note halley "Results are undefined" in Computer Science means, "the documentation is not going to explain what you'll get in Tuesday's version of the compiler on John's favorite machine." <p>In the case of <code>keys %foo</code>, the results are defined: you will get all keys. The order they're returned is defined but poorly so: it's a mistake for documentation to say they are returned in "random order," because the order has nothing to do with a PRNG. It would be better to say the order is not defined, as that is more accurate. Tuesday's compiler on John's machine will produce something inconsistent with your own experiments. <p>In the case of <code>++$m + $m++</code>, the results are undefined: no documentation claims to give an authoritative answer on such expressions, and some documentaion will specifically disavow any predictable results. <p>For those who do not program in C, here's the gist of the action. The <code>do { my $x = $m; $m++; $x }</code> mechanism is not a sufficient postincrement operator, because real post-incrementation happens <b>after the rest of the expression</b>. However, the specific moment that is defined to be after the rest of the expression is not clearly defined; various versions of a C compiler will differ, and various hardware platforms may also have something to say about it. It's undefined. <div class="pmsig"><div class="pmsig-249603"> <p>--<br><tt>&#91; e d @ h a l l e y . c c &#93;</tt> </div></div> 369247 369461