Page History

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 nonnumeric strings. Likewise, do not use the string comparison operators on 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 because $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 converted back to a string, but this time it has the value 2, so the string comparison fails.

Compliant Solution (Numbers)

This compliant solution uses ==, which interprets its arguments as numbers. 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"};

...

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 by extracting digits from the front of each argument (along with a preceding + or -). Nonnumeric data in an argument is ignored, and the number consists of whatever digits were extracted. 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 be considered equivalent.

...

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

...

...

...

Image Added Image Modified      02. Expressions       Image Modified