求问两个关于字符串的问题

qaszxc

1。怎样判断一个字符串是否为空？
   
    我用 $a =~""; 来判断，但有时会出错，有没有其他方法？

2。字符串长度有无限制，如果有，长度是多少？

   如果我需要使用的字符串很长，又不想用数组该怎么办？

3。怎样获得字符串的长度？

[ 本帖最后由 qaszxc 于 2006-3-12 20:55 编辑 ]

兰花仙子

[quote]原帖由 qaszxc 于 2006-3-12 20:18 发表
1。怎样判断一个字符串是否为空？

unless ($string){ do something; }
  

2。字符串长度有无限制，如果有，长度是多少？

长度没有限制吧？取决于你的内存大小。


3。判断字符串长度的函数是： length()

dirac_teng

1。怎样判断一个字符串是否为空？

if (defined $aa ) {
  print "It isn't null." , "\n" ;
}else {
  print "It is null." , "\n" ;
}

如果 $aa = 0 ， 上面的代码和 2楼的结果 不一样

POLOGG

原帖由 dirac_teng 于 2006-3-13 15:46 发表
1。怎样判断一个字符串是否为空？

if (defined $aa ) {
  print "It isn't null." , "\n" ;
}else {
  print "It is null." , "\n" ;
}

如果 $aa = 0 ， 上面 ...

define表示有没有定义！

和空不空是俩码子事！

ranzo

应该使用defined

ulmer

原帖由 兰花仙子 于 2006-3-12 21:14 发表
[quote]原帖由 qaszxc 于 2006-3-12 20:18 发表
1。怎样判断一个字符串是否为空？

unless ($string){ do something; }
  

Hallo,

The first question seems too simple but really not simple to answer!
It's depend on which context of the given string.
For examples:
my $string;           # undefined variable, but with my() localized.
my $string = "";    # defined variable but it's empty string.
my $string = "  ";    # defined variable but it contains space(s) or tabs.
my $string = 0;    # strange??? is boolean false? or exactly integer 0? perl treated it as undef.
my $string = "0"; # defined string with character 0 (zero). perl returns it as undef.
undef $string;     # really explicitly as undefined string.

if you don't pay attenction to the context of the given string, you will get an unexpected
result.

For better undstanding to try this:
perl -Mstrict -MData::Dumper -le 'my $s=""; print Dumper($s) unless $s'
perl -Mstrict -MData::Dumper -le 'my $s=""; print Dumper($s) unless defined $s'
perl -Mstrict -MData::Dumper -le 'my $s; print Dumper($s)'
perl -Mstrict -MData::Dumper -le 'my $s="0"; if ($s){print Dumper($s)}'
perl -Mstrict -MData::Dumper -le 'my $s="0"; if ($s ne ""){print Dumper($s)}'
perl -Mstrict -le 'my $s=0; if (defined $s){print "string: >$s<"}'
... more ...
....testing by yourself.


Best, ulmer

---------------------------
Just 4 Fun.

dirac_teng

4楼说的对，我误解了 $s=""。  

另外楼上的建议很好，我测试了一下
...
my $string = 0;    # strange??? is boolean false? or exactly integer 0? perl treated it as undef.
...
这句不对。if ( $string ) 是返回假的，但 if (defined $string)是返回真的，所以说 “perl treated it as undef”是不对的。


...
my $string = "0"; # defined string with character 0 (zero). perl returns it as undef.
...
楼上的既然说 是 “defined string”，为何又说“perl returns it as undef”？ 针对这句：if ( $string ) 是返回假的，但 if (defined $string)是返回真的。

[ 本帖最后由 dirac_teng 于 2006-3-13 18:06 编辑 ]

ulmer

原帖由 dirac_teng 于 2006-3-13 18:00 发表
4楼说的对，我误解了 $s=""。  

另外楼上的建议很好，我测试了一下
...
my $string = 0;    # strange??? is boolean false? or exactly integer 0? perl treated it as undef.
...
这句不对。i ...

You are right!
I just want somebody to test and find what's happend.
Returned defined, undefined or boolean (true/false) strongly depended on CONTEXT
and it's control condition!


Bets, ulmer