Perl UNLESS...ELSE 语句

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Perl except语句后面可以跟一个可选的else语句，该语句在布尔表达式为 true 时执行。

句法

Perl 编程语言中except...else语句的语法是 -

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

如果布尔表达式的计算结果为true，则将执行except 代码块，否则将执行else 代码块。

数字 0、字符串 '0' 和 "" 、空列表 () 和 undef在布尔上下文中都是false ，所有其他值都是true。否定真值！or not返回一个特殊的 false 值。

流程图

例子

#!/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，会产生以下结果 -

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