Re: Increase in efficiency with Module

*To*: mathgroup at smc.vnet.net*Subject*: [mg40061] Re: [mg40050] Increase in efficiency with Module*From*: Dr Bob <drbob at bigfoot.com>*Date*: Tue, 18 Mar 2003 02:21:30 -0500 (EST)*References*: <200303170835.DAA22905@smc.vnet.net>*Reply-to*: drbob at bigfoot.com*Sender*: owner-wri-mathgroup at wolfram.com

I don't know the implementation details, but without Module, the function has side-effects. It changes the values of global symbols (a functional programming no-no). With Module, the C compiler is free to keep those values in counter registers, or wherever. Here's a code that's 20 to 35 percent faster: ftauc3 = Compile[{{ry, _Real, 1}}, Block[{i = 0, j, a = 0, n = Length@ry}, Do[i++; j = i + 1; Do[a += Sign[ry[[j]] - ry[[i]]]; j++, {n - i}], {n - 1}]; a]]; test = Array[Random[Real, 100] &, 500]; Timing@ftauc3[test] Timing@ftauc2[test, Length@test] Timing@ftauc[test, Length@test] {0.06300000000000239*Second, 623} {4.922000000000004*Second, 623} {0.0940000000000012*Second, 623} test = Array[Random[Real, 100] &, 1000]; Timing@ftauc3[test] Timing@ftauc2[test, Length@test] Timing@ftauc[test, Length@test] {0.26499999999998636*Second, 1270} {20.062999999999988*Second, 1270} {0.34300000000001774*Second, 1270} test = Array[Random[Real, 100] &, 5000]; Timing@ftauc3[test] Timing@ftauc[test, Length@test] {6.4839999999999804*Second, -128684} {8.530999999999977*Second, -128684} Bobby On Mon, 17 Mar 2003 03:35:18 -0500 (EST), Aaron E. Hirsh <aehirsh at stanford.edu> wrote: > I would be grateful if someone could explain the difference in efficiency > between the following two simple programs. ry is a list of length n. For > n = 1000, the program using Module takes 1 second on my laptop, whereas > the program without Module takes 75 seconds. > > ftauc = Compile[{{ry, _Real, 1}, {n, _Real, 0}}, > Module[{i, j, a}, > i = 1; > a = 0; > Do[ > > j = i + 1; > Do[ > If[ry[[i]] < ry[[j]], a++, If[ry[[i]] > ry[[j]], a--]]; > j++, {n - i}]; > > i++, {n - 1}]; a > ]] > > ftauc2 = Compile[{{ry, _Real, 1}, {n, _Real, 0}}, > > > > i = 1; > a = 0; > Do[ > j = i + 1; > > Do[ > If[ry[[i]] < ry[[j]], a++, If[ry[[i]] > ry[[j]], a--]]; > j++, {n - i}]; > i++, {n - 1}]; a > ] > > thank you, -- majort at cox-internet.com Bobby R. Treat

**References**:**Increase in efficiency with Module***From:*"Aaron E. Hirsh" <aehirsh@stanford.edu>