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Re: Why does Through unpack?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg125609] Re: Why does Through unpack?
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Wed, 21 Mar 2012 05:47:13 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201203200720.CAA11907@smc.vnet.net>
  • Reply-to: drmajorbob at yahoo.com

Your Through syntax is not right for what you seem to want, since operator  
precedence makes it equivalent to

Through /@ ({Min, Max} /@ Transpose[dat])

But this is OK:

On["Packing"]
dat = RandomInteger[1*^7, {5000000, 2}];
{Min@#, Max@#} & /@ Transpose@dat // AbsoluteTiming // Quiet

{0.158896, {{4, 9999996}, {1, 10000000}}}

Min /@ Transpose@dat // AbsoluteTiming // Quiet

{0.118989, {4, 1}}

Max /@ Transpose@dat // AbsoluteTiming // Quiet

{0.119237, {9999996, 10000000}}

Your syntax gives

Through /@ {Min, Max} /@ Transpose[dat] // AbsoluteTiming

{1.575901, {{4, 9999996}, {1, 10000000}}}

(Correct result, but slow.)

Bobby

On Tue, 20 Mar 2012 02:20:32 -0500, Szabolcs <szhorvat at gmail.com> wrote:

> Consider the following example:
>
> On["Packing"]
>
> dat = RandomInteger[1*^7, {5000000, 2}];
>
> Max /@ Transpose[dat]; // AsboluteTiming
>
> Note that the array is only unpacked to level 1, so Max is fast.
>
> Now let's get both the Min and the Max:
>
> Through /@ {Min, Max} /@ Transpose[dat]; // AbsoluteTiming
>
> Now the full array is unpacked, significantly hurting performance.
>
> Question:
>
> Why is it necessary to unpack the array when using Through?  Here it  
> doesn't seem to be.  Is it a design decision that Through unpacks, or is  
> it an over sight?  Through does not touch the function argument (it only  
> affects the head), I don't see why it would need to unpack it.
>


-- 
DrMajorBob at yahoo.com



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