the difference between @_ and shift

joson12345

有的时候用shift，有的时候用@_，搞不明白？？？
例如：
sub ff
{
        my ($reff1, $reff2)=@_; #the difference between @_ and shift
  for ($index=0;$index<=$#$reff1; $index++) {
          $result[$index]=$reff1->[$index]+$reff2->[$index];
  }
为啥改为shift之后就不好用了呢，他俩的本质区别在哪呢？？当然还有$_，差点忘了。
谢谢了

[ 本帖最后由 joson12345 于 2006-5-8 15:55 编辑 ]

兰花仙子

sub ff
{
        my ($reff1, $reff2)=@_; #the difference between @_ and shift
  for ($index=0;$index<=$#$reff1; $index++) {
          $result[$index]=$reff1->[$index]+$reff2->[$index];
  }

这里也可以写成：
my ($reff1, $reff2)=(shift,shift);

或下面的循环直接写成：
$result[$index]= shift->[$index]+ shift->[$index];

不会有问题。

joson12345

也就是说，在子程序获得数据方面，@和shift没什么区别呀？

兰花仙子

原帖由 joson12345 于 2006-5-8 19:18 发表
也就是说，在子程序获得数据方面，@和shift没什么区别呀？

shift也就是shift @_;它从@_这个数组里弹出数据。

joson12345

这两个（shift  and @_)只要使用其中的一个就可以了，或者两者都用效果相同？
例如：
sub sss
{
   print shift @_;  #  or shift;   or @_;
}

结果应该是一样的，@_and shift 之间是不是可以相互替换？

兰花仙子

from 'perldoc perlsub' and search for '@_' :

       Any arguments passed in show up in the array @_.  Therefore, if you
       called a function with two arguments, those would be stored in $_[0]
       and $_[1].  The array @_ is a local array, but its elements are aliases
       for the actual scalar parameters.  In particular, if an element $_[0]
       is updated, the corresponding argument is updated (or an error occurs
       if it is not updatable).  If an argument is an array or hash element
       which did not exist when the function was called, that element is cre-
       ated only when (and if) it is modified or a reference to it is taken.
       (Some earlier versions of Perl created the element whether or not the
       element was assigned to.)  Assigning to the whole array @_ removes that
       aliasing, and does not update any arguments.

from 'perodoc -f shift' :

       shift ARRAY
       shift   Shifts the first value of the array off and returns it, short-
               ening the array by 1 and moving everything down.  If there are
               no elements in the array, returns the undefined value.  If
               ARRAY is omitted, shifts the @_ array within the lexical scope
               of subroutines and formats, and the @ARGV array at file scopes
               or within the lexical scopes established by the "eval ''",
               "BEGIN {}", "INIT {}", "CHECK {}", and "END {}" constructs.


I don't like to discourage the newbe as you,but I really would not rather see someone ask a basal problem stiffly here before he could read some possible documents at first.