Re: Compile forgets assigned values inside Do unless they are initialised

*To*: mathgroup at smc.vnet.net*Subject*: [mg82312] Re: Compile forgets assigned values inside Do unless they are initialised*From*: Andrew Moylan <andrew.j.moylan at gmail.com>*Date*: Wed, 17 Oct 2007 04:07:23 -0400 (EDT)*References*: <fepss7$gi3$1@smc.vnet.net><ff1pcn$8p2$1@smc.vnet.net>

I think your rule of thumb is a good one Jean-Marc, because it seems to be required in general to make compiled code behave as expected. However, I can't see any reason why the compiler can't in principle systemically successfully parse functions like Compile[{}, Module[{var}, Do[var = 0.;, {10}]; var]][] without losing track of where/whether variables like var are assigned values. Actually I wouldn't be surprised if this turned out to be just a bug related to not setting (inside the compilation of the Do block) a flag describing var as "set as a _Real" outside the Do block. On Oct 16, 5:31 pm, "Jean-Marc Gulliet" <jeanmarc.gull... at gmail.com> wrote: > Giving more thoughts, I think you are right and my "thesis" would have > been clearer if I had written, "As a rule of thumb, if one wants to > compile an expression that uses local symbols, then these symbols > should be initialized preferably at the beginning of the Module or > Block construct, or as soon as possible." > > Why that? Because, as we are warned by the online documentation, > "Compiled code does not handle numerical precision and local variables > in the same way as ordinary Mathematica code." > > Moreover, "The number of times and the order in which objects are > evaluated by Compile may be different from ordinary Mathematica code." > > In other words, ordinary evaluation rules do not necessarily and/or > usually apply to compiled code. > > In[1]:= Block[{x}, Do[x = 2, {10}]; x] > > Out[1]= 2 > > In[2]:= Compile[{}, Block[{x}, Do[x = 2, {10}]; x]][] > > During evaluation of In[2]:= Compile::initvar: The variable x has not \ > been initialized or has been initialized to Null. >> > > In[3]:= Compile[{}, Block[{x}, x = 2; Do[x = 2, {10}]; x]][] > > Out[3]= 2 > > Regards, > Jean-Marc > > On 10/15/07, DrMajorBob <drmajor... at bigfoot.com> wrote: > > > > > > Yes. If one wants to compile an expression that uses local symbols, then > > > these local symbols must be initialized at the beginning of the Module > > > (or Block) construct. See ref/message/Compile/initvar. > > > ref/message/Compile/initvar is precious little to go on, and the following > > seems to contradict your thesis: > > > Compile[{},Block[{x},x=2]] > > %[] > > > CompiledFunction[{},Block[{x},x=2],-CompiledCode-] > > 2 > > > Precisely how "late" is "too late"? > > > Bobby > > > On Mon, 15 Oct 2007 00:18:55 -0500, Jean-Marc Gulliet > > <jeanmarc.gull... at gmail.com> wrote: > > > > Andrew Moylan wrote: > > > >> In the following example, although var is assigned a value, these are > > > > The value is set to late in your example (see below). > > > >> "forgotten" and an error occurs when it is returned as the result of the > > > > The value has not been "forgotten" since it has never been assigned to > > > the symbol. > > > >> compiled function: > > > >> Compile[{}, > > >> Module[ > > >> {var}, > > >> Do[ > > >> var = 0.;, > > >> {10} > > >> ]; > > >> var > > >> ] > > >> ][] > > > >>>> Compile::initvar: The variable var has not been initialized or has > > >>>> been > > >> initialized to Null. > > > >> But if var is initialised (to any Real value at all), then all > > >> subsequents > > >> assignments to it are "remembered": > > > >> Compile[{}, > > >> Module[ > > >> {var = 1.}, > > >> Do[ > > >> var = 0.;, > > >> {10} > > >> ]; > > >> var > > >> ] > > >> ][] > > > >>>> 0. > > > >> Is this the expected behaviour? > > > > Yes. If one wants to compile an expression that uses local symbols, then > > > these local symbols must be initialized at the beginning of the Module > > > (or Block) construct. See ref/message/Compile/initvar. > > > > Regards, > > > -- > > DrMajor... at bigfoot.com > > -- > Jean-Marc