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。 ! 或 not 对真值的否定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";
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 :
