# update prices
    foreach my $stock ( @ticker{@stock_names} ) {
        $$stock{price} += generate_random($$stock{volitility});
        $$stock{price} = 0 if $$stock{price} < 1;
    }
    # liquidate holdings if price hits 0
    foreach my $stock ( @stock_names ) {
        $portfolio{$stock} = 0 if $ticker{$stock}{price} == 0
    }