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Perl DISCUSSION
I am confused by how Perl treats arrays. With my @pins = (2, 3, 5, 7); the line my $n = @pins; gives 4, but my ($n) = @pins; gives 2. print "Pins: @pins\n"; prints the values, while print "Pins: " . @pins . "\n"; prints 4.
What rule decides whether I get the length or the elements, and what is the safe way to get the number of elements every time?
Every Perl expression is evaluated in either scalar or list context, decided by what surrounds it. An array in list context yields its elements; in scalar context it yields its element count. Assignment to a plain scalar, my $n = @pins;, is scalar context, so you get 4. Parentheses on the left, my ($n) = @pins;, make it a list assignment: the elements are copied position by position, $n receives the first one (2) and the rest are dropped.
Your two print lines follow the same rule. Inside double quotes an array is interpolated as its elements joined with a space (the value of $"). The concatenation operator . needs a single string on each side, so it puts @pins in scalar context and you get the count. Note that print "Pins: ", @pins; with a comma is different again: the arguments of print are a list, so the elements are printed with nothing between them, giving Pins: 2357.
To state the intent explicitly, use scalar(@pins); it forces scalar context anywhere, for example print "Count: ", scalar(@pins), "\n";. Numeric comparisons already supply scalar context, so if (@pins > 3) and while (@queue) are fine as they are.
$#pins is the index of the last element, not the count: here it is 3, and it is -1 for an empty array. Loops written as for my $i (0 .. $#pins) rely on that.
Context also changes what functions return, which is where real bugs come from. my $t = localtime; gives a readable date string, while my @t = localtime; gives nine numbers (seconds, minutes, hours, day and so on). A subroutine that ends with return @results; returns the count to a caller that writes my $r = get_results();. Inside a sub you can check which context you were called in with wantarray.
A list literal is not an array: my $x = (4, 5, 6); sets $x to 6, because in scalar context the comma operator evaluates each item and keeps the last. With use warnings Perl flags that line.
Two idioms build on this. my $count = () = $text =~ /\d+/g; counts regex matches: the empty list forces the match into list context, and a list assignment evaluated in scalar context yields the number of items on its right side. And for hashes, use scalar(keys %h) for the number of keys; it behaves the same on every Perl version, whereas a bare hash in scalar context returned a bucket-usage string such as 3/8 on older releases.
