);
Perl will now generate a warning when it sees defined(@a) and
defined(%h).
- Glob assignment from localized variable to variable
glob assignment from variable to variable will fail if the assigned
variable is localized subsequent to the assignment
@a = ("This is Perl 4");
*b = *a;
local(@a);
print @b,"\n";
- Assigning
undef to glob
Assigning undef to a glob has no effect in Perl 5. In Perl 4
it undefines the associated scalar (but may have other side effects
including SEGVs). Perl 5 will also warn if undef is assigned to a
typeglob. (Note that assigning undef to a typeglob is different
than calling the undef function on a typeglob (undef *foo), which
has quite a few effects.
$foo = "bar";
*foo = undef;
print $foo;
- Changes in unary negation (of strings)
Changes in unary negation (of strings)
This change effects both the return value and what it
does to auto(magic)increment.
$x = "aaa";
print ++$x," : ";
print -$x," : ";
print ++$x,"\n";
- Modifying of constants prohibited
perl 4 lets you modify constants:
$foo = "x";
&mod($foo);
for ($x = 0; $x < 3; $x++) {
&mod("a");
}
sub mod {
print "before: $_[0]";
$_[0] = "m";
print " after: $_[0]\n";
}
defined $var behavior changed
The behavior is slightly different for:
print "$x", defined $x
- Variable Suicide
Variable suicide behavior is more consistent under Perl 5.
Perl5 exhibits the same behavior for hashes and scalars,
that perl4 exhibits for only scalars.
$aGlobal{ "aKey" } = "global value";
print "MAIN:", $aGlobal{"aKey"}, "\n";
$GlobalLevel = 0;
&test( *aGlobal );
sub test {
local( *theArgument ) = @_;
local( %aNewLocal );
$aNewLocal{"aKey"} = "this should never appear";
print "SUB: ", $theArgument{"aKey"}, "\n";
$aNewLocal{"aKey"} = "level $GlobalLevel";
$GlobalLevel++;
if( $GlobalLevel<4 ) {
&test( *aNewLocal );
}
}
- Elements of argument lists for formats evaluated in list context
The elements of argument lists for formats are now evaluated in list
context. This means you can interpolate list values now.
@fmt = ("foo","bar","baz");
format STDOUT=
@<<<<< @||||| @>>>>>
@fmt;
.
write;
caller() returns false value in scalar context if no caller present
The caller() function now returns a false value in a scalar context
if there is no caller. This lets library files determine if they're
being required.
caller() ? (print "You rang?\n") : (print "Got a 0\n");
- Comma operator in scalar context gives scalar context to args
The comma operator in a scalar context is now guaranteed to give a
scalar context to its arguments.
@y= ('a','b','c');
$x = (1, 2, @y);
print "x = $x\n";
sprintf() prototyped as ($;@)
sprintf() is prototyped as ($;@), so its first argument is given scalar
context. Thus, if passed an array, it will probably not do what you want,
unlike Perl 4:
@z = ('%s%s', 'foo', 'bar');
$x = sprintf(@z);
print $x;
printf() works the same as it did in Perl 4, though:
@z = ('%s%s', 'foo', 'bar');
printf STDOUT (@z);
Perl4-to-Perl5 traps involving precedence order.
Perl 4 has almost the same precedence rules as Perl 5 for the operators
that they both have. Perl 4 however, seems to have had some
inconsistencies that made the behavior differ from what was documented.
- LHS vs. RHS of any assignment operator
LHS vs. RHS of any assignment operator. LHS is evaluated first
in perl4, second in perl5; this can affect the relationship
between side-effects in sub-expressions.
@arr = ( 'left', 'right' );
$a{shift @arr} = shift @arr;
print join( ' ', keys %a );
- Semantic errors introduced due to precedence
These are now semantic errors because of precedence:
@list = (1,2,3,4,5);
%map = ("a",1,"b",2,"c",3,"d",4);
$n = shift @list + 2;
print "n is $n, ";
$m = keys %map + 2;
print "m is $m\n";
- Precedence of assignment operators same as the precedence of assignment
The precedence of assignment operators is now the same as the precedence
of assignment. Perl 4 mistakenly gave them the precedence of the associated
operator. So you now must parenthesize them in expressions like
/foo/ ? ($a += 2) : ($a -= 2);
Otherwise
/foo/ ? $a += 2 : $a -= 2
would be erroneously parsed as
(/foo/ ? $a += 2 : $a) -= 2;
On the other hand,
$a += /foo/ ? 1 : 2;
now works as a C programmer would expect.
open requires parentheses around filehandle
open FOO || die;
is now incorrect. You need parentheses around the filehandle.
Otherwise, perl5 leaves the statement as its default precedence:
open(FOO || die);
$: precedence over $:: gone
perl4 gives the special variable, $: precedence, where perl5
treats $:: as main package
$a = "x"; |
