无涯教程-Perl - unless 语句函数

Perl的除非unless语句由一个布尔表达式和一个或多个语句组成。

unless - 语法

Perl编程语言中的exclude语句的语法是-

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

如果布尔表达式的输出为 false ，则将执行exclude语句中的代码块。

unless - 流程图

unless - 示例

#!/usr/local/bin/perl
 
$a=20;
# check the boolean condition using unless statement
unless( $a < 20 ) {
   # if condition is false then print the following
   printf "a is not less than 20\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";
}
print "value of a is : $a\n";

首先，除非语句使用小于运算符(<)，该运算符比较两个操作数，如果第一个操作数小于第二个操作数，则返回true，否则返回false。 

a is not less than 20
value of a is : 20
a has a false value
value of a is : 

参考链接

https://www.learnfk.com/perl/perl-unless-statement.html