8. Literals and Operators

Example: Numbers and Characters

    #!/usr/local/bin/perl
    print '007',' has been portrayed by at least ', 004, ' actors. ';
    print 7+3, ' ', 7*3, ' ', 7/3, ' ', 7%3, ' ', 7**3, ' ';
    $x = 7;
    print $x;
    print '   Doesn\'t resolve variables like $x and backslashes \n. ';
    print "Does resolve $x and backslash\n";
    $y = "A line containing $x and ending with line feed.\n";
    print $y;
    $y = "Con" . "cat" . "enation!\n";
    print $y;

This produces:

    007 has been portrayed by at least 4 actors. 10 21
    2.3333333333333335 1 343 7
    Doesn't resolve variables like $x and backslashes \n. Does
    resolve 7 and backslash
    A line containing 7 and ending with line feed.
    Concatenation!

Questions:

1. Why does the output from the first few print statements run together?
2. Is it necessary to declare variables in Perl? Is it possible?

Example: Comparisons

    # The following "<<" variation of
    # data input simplifies CGI forms.
    $x = 'operator';
    print <<THATSALL;
    A common mistake:  Confusing the assignment $x =
    and the numeric comparison $x ==, and the character
    comparison $x eq.
    THATSALL
    $x = 7;
    if ($x == 7) { print "x is $x\n"; }
    if ($x = 5)  {
        print "x is now $x,",
        "the assignment is successful.\n";
    }
    $x = 'stuff';
    if ($x eq 'stuff') {
        print "Use eq, ne, lt, gt, etc for strings.\n";
    }

This produces:

    A common mistake:  Confusing the assignment operator =
    and the numeric comparison operator ==, and the character
    comparison operator eq.
    x is 7
    x is now 5, the assignment is successful.
    Use eq, ne, lt, gt, etc for strings.

Example: Ordinary Arrays

    @stuff = ('This', 'is', 'a', 'list.');
    print "Lists and strings are indexed from 0.\n";
    print "So \$stuff[1] = $stuff[1], ",
         "and \$#stuff = $#stuff.\n";
    print @stuff,"\n";
    print join('...',@stuff),"\n";
    splice(@stuff, 3, 0, ('fine', 'little'));
    print join('...',@stuff),"\n";

This produces:

    Lists and strings are indexed from 0.
    So $stuff[1] = is, and $#stuff = 3.
    Thisisalist.
    This...is...a...list.
    This...is...a...fine...little...list.

Homework: Validate a date.

    #!/usr/local/bin/perl
    print "Enter numeric:  month  day  year\n";
    $_ = <STDIN>;
    ($month,$day,$year) = split;

Approach this incrementally. On the first draft, assume that the user enters 3 numbers separated by spaces and that February has 28 days. Subsequent refinements should account for bad input and leap year. Finally, find a Perl builtin function that converts a date to system time, and see how to use that to validate time data generally.

Homework: Play with associative arrays.

Start with a few assignments like:

    $name{12345} = 'John Doe';
    $name{24680} = 'Jane Smith';

    $blunk = %name;
    @persons = %name;
    print '$blunk=',$blunk,', @persons=',
        join(', ',@persons),"\n";