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Re: Function with optional default argument cannot cache

• To: mathgroup at smc.vnet.net
• Subject: [mg130682] Re: Function with optional default argument cannot cache
• From: Dan O'Brien <danobrie at gmail.com>
• Date: Wed, 1 May 2013 21:41:31 -0400 (EDT)
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• References: <20130501073754.8ADA36A27@smc.vnet.net> <E2002B9A-B548-4C4B-9EE9-D9479239DA0D@gmail.com>

Thanks for your response.  What you say was clear to start with, I 
should have asked a better question.  The question is why wouldn't 
mathematica assign downvalues to function evaluations when the default 
arguments (y_:2) are used?

The reason I ask this is because I was playing with a complicated 
function I defined where I would cache the values.  It occurred to me 
that one of the parameters typically didn't change and so I gave it a 
default value so that I didn't need to specify it every time. Then I 
quickly realized the function evalutations where not being cached 
(downvalues were not being assigned).  That is when I specify f[1], 
mathematica should know that that actually means f[1,2] and give it a 
downvalue upon  evaluation.  So my question is, why wouldn't mathematica 
do this?  Would it break some other intended functionality, or, to 
implement that sort of recognition is just not worth it?

-Dan


On 5/1/2013 7:46 AM, Sseziwa Mukasa wrote:
> On May 1, 2013, at 3:37 AM, Dan O'Brien <danobrie at gmail.com> wrote:
>
>> Is there a discussion somewhere on why this is?
>>
>> In[1]:= $Version
>> f[x_, y_: 2] := f[x, y] = {Pause[1], x, y}
>> f[1, 2] // AbsoluteTiming
>> f[1, 2] // AbsoluteTiming
>> f[1] // AbsoluteTiming
>> f[1] // AbsoluteTiming
>>
>> Out[1]= "9.0 for Microsoft Windows (64-bit) (January 25, 2013)"
>>
>> Out[3]= {1.014002, {Null, 1, 2}}
>>
>> Out[4]= {0., {Null, 1, 2}}
>>
>> Out[5]= {1.029602, {Null, 1, 2}}
>>
>> Out[6]= {1.014002, {Null, 1, 2}}
>>
> The value that's cached for f[1,2] is {Null,1,2} not {Pause[1],1,2}.  Out[4] returns the cached value, f[1] doesn't match the cached pattern so f[x_,y_:2] is evaluated again in the case of Out[5] and Out[6].  You can see this by looking at the DownValues of f:
>
> (Debug) In[7]:= DownValues[f]
> (Debug) Out[7]= {HoldPattern[f[1, 2]] :> {Null, 1, 2},
>   HoldPattern[f[x_, y_ : 2]] :> (f[x, y] = {Pause[1], x, y})}
>
> It's not clear what you are trying to accomplish, perhaps you want:
>
> (Debug) In[39]:= f[x_, y_: 2] := f[x, y] = {Hold[Pause[1]], x, y}
> ReleaseHold[f[1, 2]] // AbsoluteTiming
> ReleaseHold[f[1, 2]] // AbsoluteTiming
> ReleaseHold[f[1]] // AbsoluteTiming
> ReleaseHold[f[1]] // AbsoluteTiming
> (Debug) Out[40]= {1.000602, {Null, 1, 2}}
> (Debug) Out[41]= {1.000028, {Null, 1, 2}}
> (Debug) Out[42]= {1.000572, {Null, 1, 2}}
> (Debug) Out[43]= {1.000699, {Null, 1, 2}}

• References:
  • Function with optional default argument cannot cache results
    • From: Dan O'Brien <danobrie@gmail.com>