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8.2 Common Goofs for Novices

The biggest goof of all is forgetting to use the -w switch, which points out many errors. The second biggest goof is not using use strict when it's appropriate.

Apart from those, there are certain traps that almost everyone falls into, and other traps you'll fall into only if you come from a particular culture. We've separated these out in the following sections.

8.2.1 Universal Blunders

  • Putting a comma after the filehandle in a print statement. Although it looks extremely regular and pretty to say:

    print STDOUT, "goodbye", $adj, "world!\n";    # WRONG

    this is nonetheless incorrect, because of that first comma. What you want instead is:

    print STDOUT "goodbye", $adj, "world!\n";     # ok

    The syntax is this way so that you can say:

    print $filehandle "goodbye", $adj, "world!\n";

    where $filehandle is a scalar holding the name of a filehandle at run-time. This is distinct from:

    print $notafilehandle, "goodbye", $adj, "world!\n";

    where $notafilehandle is simply a string that is added to the list of things to be printed. See Indirect Object in the glossary.

  • Using == instead of eq and != instead of ne . The == and != operators are numeric tests. The other two are string tests. The strings "123" and "123.00" are equal as numbers, but not equal as strings. Also, any non-numeric string is numerically equal to zero. Unless you are dealing with numbers, you almost always want the string comparison operators instead.

  • Forgetting the trailing semicolon. Every statement in Perl is terminated by a semicolon or the end of a block. Newlines aren't statement terminators as they are in awk or Python.

  • Forgetting that a BLOCK requires braces. Naked statements are not BLOCK s. If you are creating a control structure such as a while or an if that requires one or more BLOCK s, you must use braces around each BLOCK .

  • Not saving $1 , $2 , and so on, across regular expressions. Remember that every new m/atch/ or s/ubsti/tute/ will set (or clear, or mangle) your $1 , $2 ... variables, as well as $ ` , $ ' , and $& . One way to save them right away is to evaluate the match within a list context, as in:

    ($one,$two) = /(\w+) (\w+)/;
  • Not realizing that a local also changes the variable's value within other subroutines called within the scope of the local. It's easy to forget that local is a run-time statement that does dynamic scoping, because there's no equivalent in languages like C. See local in Chapter 3, Functions . Usually you wanted a my anyway.

  • Losing track of brace pairings. A good text editor will help you find the pairs. Get one.

  • Using loop control statements in do {} while . Although the braces in this control structure look suspiciously like part of a loop BLOCK , they aren't.

  • Saying @foo[1] when you mean $foo[1] . The @foo[1] reference is an array slice , and means an array consisting of the single element $foo[1] . Sometimes, this doesn't make any difference, as in:

    print "the answer is @foo[1]\n";

    but it makes a big difference for things like:

    @foo[1] = <STDIN>;

    which will slurp up all the rest of STDIN , assign the first line to $foo[1] , and discard everything else. This is probably not what you intended. Get into the habit of thinking that $ means a single value, while @ means a list of values, and you'll do okay.

  • Forgetting to select the right filehandle before setting $^ , $~ , or $| . These variables depend on the currently selected filehandle, as determined by select ( FILEHANDLE ). The initial filehandle so selected is STDOUT . You should really be using the filehandle methods from the FileHandle module instead. See Chapter 7, The Standard Perl Library .

8.2.2 Frequently Ignored Advice

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. Chapter 3 has all the details.

  • Avoid barewords if you can, especially all lowercase ones. You can't tell just by looking at it whether a word is a function or a bareword string. By using quotes on strings and parentheses around function call arguments, you won't ever get them confused. In fact, the pragma use strict at the beginning of your program makes barewords a compile-time error - probably a good thing.

  • You can't tell just by looking which built-in functions are unary operators (like chop and chdir ), which are list operators (like print and unlink ), and which are argumentless (like time ). You'll want to learn them from Chapter 2, The Gory Details . Note also that user-defined subroutines are by default list operators, but can be declared as unary operators with a prototype of ($) .

  • People have a hard time remembering that some functions default to $_ , or @ARGV , or whatever, while others do not. Take the time to learn which are which, or avoid default arguments.

