Perl Learning - 4 (pop/push/shift/unshift, foreach, $_)
发布时间：2020-12-15 21:03:00 所属栏目：大数据 来源：网络整理
导读：pop() push() ? pop() get the last element out of array,and returns that element. ? @array=5..9; $fred=pop(@array);?# $fred is 9,@array now is (5,6,7,8) $barney=pop @array;?# $barney is 8,7) pop @array;??# @array now is (5,6) ? Retuned valu
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pop() & push()
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pop() get the last element out of array,and returns that element.
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@array=5..9;
$fred=pop(@array);?# $fred is 9,@array now is (5,6,7,8) $barney=pop @array;?# $barney is 8,7) pop @array;??# @array now is (5,6)
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Retuned value can be throw away,it's ligal in Perl.
If array is empty,pop() do nothing and returns undef. The ( ) after pop can be skipped,Perl says if skipping something doesn't make difference,then let's skip it.
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push() do the oppsite things to pop(),it adds elements to the end of an array.
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push(@array,0);??# @array now is (5,0)
push @array,8;??# @array now is (5,8) push @array,1..10;?# 10 more elements @others=qw/9 0 2 1 0/; push @array,@others;?# 5 more elements
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Note the fist parameter of push() and the only parameter of pop() must be an array.
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shift() & unshift()
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Shift() removes the first element out of an array,unshift() adds elements to the beginning of an array.
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@array=qw#dino fred barney#;
$m=shift(@array);??# $m is 'dino',@array now is ('fred','barney') $n=shift @array;??# $n is 'fred',@array now is ('barney') shift @array;???# @array now is empty () $o=shift @array;??# @array still empty,$o is undef unshift(@array,5);??# @array becomes (5) unshift @array,4;??# @array becomes (4,5) @others=1..3; unshift @array,@others;??# @array becomes (1,2,3,4,5)
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Array cen be inserted to pire of " ",element will be serarated by space automatically.
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@rocks=qw{flintstone slate rubble};
print "quartz @rocks limestonen";?# prints "quartz flintstone slate rubble limestone"
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It will not present spaces before the head or after the end of array,just add space between elements.
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foreach() can go through all elements of an array and produce a loop.
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foreach $rock (qw/bedrock slate lava/){
??? print "One rock is $rock.n"; ??? }
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The contral variable $rock get an element every time it loops,until the last element it ends.
The contral variable changes the values of elements when it loops,not only getting a copy.
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If there is another variable with same name as the contral variable,when loop starts the varable cannot change.
Until loop ends,the variable recovers it's original value ahead of loop.
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If no contral variable is determined,Perl uses $_ as contral variable.
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foreach(1..10){
??? print "I can count to $_!n"; ??? }
$_="Yabba dabba doon";
print;???# $_ will be printed
Whenever you cannot find a varable,it could be a $_ working there ^_^
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