Perl除非...否则语句

Perlunless语句后面可以跟一个可选的else语句，当布尔表达式为真时执行该语句。

语法

Perl编程语言中unless...else 语句的语法为：

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

如果布尔表达式求值为真，则执行unless代码块，否则执行else代码块。

在布尔上下文中，数字0、字符串“0”和“”，空列表()和undef都是假，所有其他值都是真。通过!或not对真值进行否定会返回一个特殊的假值。

流程图

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";

执行上述代码时，会生成以下结果：

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