"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."

In the case of keys %foo, 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.

In the case of ++$m + $m++, the results are undefined: no documentation claims to give an authoritative answer on such expressions, and some documentaion will specifically disavow any predictable results.

For those who do not program in C, here's the gist of the action. The do { my $x = $m; $m++; $x } mechanism is not a sufficient postincrement operator, because real post-incrementation happens after the rest of the expression. 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.

---

---

• Are you posting in the right place? Check out Where do I post X? to know for sure.
• Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:

  <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>

• Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
• Want more info? How to link or How to display code and escape characters are good places to start.