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Re: new way to code html?

by muba (Priest)
on Nov 21, 2012 at 01:51 UTC ( #1004827=note: print w/ replies, xml ) Need Help??


in reply to new way to code html?

?: should really be used to let Perl choose one or another value based on the truthfullness of an expression, not as a flow control mechanism. If you want flow control, use if and else

# Ok - pick a value based on the value of $bar $foo = $bar eq "baz" ? "foobaz" : "foobar"; print "I am the ", ($sex eq "male" ? "king" : "queen"), "of rock & rol +l"; some_subroutine( ref($foo) eq "ARRAY" ? $foo : [$foo] );
# Not ok - ?: flow control $bar eq "baz" ? $foo = "foo" : $bar = "baz";
# Ok - if/else flow control if ($bar eq "baz") { $foo = "foo"; } else { $bar = "baz"; }
# Ok - if/else flow control, if a bit verbose. if ($bar eq "baz") { $foo = "foo"; } else { $foo = "baz"; }


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Re^2: new way to code html?
by tobyink (Abbot) on Nov 21, 2012 at 07:05 UTC

    Using the ternary operator (?:) over if/else can sometimes be a useful optimisation. Especially if it's in a deeply nested loop that gets executed thousands of times.

    On my machine, ternary is consistently about 25% faster.

    use Benchmark ':all'; my ($foo, $bar); cmpthese(-1, { ternary => sub { int(rand(2)) ? ( $foo = 1 ) : ( $bar = 1 ) + }, if_else => sub { if(int(rand(2))) { $foo = 1 } else { $bar = 1 } + }, });

    Why is this? It's because the braces used in if/else structures introduce a new lexical scope, so when they're encountered Perl needs to create a new pad (i.e. a structure where it keeps my variables) even if that pad doesn't get used.

    Of course, generally speaking clarity should win over speed. But in subs which are executed many thousands of times, use of the ternary for simple conditionals is a handy trick to have up your sleeve.

    See also: the block forms of map and grep; control structures versus statement modifiers.

    Update: thanks anonymous monk below for pointing out the precedence error in my original code for the ternary sub. Corrected above now. I think this illustrates perfectly my statement that "generally speaking clarity should win over speed". :-)

    perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

      use of the ternary for simple conditionals is a handy trick to have up your sleeve.

      I'd worry about having a grasp of the language (context, syntax) before worrying about micro-optimizations

      -MO=Deparse,-p says

      use Benchmark (':all'); my($foo, $bar); cmpthese((-1), {'ternary', sub { ((int(rand(2)) ? ($foo = 1) : $bar) = 1); } , 'if_else', sub { if (int(rand(2))) { ($foo = 1); } else { ($bar = 1); } } });

      More clearly  ((int(rand(2)) ? ($foo = 1) : $bar) = 1);

      is

      ( ( int(rand(2)) ? ($foo = 1) : $bar ) = 1 );
      which is an error

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