Re^5: modify the contents of an array

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Re^6: modify the contents of an array by Anonymous Monk on Sep 29, 2005 at 13:22 UTC
Let's benchmark: #!/usr/bin/perl use strict; use warnings; use Benchmark qw 'cmpthese'; our @dates = qw [092205 092305 092605]; our (@mb, @me, @fb, @fe); cmpthese(-1, { map_block => '@mb = @dates; map {s{(\d{2})(\d{2})(\d{2})}{$1/$2/$3 +}} @mb', map_expr => '@me = @dates; map s{(\d{2})(\d{2})(\d{2})}{$1/$2/$3 +}, @me', for_block => '@fb = @dates; for (@fb) {s{(\d{2})(\d{2})(\d{2})}{$1 +/$2/$3}}', for_expr => '@fe = @dates; s{(\d{2})(\d{2})(\d{2})}{$1/$2/$3} for + @fe', }); die unless "@mb" eq "@me" && "@mb" eq "@fb" && "@mb" eq "@fe"; __END__ Rate for_block for_expr map_block map_expr for_block 43115/s -- -1% -2% -6% for_expr 43530/s 1% -- -1% -5% map_block 43840/s 2% 1% -- -5% map_expr 45948/s 7% 6% 5% -- [download] Running it a few more times shows the expression form of map being slightly faster than any of the others, and even the block version of map not doing too bad: `Rate map_block for_expr for_block map_expr map_block 43050/s -- -1% -5% -7% for_expr 43631/s 1% -- -4% -6% for_block 45271/s 5% 4% -- -2% map_expr 46419/s 8% 6% 3% -- Rate for_expr for_block map_block map_expr for_expr 42309/s -- -3% -5% -7% for_block 43530/s 3% -- -3% -5% map_block 44660/s 6% 3% -- -2% map_expr 45583/s 8% 5% 2% -- Rate for_block for_expr map_block map_expr for_block 43115/s -- -1% -3% -7% for_expr 43530/s 1% -- -2% -6% map_block 44246/s 3% 2% -- -5% map_expr 46419/s 8% 7% 5% --` [download] Things might be different on your machine, but the difference in performance is too small to regard one as better than the other. Now, if you really want to go for speed, your concern should not be the difference between 'for' and 'map', but the difference between 's///' and something else. For instance, let's introduce an additional clause in the benchmark: `pack => '@pk = @dates; @pk = map {join "/", unpack "A2 A2 A2", $_} @pk +'` [download] This leads to the results: `Rate for_expr for_block map_block map_expr pack for_expr 42309/s -- -0% -2% -9% -48% for_block 42309/s -0% -- -2% -9% -48% map_block 43115/s 2% 2% -- -7% -47% map_expr 46394/s 10% 10% 8% -- -43% pack 80736/s 91% 91% 87% 74% --` [download] It dwarves the difference between map and for.	[reply] [d/l] [select]
Re^6: modify the contents of an array by blazar (Canon) on Sep 29, 2005 at 13:06 UTC
briefly, map and grep are to be used for their return value. Or else a `for` loop will do. Even the perlstyle page mentions that...	[reply] [d/l]
Re^7: modify the contents of an array by Anonymous Monk on Sep 29, 2005 at 13:35 UTC
Eh, no, you completely misunderstand the purpose of perlstyle. Perlstyle lists Larry's preferences of how he codes. And there's only one thing he strongly cares about - which isn't the map issue. The remark you quote is listed under Here are some other more substantive style issues to think about. It's telling you to think about map/grep in void context - not ordering you to avoid it. But what's more important is the reasoning - it's referring to the fact that map in void context is expensive. And it was - at the time perlstyle was written. But perlstyle was written many years ago, long before the map issue was fixed. It's fixed now, so there's nothing left over from the reasoning in perlstyle.	[reply]
Re^8: modify the contents of an array by nothingmuch (Priest) on Sep 29, 2005 at 21:55 UTC
uh, seriously, wtf? `map { s/// } @array; # ... s/// for @array;` [download] They're the same, except one is more confusing to read, because most people expect it's return type to be used. Just because larry likes something is not a reason everyone should be following him (we should all be blindingly following Perl Best Practices instead ;-), but map in void context has no advantage over for, but does add an element of confusion (did the author mean to assign to a result array? did the author not know about for? am I reading this badly? let me go up one line and see if it's a multi line assignment statement). Optimizing for readability is universal good style. -nuffin zz zZ Z Z #!perl	[reply] [d/l]