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";
}
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";
}
# 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";
}
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;
}
$j *= 2;
}
if expression can be written without a block of {}
print "$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;
$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;
}
do something;
}elsif(condition){
do something;
}elsif(condition){
do something;
}else{
do something;
}
my $bedrock=42;
$bedrock++;
$bedrock++;
my @people=qw{ Fred barney Fred wilma dino barney Fred pebbles };
my %count;
$count{$_}++ foreach @people;
my %count;
$count{$_}++ foreach @people;
++ means +1 byself, undef++ gets 1
my $n=++$m;
$m add 1, then give value to $n.
$m add 1, then give value to $n.
my $n=$m++;
give value to $n, then $m add 1.
give value to $n, then $m add 1.
for (init;test;increment){
body;
body;
}
body;
body;
}
for(i=1;$i<=10;$i++){
print "I can count to $i!\n";
}
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: $1\n";
}
print "One character is: $1\n";
}
for(1..10){
print "I can count to $_!\n":
}
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.
