2. 区分散列的 undef 值， 和手动赋值 0 不一样。1. 使用exists函数，散列中有这个键（必须是keys %hash 有这结果），则返回真值，

2.

    123 my %vertical_alignment;
    124 $vertical_alignment{"subscripting"} = 0;
    125 unless($vertical_alignment{"subscripting"}){
    126     print "0 ";
    127 }
    128 unless($vertical_alignment{"anonymous"}){
    129     print "undef. ";
    130 }

124行： 不能对散列中的  元素？  再加  my 关键字， 直接操作就可以

如下，散列中的值需手动定义为undef

1.

      6 my %vertical_alignment;
      7 $vertical_alignment{"subscripting"} = 0;
      8 unless($vertical_alignment{"array_ref"}){
      9     print "0 ";
     10 }   
     11 printf keys %vertical_alignment;
     12 if( exists $vertical_alignment{"array_ref"}){
     13     print " there is a library card for subscripting ";
     14 }
     15 else{
     16     print " no card. ";
     17 }
     18 $vertical_alignment{"array_ref"} = undef;
     19 printf keys %vertical_alignment;
     20 if( exists $vertical_alignment{"array_ref"}){
     21     print " there is a library card for subscripting ";
     22 }
     23 else{
     24     print "a o. ";
     25 }

输出：

0
subscripting
no card.
subscripting
there is a library card for subscripting