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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

(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
or
```\$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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