hash 的一点问题

minmig

1. [root@localhost ~]# perl -e '%a=(a,aa,b,bb,c,cc,d,dd,e,ee);$b=%a;print $b,"\n"'
   
2. 5/8
   
3. [root@localhost ~]# perl -e '%a=(a,aa,b,bb,c,cc,d,dd,e,ee,f,ff);$b=%a;print $b,"\n"'
   
4. 6/8
   
5. [root@localhost ~]# perl -e '%a=(a,aa,b,bb,c,cc,d,dd,e,ee,f,ff,g,gg);$b=%a;print $b,"\n"'
   
6. 6/8

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为啥后面2个是一样的，明显多了一个。
问了几个人都说诡异。

flw

502 只住两个人，602 合租住了 8 个人，房子面积居然是一样的。诡异。后来 602 又搬走了 2 个人，只剩下 6 个人，面积居然没有减少！诡异！

zhlong8

明显 'g' 和别人同房了

Perl_Er

回复 1# minmig


   

http://perldoc.perl.org/perldata.html#Scalar-values

If you evaluate a hash in scalar context, it returns falseif the hash is empty. If there are any key/value pairs, it returns true; moreprecisely, the value returned is a string consisting of the number of usedbuckets and the number of allocated buckets, separated by a slash. This ispretty much useful only to find out whether Perl's internal hashing algorithmis performing poorly on your data set. For example, you stick 10,000 things ina hash, but evaluating %HASH in scalar context reveals "1/16", which means only one out of sixteen buckets has beentouched, and presumably contains all 10,000 of your items. This isn't supposedto happen. If a tied hash is evaluated in scalar context, a fatal error willresult, since this bucket usage information is currently not available for tiedhashes.

You can preallocate space for a hash by assigning to thekeys() function. This rounds up the allocated buckets to the next power of two:

1.      keys(%users)= 1000;        # allocate 1024 buckets



Also refer to <<Professional Perl Programming>> Page 137