defined

defined EXPR

defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value undef- If EXPR is not present, $_ is checked.

Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions- This function allows you to distinguish undef from other values- (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0"登录查看完整内容






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