"if" and compound statement

[juliosergio]

I'm new here, and in Perl as well. I thought I'd learnt that

if (1) {
  statement;
}

is more or less the same as:

statement if (1);

However, I'm puzzled with the following, that uses the compound statement and doesn't work

{print "one\n"; print "two\n";} if (1);

Do you have any comments on this?

Thanks! -Sergio.

[BrowserUk]

The postfix form of if is a statement modifier, not a block modifier. And unadorned curlies form a block.

You could do either:

print( "one\n"), print( "two\n" ) if (1);

Or:

do{ print "one\n"; print "two\n"; } if (1);

Though there is little advantage of the latter over:

if( 1 ){ print "one\n"; print "two\n"; }

But most people will generally dislike it if you do either.

As an aside, your example is not a good one, as two print statements can always be combined:

print "one\ntwo\n" if (1);

Personally, I find that there are cases for which compounding statements with a postfix if makes sense. Eg:

while( my $input = <> ) {
    warn( "Bad input" ), next unless $input =~ m[...];
    ...
}

[JavaFan]

As an aside, your example is not a good one, as two print statements can always be combined

If you're going to nitpick about examples, I'm going to nitpick about your use of always. If the default filehandle is tied, if autodie is enabled, or if strings are overloaded, combining the given statements can lead the different results.

[BrowserUk]

Consider me duly chastened. Even though I only added the aside in an attempt to avoid the nitpickers :)

[juliosergio]

Thanks! I really appreciate your comments..
-Sergio.

[choroba]

From perlsyn:

Any simple statement may optionally be followed by a SINGLE modifier, just before the terminating semicolon (or block ending).

Emphasis mine (not upper-case).

[educated_foo]

You can combine statements with "do", like so:

do { statement; statement } if $condition;

[tobyink]

Your question has already been well and truly answered, but as an aside, the parentheses around the condition with postfix if are optional. So you can write:

statement if 1;

They are not optional for the block form of if though. The following is incorrect:

if 1 {
  statement;
}

It's also worth mentioning that Damian Conway's Perl Best Practices suggests that you never use the postfix form of if, and always use the block form. (PBP #63) I happen to disagree with that particular PBP; most people have some PBPs that they disagree with, but PBP is usually a good starting point when developing your own Perl coding style.

[GrandFather]

Not just "doesn't work" - it's a syntax error which should suggest to you that the animal you have by the tail is not the one you thought you had.

As a general thing we often suggest to newbies to always use strictures (use strict; use warnings;), but in this case it fails without needing them - however the advice is still good!

[JavaFan]

As a general thing, it's often recommended to eat a serving of vegetables, and two pieces of fruit everyday. The OP's code would have failed regardless how much veggies and fruit he has consumed, however, the advice is still good!

[GrandFather]

Sure, and if the OP had said he was a six year old Perl newbie I might have offered other good life advice too. But this is a Perl oriented site and the OP gave no such indication so I didn't. Strictures, strict in particular, are such useful tools for catching typos and other errors, that suggesting their use to self avowed Perl newbies is pretty much always good advice.

[ikegami]

It's

EXPR if EXPR;

and not

STATEMENT if EXPR;

You can use do (do { ... }if EXPR;), but not a bare loop ({ ... } if EXPR;).