Perl Learning - 18 (unless, until, elsif, for)

发布时间：2020-12-15 21:01:38 所属栏目：大数据来源：网络整理

导读：unless expression: when condition is false execute the block. ? unless($fred=~/^[A-Z_]w*$/i){ ?print "The value of $fred doesn't look like a perl identifier name.n"; } ? It's the same as written like this: ? if($fred=~/^[A-Z_]w*$/i){ ?

unless expression: when condition is false execute the block.

unless($fred=~/^[A-Z_]w*$/i){
?print "The value of $fred doesn't look like a perl identifier name.n";
}

It's the same as written like this:

if($fred=~/^[A-Z_]w*$/i){
?# do nothing
}else{
?print "The value of $fred doesn't look like a perl identifier name.n";
}

Or:

if(!($fred=~/^[A-Z_]w*$/i)){
?print "The value of $fred doesn't look like a perl identifier name.n";
}

until expression: when condition is false execute the loop.

until($j>$i){
?$j *= 2;
}

if expression can be written without a block of {}

print "$n is a negative number.n" if $n < 0;

Perl execute 'if $n < 0' first,if it's true then execute the print expression,if false do nothing.

Those styles are all popular in Perl:

&error("Invalid input") unless &valid($input);
$i *=2 until $i > $j;
print " ",($n += 2) while $n < 10;
&greet($_) foreach @person;

If there are more than one expression,better to write them in {} blocks.

elsif expression:

if(condition){
?do something;
}elsif(condition){
?do something;
}elsif(condition){
?do something;
}else{
?do something;
}

my $bedrock=42;
$bedrock++;

my @people=qw{ Fred barney Fred wilma dino barney Fred pebbles };
my %count;
$count{$_}++ foreach @people;

++ means +1 byself,undef++ gets 1

my $n=++$m;
$m add 1,then give value to $n.

my $n=$m++;
give value to $n,then $m add 1.

for (init;test;increment){
?body;
?body;
}

for(i=1;$i<=10;$i++){
?print "I can count to $i!n";
}

Any of init,test,increment can be empty,but the symbol ';' should be there.

for($_="bedrock";s/(.)//;){ # whenever s/// successes,loop continue
?print "One character is: $1n";
}

for(1..10){
?print "I can count to $_!n":
}

Here 'for' is the same as foreach. for and foreach are the same in Perl,Perl will judge by the contents in (). If there're ';' in () then it's for,otherwise it's foreach.

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!