Beefy Boxes and Bandwidth Generously Provided by pair Networks
Problems? Is your data what you think it is?
 
PerlMonks  

Why does $string++ work the way it does?

( #38529=categorized question: print w/ replies, xml ) Need Help??
Contributed by spaz on Oct 26, 2000 at 04:33 UTC
Q&A  > strings


Description:

The following code produces some bizzare output. Apparently, "a"++ = "b", and "z"++ = "aa". But if I ever try to convert $string to a number "$number = (int $string)" I get 0. Anybody know why? --snip!--
#!/usr/bin/perl my $string = "a"; for( $i = 0; $i < 64; $i++ ) { $string++; print "string = '$string'\n"; }
--snip!--

Answer: Why does $string++ work the way it does?
contributed by extremely

Well, you've discovered that '++' is magic on strings of letters. The magic works on strings like "aaaa" and "aaa111" but on strings like "1111aaaa" you will get 1112. If the string begins with a number then perl will whack off the text and treat it as a number. All the other math ops will do the same for a string that begins with a number.

BTW, perl's definition of a number is pretty complex. "-1e3garg" + "+4ee3" will net you 1004.

$a="-1e3ble"; $b="+4.33eeq"; $c="aaa999"; $d="a9z9z9"; print "a = $a and b = $b\n"; print "c = $c and d = $d\n"; print "a + b = ". $a+$b . "\n"; print "c + d = ". $c+$d . "\n"; print "int(b) = ". int($b) . "\n"; print "++a = ". ++$a . "\n"; print "++b = ". ++$b . "\n"; print "++c = ". ++$c . "\n"; print "++d = ". ++$d . "\n";

try out that code and see what I mean. Note what happens to $c when the numbers flip and notice that $d goes poof. =)

Answer: Why does $string++ work the way it does?
contributed by Fastolfe

++ is magic in that it knows how to deal with both numbers and strings. With numbers, it behaves just like a numeric increment, but since Perl is smart, it assumes that if you are ++'ing a string, that you want to change the letters around a bit.

The alternative would be to convert it to a number (0) and increment that (1), which doesn't make a lot of sense if you start off with a string, so why not make something useful out of it?

Note that this is the way that ++ differs from + and +=. The latter two operations require another argument, which can't really be anything but a number, which would convert your string to numeric form first. So if you really do want to end up with a 1 and start from a string, just use +1 or +=1 to get it.

Answer: Why does $string++ work the way it does?
contributed by davido

The ++ operator can act upon both numbers and strings in a predictable fashion. With strings, 'A' increments to 'B', and 'Z' to 'AA'. ...and so on.

The magic of the ++ operator with respect to strings also exherts itself by providing the .. operator (range operator) with similar magic.

Try this at home...

my $x = 1; my $b = 'A'; $x++; # $x now equals 2. $b++; # $b now equals 'B'. my @numbers = (1..10); # @numbers now contains 1, 2, 3, .. 10. my @letters = ('A'..'Z'); # @letters now contains A, B, C, .. Z. my @more = ('A'..'AZ'); # @more now contains A, B, .. Z, AA, AB, ..AZ.

It seems that ++ magic, and .. magic, are related. Don't be fooled into expecting -- to be magic too though, it isn't. Neither can you use the .. operator alone to create descending lists.

my $b = 'Z'; $b--; # You don't get Y. my @array = (10..1); # You don't get 10, 9..1. my @letters = ('Z'..'A'); # You don't get the letters reversed.

Have fun with ++ and .. but take care not to expect magic from --. -- is for numbers.

Please (register and) log in if you wish to add an answer



  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • 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, 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, 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.
  • Log In?
    Username:
    Password:

    What's my password?
    Create A New User
    Chatterbox?
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others exploiting the Monastery: (10)
    As of 2015-07-29 23:20 GMT
    Sections?
    Information?
    Find Nodes?
    Leftovers?
      Voting Booth?

      The top three priorities of my open tasks are (in descending order of likelihood to be worked on) ...









      Results (269 votes), past polls