perltutorial runrig <h3>When you want to use strict</h3> Whenever your program gets over a few lines long, definitely when you can't view the whole program on one page, or sometimes when you just can't figure out what else could be wrong. <h3>Why do you want to use strict?</h3> To help you catch typos so you can quickly get on to finding more significant problems (and so we don't have to catch the typos for you either), among other reasons. <br>Its difficult to spot '$recieve_date' when on the previous page you've been calling it '$receive_date'. Also, to give your variables as small a scope as possible so that you don't have to worry about what they're doing to other parts of your program (although that's the function of [my], it forces you to use [my] which when properly used helps achieve this goal). <h3>Why it's not 'too much trouble' to use strict</h3> It's just 11 extra characters at the top of your script(use [strict];), and two extra characters throughout your script([my]). <h3>That sounds great. How do I use strict?</h3> Put this line at the top of your script (after the shebang, e.g., '#!/usr/bin/perl' line):<code> use strict; </code> <h3>Now my program's broken. What do I do?</h3> The most common error showing up looks something like this:<code> Global symbol "$xxx" requires explicit package name at ./tst line 5.</code> This is the error we're going to focus on fixing. <br>(If you are getting 'Server Error' or the like, then either check your web server error logs or run your script from the command line, or look into using [cpan://CGI::Carp]). <br>Whenever you first use a variable, put 'my' in front of it, e.g.:<code> # Change this: $string = "hello world"; @array = qw(ABC DEF); %hash = (A=>1, B=>2); # To this: my $string = "hello world"; my @array = qw(ABC DEF); my %hash = (A=>1, B=>2); # Change this: # '$name' is global here @names = qw(Bob Fred); foreach $name (@names) { print "Name: $name\n"; } # To this: my @names = qw(Bob Fred); foreach my $name (@names) { # Now '$name' only exists in this block print "Name: $name\n"; } # Change this: # Likewise, '$digit' is global here @digits = (5,3,4); foreach $digit (@digits) { $number = 10*$number + $digit; } print "Number: $number\n"; # To this (variables used in an outer scope ('$number') # will have to be declared in an outer scope): my @digits = (5,3,4); my $number = 0; foreach my $digit (@digits) # Now '$digit' only exists in this block $number = 10*$number + $digit; } print "Number: $number\n"; # Change this: sub my_sub { ($arg1, $arg2) = @_; print "Arg1: $arg1 Arg2: $arg2\n"; } # To this: sub my_sub { my ($arg1, $arg2) = @_; print "Arg1: $arg1 Arg2: $arg2\n"; } # Using DBI? You can change this: $sth->bind_columns(\$field1, \$field2); while ($sth->fetch) { print "F1: $field1 F2: $field2\n"; } # To this (the '\' is distributed over a list of values): $sth->bind_columns(\my ($field1, $field2)); while ($sth->fetch) { print "F1: $field1 F2: $field2\n"; } </code> <h3>That seems like too much trouble. Isn't laziness a virtue?</h3> Sure we're lazy. And we don't like spending time looking for simple mistakes in your program that you could have found yourself with 'use [strict]'. <h3>What about warnings?</h3> Oh yeah. In version 5.6 or later you can put this right around the same place you put 'use strict;':<code> use warnings; </code> In perl's before 5.6 (or if you just want to be portable between the versions), you can put '-w' on the 'shebang' line, or set the $^W variable (however, setting $^W will not catch compile time warnings unless its in a BEGIN{} block, so '-w' is usually preferable):<code> #!/usr/local/bin/perl -w # Or $^W = 1; # Or BEGIN { $^W = 1 } </code> If you know you want to disable warnings, you can do it in a limited scope:<code> # Change this: sub add_two_numbers_which_might_be_undef { $_[0] + $_[1]; } # To one of these (depending on perl version): # 1 sub add_two_numbers_which_might_be_undef { # See 'perldoc perllexwarn' for all the categories of warnings # because its better to only disable the warnings you're expecting no warnings "uninitialized"; $_[0] + $_[1]; } # 2 sub add_two_numbers_which_might_be_undef { local $^W; $_[0] + $_[1]; } </code> Or sometimes you'll have to initialize variables as in the example above that uses '$number'. <h2>See Also:</h2> <p>Also read [Ovid]'s excellent ['use strict' is not Perl].</p> <br>And (as [wog] pointed out): [Use strict warnings and diagnostics]. <br>And [id://87628]. <br>And [id://686571]. <p>And that's it! Now you have no excuse for not using [strict] or warnings. And it'll make life easier for all of us :)</p>