note BrowserUk <blockquote><i><c>return $fib3_cache{$n} ||= do {</c></i></blockquote> <p>I'd strongly recommend trading <c>//=</c> for <c>||=</c>. Any function that can return 0, but costs to determine that, looses out as is. <p>You can get greater benefits still for functions that take low integers as inputs by using an array instead of a hash for the cache:<code> my @fib4_cache; sub fib4 { my $n = shift; return $fib4_cache[$n] //= do { $n < 2 ? $n : fib4($n-1) + fib4($n-2) }; } __END__ C:\test>memofib.pl Rate fib1 fib2 fib3 fib4 fib1 88.7/s -- -100% -100% -100% fib2 219835/s 247838% -- -89% -91% fib3 2073455/s 2338415% 843% -- -19% fib4 2551114/s 2877136% 1060% 23% -- </code> <p>And see the benefits more clearly if you do away with some of the benchmark overheads by loosing the sub wrappers (The module adds its own wrapper internally):<code> cmpthese(-1, { fib1 => q{ fib1 20 }, fib2 => q{ fib2 20 }, fib3 => q{ fib3 20 }, fib4 => q{ fib4 20 }, }); __END__ C:\test>memofib.pl Rate fib1 fib2 fib3 fib4 fib1 87.4/s -- -100% -100% -100% fib2 223254/s 255377% -- -89% -91% fib3 2071504/s 2370384% 828% -- -20% fib4 2579867/s 2952119% 1056% 25% -- </code> <P>And finally, there's a couple more % to be had:<code> my @fib5_cache; sub fib5 { my $n = shift; return $fib5_cache[ $n ] //= do { $n < 2 ? $n : ( $fib5_cache[ $n-1 ] //= fib5($n-1) ) + ( $fib5_cache[ $n-2 ] //= fib5($n-2) ) ## corrected; thanks to moriitz }; } __END__ C:\test>memofib.pl Rate fib1 fib2 fib3 fib4 fib5 fib1 89.0/s -- -100% -100% -100% -100% fib2 218452/s 245352% -- -89% -91% -91% fib3 2022174/s 2272005% 826% -- -18% -20% fib4 2478824/s 2785096% 1035% 23% -- -2% fib5 2537045/s 2850512% 1061% 25% 2% -- </code> <div class="pmsig"><div class="pmsig-171588"> <hr /> <font size=1 > <div>With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'</div> <div>Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.</div> <div>"Science is about questioning the status quo. Questioning authority". </div> <div>In the absence of evidence, opinion is indistinguishable from prejudice. </div> </font> </div></div> 1013033 1013033