Perl除非...否則語句

Perlunless語句後面可以跟一個可選的else語句，當布林表示式為真時執行該語句。

語法

Perl程式語言中unless...else 語句的語法為：

unless(boolean_expression) {
   # statement(s) will execute if the given condition is false
} else {
   # statement(s) will execute if the given condition is true
}

如果布林表示式求值為真，則執行unless程式碼塊，否則執行else程式碼塊。

在布林上下文中，數字0、字串“0”和“”，空列表()和undef都是假，所有其他值都是真。透過!或not對真值進行否定會返回一個特殊的假值。

流程圖

示例

#!/usr/local/bin/perl
 
$a = 100;
# check the boolean condition using unless statement
unless( $a == 20 ) {
   # if condition is false then print the following
   printf "given condition is false\n";
} else { 
   # if condition is true then print the following
   printf "given condition is true\n";
}
print "value of a is : $a\n";

$a = "";
# check the boolean condition using unless statement
unless( $a ) {
   # if condition is false then print the following
   printf "a has a false value\n";
} else {
   # if condition is true then print the following
   printf "a has a true value\n";
}
print "value of a is : $a\n";

執行上述程式碼時，會生成以下結果：

given condition is false
value of a is : 100
a has a false value
value of a is :