看不懂，求解！

#!/usr/bin/perl while ($line = <STDIN>){          $line =~ tr/A-Z/a-z/;     $line =~ s/[^a-z]//g;     @letters=split(//,$line);     foreach $letter (@letters){         $lettercount{$letter}+=1;     } } $~="WRITEHEADER"; write; $count =0; foreach $letter (reverse sort occurrences(keys(%lettercount))){     &write_letter($letter,$lettercount{$letter});     last if (++$count ==5); } sub occurrences{     $lettercount{$a}<=>$lettercount{$b}; } sub write_letter{     local($letter,$value)=@_;     $~="WRITELETTER";     write; } format WRITEHEADER = The five most frequently occurring letters are: . format WRITELETTER =   @: @<<<<<<   $letter, $value .

其中 ：

foreach $letter (reverse sort occurrences(keys(%lettercount))){
        &write_letter($letter,$lettercount{$letter});
        last if (++$count ==5);
}

sub occurrences{
        $lettercount{$a}<=>$lettercount{$b};
}

看不懂，求解？occurrences 什么作用？

对lettercount这个hash的键进行升序排列

xiexie，是按值排列。

测试代码：

#!/usr/bin/perl print ("\t统计字符出现次数\n请输入英文字符：回车ctl+d 结束输入：\n"); while ($line = <STDIN>){          $line =~ tr/A-Z/a-z/;     $line =~ s/[^a-z]//g;     @letters=split(//,$line);     foreach $letter (@letters){         $lettercount{$letter}+=1;     } } $i=0; sub occurrences{     #print ("调用第[".++$i."]次:$a=>$lettercount{$a} $b=>$lettercount{$b}\n");     $lettercount{$a}<=>$lettercount{$b}; } print ("\n原始哈希表元素列表 ：\n"); while (($key, $var) = each(%lettercount)) {   print ("$key=>$var\n"); } print ("\n哈希表按字符排序：\n"); foreach $word (sort keys(%lettercount)) {     print ("$word=>$lettercount{$word}\n");     } print("\n按字符出现次数排序\n"); foreach $word (reverse sort occurrences(sort keys(%lettercount))) {         print ("$word=>$lettercount{$word}\n");     }

perldoc -f sort

reverse sort occurrences(keys(%lettercount))这么写有点傻，直接写成密名函数看着就清楚多了