  • < FH > is not the name of a filehandle, but an angle operator that does a line-input operation on the handle. This confusion usually manifests itself when people try to print to the angle operator:

    print <FH> "hi";    # WRONG, omit angles

  • Remember also that data read by the angle operator is assigned to $_ only when the file read is the sole condition in a while loop:

    while (<FH>)      { }
    while ($_ = <FH>) { }..
    <FH>;  # data discarded!

  • Remember not to use = when you need =~ ; the two constructs are quite different:

    $x =  /foo/;  # searches $_, puts result in $x
    $x =~ /foo/;  # searches $x, discards result

  • Use my for local variables whenever you can get away with it (but see "Formats" in Chapter 2 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.

  • Don't localize a module's exported variables. If you localize an exported variable, 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.

8.2.3 Awk Traps

Accustomed awk users should take special note of the following:

  • The English module, loaded via

    use English;

    allows you to refer to special variables (like $RS ) using their awk names; see the end of Chapter 2 for details.

  • Semicolons are required after all simple statements in Perl (except at the end of a block). Newline is not a statement delimiter.

  • Braces are required on if and while blocks.

  • Variables begin with $ or @ in Perl.

  • Arrays index from 0 , as do string positions in substr and index .

  • You have to decide whether your array has numeric or string indices.

  • You have to decide whether you want numeric or string comparisons.

  • Hash values do not spring into existence upon reference.

  • Reading an input line does not split it for you. You get to split it yourself to an array. And the split operator has different arguments than you might guess.

  • 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 Chapter 2 .

  • $1 , $2 , and so on, do not refer to fields - they refer to substrings matched by the last pattern match.

  • The print operator does not add field and record separators unless you set $, and $\ . ( $OFS and $ORS if you're using English.)

  • You must open your files before you print to them.

  • The range operator is .. rather than comma. The comma operator works (more or less) as in does C.

  • The match binding operator is =~ , not ~ . ( ~ is the 1's complement operator, as in C.)

  • The exponentiation operator is ** , not ^ . ^ is the bitwise XOR operator, as in C. (You know, one could get the feeling that awk is basically incompatible with C.)

  • The concatenation operator is dot ( . ), not "nothing". (Using "nothing" as an operator would render /pat/ /pat/ unparsable, since the third slash would be interpreted as a division operator - the tokener is in fact slightly context sensitive for operators like / , ? , and < . And, in fact, a dot itself can be the beginning of a number.)

  • The next , exit , and continue keywords work differently.

  • The following variables work differently:

    awk Perl
    ARGC

    $#ARGV or scalar @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.

8.2.4 C Traps

Cerebral C programmers should take note of the following:

  • Curlies are required for if and while blocks.

  • You must use elsif rather than "else if" or "elif". Syntax like:

    if (expression) {
        block;
    }
    else if (another_expression) {
        another_block;
    }

    is illegal. The else part is always a block, and a naked if is not a block. You mustn't expect Perl to be exactly the same as C. What you want instead is:

    if (expression) {
        block;
    }
    elsif (another_expression) {
        another_block;
    }

    Note also that "elif" is "file" spelled backward. Only Algol-ers would want a keyword that was the same as another word spelled backward.

  • 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.

  • There's no switch statement. (But it's easy to build one on the fly; see "Bare Blocks and Case Structures" in Chapter 2 .)

  • Variables begin with $ , @ , or % in Perl.

  • printf does not implement the * format for interpolating field widths, but it's trivial to use interpolation of double-quoted strings to achieve the same effect.

  • Comments begin with # , not /* .

  • 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 C's argv[0] ends up in $0 .

  • Functions such as link , unlink , and rename return true for success, not 0 .

  • Signal handlers deal with signal names, not numbers.

8.2.5 Sed Traps

Seasoned sed programmers should take note of the following:

  • Backreferences in substitutions use $ rather than \ .

  • The pattern matching metacharacters ( , ) , and | do not have backslashes in front. The corresponding literal characters do.

  • The range operator in Perl is ... rather than a comma.

