Perl's operators are monomorphic, and its data is polymorphic. Look at this:

print "Yes\n" if 1 == "1"; # Yes
print "Yes\n" if "0" eq 0; # Yes

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In the first test, the number 1 is compared numerically to the string "1". Perl converts that string to a number using rules defined in perldata, and perlop.

In the second test, the string "0" is compared to the number 0 using stringwise comparison. So the number 0 is promoted to a string for the comparison.

If you compare undef to undef numerically, undef is treated as 0. If you compare undef to undef stringwise, undef is treated as empty string. So undef == undef produces the same result (true) as 0 == 0. And undef eq undef produces the same value (a Boolean true value) as '' eq '' (empty string is equal to empty string).

To put it even more plainly:


Dave

In reply to Re^4: Learning Programming, desperately need to know what information is contained in scalar variables by davido
in thread Learning Programming, desperately need to know what information is contained in scalar variables by Anonymous Monk

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