#!/bin/perl

# SCALARS (continued)

# Undefined variables start out with a special value "undef"
# which is 0 in the context of numbers
#     and '' in the context of strings

while ($n <= 10) {
    $sumString .= "+ $n ";
    $sum += $n;
    print "$sumString = $sum\n";
    $n += 1;
}

$n = 0;
$sum = undef; # obscure undef operator
$sumString = undef;
while ($n <= 10) {
    $sumString .= "+ $n ";
    $sum += $n;
    print "$sumString = $sum\n";
    $n += 1;
}

print("Now it's time to sum your numbers (one per line; ^D to end):\n");

$sum = undef; # obscure undef operator
$sumString = undef;
chomp($n = <STDIN>);
while (defined($n)) { # <STDIN> returns undef at end of input
    $sumString .= "+ $n ";
    $sum += $n;
    print "$sumString = $sum\n";
    chomp($n = <STDIN>);
}
print "Done!\n";