When comparing a value against a constant, always write the constant to the left side like this:

if (undef = $WCkey) { ...

[download]

If you make the error you made in your original post, Perl will immediately tell you.

Also, the function for checking whether a value is undef or not is the defined function.

Update: The if (undef = $WCkey) { ... is not a typo but intended for demonstration. Perl raises a runtime error when you use

if (undef = $WCkey) { ...

[download]

instead of

if (undef == $WCkey) { ...

[download]

while it doesn't even raise a warning if you write the "comparison" the other way around:

>perl -we "if (undef = $foo) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
Modification of a read-only value attempted at -e line 1.

>perl -we "if ($foo = undef) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.

[download]

So, in the spirit of building a defensive habit, write the constant to the left side of comparisons for equality.

In reply to Re: check undef by Corion
in thread check undef by Sun751

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.