Perl UNLESS...ELSE 语句

Perl unless 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。


语法

Perl 编程语言中 unless...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 unless 块 代码,否则将执行 else 块 代码。

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!not 对真值的否定返回一个特殊的假值。


流程图

Perl unless...else 语句

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";

当上面的代码被执行时,它会产生下面的结果 −

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

❮ Perl 条件语句