Re: For Loop and Array related

*To*: mathgroup at smc.vnet.net*Subject*: [mg58015] Re: For Loop and Array related*From*: Paul Abbott <paul at physics.uwa.edu.au>*Date*: Thu, 16 Jun 2005 06:43:50 -0400 (EDT)*Organization*: The University of Western Australia*References*: <d8oucl$t6q$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

In article <d8oucl$t6q$1 at smc.vnet.net>, "mchangun at gmail.com" <mchangun at gmail.com> wrote: > I have an array with 100 elements, all set to zero initially. Then I > want to randomly choose one element and increase its value by one, and > repeat this 16000 times. Here is my code: > > Lattice = Table[0, {n, 100}]; > For[i = 1, i = 16000, i++, Lattice[[Random[Integer, {1, 100}]]]++] > > So now if I add all the elements in the list Lattice together, I should > get 16000 (I use Total[Lattice] to get the sum of the list). But this > doesn't happen, and strangely, each time I run this, the sum of the > list is different! What am I doing wrong? A syntax error (use of == vs. =) and a subtlety. Here is the modified code: Lattice = Table[0, {n, 100}]; SeedRandom[1]; Module[{t}, For[i = 1, i <= 16000, i++, t = Random[Integer, {1, 100}]; Lattice[[t]]++]]; Lattice I've included SeedRandom[1]; so that we get a "repeatable" random sequence. Note the use of the local variable t. If you write Lattice[[Random[Integer, {1, 100}]]]++ then, essentially, you are evaluating Lattice[[Random[Integer, {1, 100}]]] = Lattice[[Random[Integer, {1, 100}]]] + 1 which involves two _different_ random numbers. > Also I'm aware that a lot of Mathematica newbies try and write code > like it were C++ and I think i've fallen into this trap as well. So is > there a different (more Mathematica) way which I can implement the > above? First compute a list of 16000 numbers: SeedRandom[1]; t = Table[Random[Integer, {1, 100}], {16000}]; Here are three ways to compute Lattice: [1] Count[t, #]& /@ Range[100] [2] << "Statistics`"; BinCounts[t, {0, 100}] [3] Length /@ Split[Sort[t]] These 3 codes give the same result as above. The last code will not work with short lists because it does not return any zero counts. Cheers, Paul -- Paul Abbott Phone: +61 8 6488 2734 School of Physics, M013 Fax: +61 8 6488 1014 The University of Western Australia (CRICOS Provider No 00126G) AUSTRALIA http://physics.uwa.edu.au/~paul http://InternationalMathematicaSymposium.org/IMS2005/