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Learning Perl: 10.4. The Naked Block Control Structure

2010-07-21 18:02 141 查看

10.4. The Naked Block Control Structure

The so-called "naked" block is one without a keyword or condition. That is, suppose you start with a while loop, which looks something like this:

while (condition) {
body;
body;
body;
}

Remove the while keyword and the conditional expression, and you'll have a naked block:

{
body;
body;
body;
}

The naked block is like a while or foreach loop, except that it doesn't loop; it executes the body of the loop once, and it's done. It's an un-loop!
You'll see other uses for the naked block, but one of its features is providing a scope for temporary lexical variables:

{
print "Please enter a number: ";
chomp(my $n = <STDIN>);
my $root = sqrt $n;  # calculate the square root
print "The square root of $n is $root./n";
}

In this block, $n and $root are temporary variables scoped to the block. As a general guideline, all variables should be declared in the smallest scope available. If you need a variable for a few lines of code, you can put those lines into a naked block and declare the variable inside that block. If you need the value of $n or $root later, you will need to declare them in a larger scope.
You may have noticed the sqrt function in that code and wondered about it; yes, we haven't shown this function before. Perl has many built-in functions beyond the scope of this book. When you're ready, check the perlfunc manpage to learn about more of them.

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