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Perl provides two sets of comparison operators; : one set for working with numbers , and one set for working with strings.

Do not use the number comparison operators on non-numeric nonnumeric strings. Likewise, do not use the string comparison operators on numbers.

Noncompliant Code Example (

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Numbers)

This noncompliant code example improperly uses eq to test two numbers for equality. Counterintuitively, this code prints false.

my $num = 2;
print "Enter a number\n";
my $num$user_num = 02<STDIN>;
# ...chomp $user_num;
if ($num eq "02"$user_num) {print "true\n"} else {print "false\n"};

This code will print true if the user enters 2, but false if the user enters 02,The counterintuitive result arises from the fact that $num is interpreted as a number. When it is initialized, the 02 string is converted to its numeric representation, which is 2. When it is compared, it is convered back to a string, but this time it has the value 2, and so the string comparison fails.

Compliant Solution (

...

Numbers)

This compliant colution solution uses ==, which interprets its arguments as numbers. Thus this This code therefore prints true, even though if the right argument to == is explicitly provided as a stringinitialized to some different string like 02.

my $num = 2;
print "Enter a number\n";
my $num$user_num = 02<STDIN>;
# ...chomp $user_num;
if ($num == "02"$user_num) {print "true\n"} else {print "false\n"};

Noncompliant Code Example (

...

Strings)

This noncompliant code example improperly uses == to test two strings for equality.

sub check_password {
  my $correct = shift;
  my $password = shift;
  # encrypt password
  if ($password == $correct) {
    return true;
  } else {
    return false;
  }
}

The == operator first converts its arguments into numbers . This is done by extracting digits from the front of each arguments argument (along with a preceding + or -). Non-numeric Nonnumeric data in an argument is ignored, and the number consists of whatever digits were extractextracted. A string such as "goodpass" has no leading digits, and so it is thus converted to the numeral 0. Consequently, unless either $password or $correct contains leading digits, they will both be converted to 0 , and will therefore be considered equivalent.

Compliant Solution (

...

Strings)

This compliant colution solution uses eq, which interprets its arguments as strings.

sub check_password {
  my $correct = shift;
  my $password = shift;
  # encrypt password
  if ($password eq $correct) {
    return true;
  } else {
    return false;
  }
}

...

Confusing the string comparison operators with numeric comparison operators can lead to incorrect program behavior or incorrect program data.

Automated Detection

Bibliography

\[[Wall 2011|AA. Bibliography#Manpages]\] [perlfunc|http://perldoc.perl.org/perlop.html] 

[CPAN]	Elliot Shank, Perl-Critic-1.116 ProhibitMismatchedOperators
[Wall 2011]	perlop

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Image Added Image Added Image AddedEXP11-C. Do not apply operators expecting one type to data of an incompatible type      03. Expressions (EXP)      EXP13-C. Treat relational and equality operators as if they were nonassociative