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Re^5: scanning hash

by ysth (Canon)
on Aug 02, 2004 at 19:50 UTC ( #379404=note: print w/replies, xml ) Need Help??

in reply to Re^4: scanning hash
in thread scanning hash

(puzzled) If you really want...I'll start us off.

You can't determine the equality of two values from the equality of their string representations.

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Re^6: scanning hash
by Aristotle (Chancellor) on Aug 02, 2004 at 20:13 UTC

    To illustrate the point:

    $ perl -le '$a = $b = "foo"; print \$a eq \$b ? "true" : "false"' false

    The strings are identical, but they're two different strings. The string representations of references to them differ. Are they equal or not? This is a question of semantics that you can't answer without additional context.

    Indeed, if we just accept the string representation as the universal measure of equality, then ambrus is right: the empty string and undef are equal, and thus all of my arguing has been moot. However, experience tells me it is a prudent bet to assume that in most situations, undef needs to be treated differently from an empty string.

    Makeshifts last the longest.

      I was thinking of something simpler, such as:
      $x = 1.23456; $y = 1.234560000000001; print 0+($x == $y); # prints 0 for me print 0+($x eq $y); # prints 1 for me
      $x = 0; $y = "0";
      Are they equal? Yes and no.

        That is a very similar class of problem, although not exactly the same.

        I was dealing with the question of whether to compare "by reference" or "by value". A typical situation where this is an issue is comparing objects. What is the criterion then, identity or equality? The answer is much less clear than with the strings I used to demonstrate the point, because objects manifest as references by definition.

        Makeshifts last the longest.

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