8.2.6 Shell Traps

Sharp shell programmers should take note of the following:

  • Variables are prefixed with $ or @ on the left side of the assignment as well as the right. A shellish assignment like:

    camel='dromedary';      # WRONG

    won't be parsed the way you expect. You need:

    $camel='dromedary';     # ok

  • The loop variable of a foreach also requires a $ . Although csh likes:

    foreach hump (one two)
    stuff_it $hump
    end

    in Perl this is written as:

    foreach $hump ("one", "two") {
        stuff_it($hump);
    }

  • The backtick operator does variable interpretation without regard to the presence of single quotes in the command.

  • The backtick operator does no translation of the return value. In Perl, you have to trim the newline explicitly, like this:

    chop($thishost = `hostname`);

  • Shells (especially csh ) do several levels of substitution on each command line. Perl does substitution only within certain constructs such as double quotes, backticks, angle brackets, and search patterns.

  • Shells tend to 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 , and so on.

  • The environment is not automatically made available as separate scalar variables. But see the Env module.

8.2.7 Previous Perl Traps

Penitent Perl 4 (and Prior) Programmers should take note of the following changes between Release 4 and Release 5 that might affect old scripts:

  • @ now always interpolates an array in double-quotish strings. Some programs may now need to use backslash to protect any @ that shouldn't interpolate.

  • Barewords that used to look like strings to Perl will now look like subroutine calls if a subroutine by that name is defined before the compiler sees them. For example:

    sub SeeYa { die "Hasta la vista, baby!" }
    $SIG{'QUIT'} = SeeYa;

    In prior versions of Perl, that code would set the signal handler. Now, it actually calls the function! You may use the -w switch to find such risky usage.

  • Symbols starting with "_" are no longer forced into package main, except for $_ itself (and @_ , and so on).

  • Double-colon is now a valid package separator in an identifier. Thus, the statement:

    print "$a::$b::$c\n";

    now parses $a:: as the variable reference, where in prior versions only the $a was considered to be the variable reference. Similarly,

    print "$var::abc::xyz\n";

    is now interpreted as a single variable $var::abc::xyz , whereas in prior versions, the variable $var would have been followed by the constant text ::abc::xyz .

  • s'$lhs'$rhs' now does no interpolation on either side. It used to interpolate $lhs but not $rhs .

  • The second and third arguments of splice are now evaluated in scalar context (as documented) rather than list context.

  • These are now semantic errors because of precedence:

    shift @list + 20; # now parses like shift(@list + 20), illegal!
    $n = keys %map + 20; # now parses like keys(%map + 20), illegal!

    Because if those were to work, then this couldn't:

    sleep $dormancy + 20;

  • The precedence of assignment operators is now the same as the precedence of assignment. Previous versions of Perl 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 FOO || die is now incorrect. You need parentheses around the filehandle, because open has the precedence of a list operator.

  • The elements of argument lists for formats are now evaluated in list context. This means you can interpolate list values now.

  • You can't do a goto into a block that is optimized away. Darn.

  • 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.

  • The caller function now returns a false value in a scalar context if there is no caller. This lets library modules determine whether they're being required or run directly.

  • m//g now attaches its state to the searched string rather than the regular expression. See "Regular Expressions" in Chapter 2 for further details.

  • reverse is no longer allowed as the name of a sort subroutine.

  • taintperl is no longer a separate executable. There is now a -T switch to turn on tainting when it isn't turned on automatically.

  • Double-quoted strings may no longer end with an unescaped $ or @ .

  • The archaic if BLOCK BLOCK syntax is no longer supported.

  • Negative array subscripts now count from the end of the array.

  • The comma operator in a scalar context is now guaranteed to give a scalar context to its arguments.

  • The ** operator now binds more tightly than unary minus. It was documented to work this way before, but didn't.

  • Setting $#array lower now discards array elements immediately.

  • delete is not guaranteed to return the deleted value for tie d arrays, since this capability may be onerous for some modules to implement.

  • The construct "this is $$x" , which used to interpolate the pid at that point, now tries to dereference $x . $$ by itself still works fine, however.

  • 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 for efficiency it no longer does so. 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. To retain prior Perl semantics you'd 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 /x/, @list) { ... }

    to:

    foreach $var (my @tmp = grep /x/, @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 $_ .)

  • Some error messages will be different.

  • Some bugs may have been inadvertently removed.[ 4 ]

    [4] Much to the consternation of Perl poets.







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