Re: Mapping tag-value lists to a third
- To: mathgroup at smc.vnet.net
- Subject: [mg132470] Re: Mapping tag-value lists to a third
- From: Ray Koopman <koopman at sfu.ca>
- Date: Mon, 24 Mar 2014 00:20:11 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
- Delivered-to: l-mathgroup@wolfram.com
- Delivered-to: mathgroup-outx@smc.vnet.net
- Delivered-to: mathgroup-newsendx@smc.vnet.net
On Sun, Mar 23, 2014 @ 02:00 AM, Bill Rowe <readnews at sbcglobal.net> wrote:
> On 3/22/14 at 12:06 AM, koopman at sfu.ca (Ray Koopman) wrote:
>
>> For v5+ users,
>>
>> val[[ SparseArray[tag] /. SparseArray[_,_,_,d_] :> Flatten@d[[2,2]]
>> ]]
>>
>> may be faster than Pick[val,tag,1].
>
> For what it is worth
>
> In[1]:= n = 10^7;
> val = RandomReal[1, n];
> tag = RandomInteger[1, n];
>
> In[4]:= Timing[c = Pick[val, tag, 1];]
>
> Out[4]= {0.137875,Null}
>
> In[5]:= Timing[
> b = val[[SparseArray[tag] /.
> SparseArray[_, _, _, d_] :> Flatten@d[[2, 2]]]];]
>
> Out[5]= {0.255513,Null}
>
> In[6]:= b == c
>
> Out[6]= True
>
> In[7]:= $Version
>
> Out[7]= 9.0 for Mac OS X x86 (64-bit) (January 24, 2013)
Timing appears to be release-dependent.
In[1]:= n = 10^6;
val = Table[Random[Real],{n}];
tag = Table[Random[Integer],{n}];
In[4]:= Timing[a = Pick[val,tag,1];]
Out[4]= {0.65 Second,Null}
In[5]:= Timing[b = val[[ SparseArray[tag] /.
SparseArray[_,_,_,d_] :> Flatten@d[[2,2]] ]];]
Out[5]= {0.1 Second,Null}
In[6]:= a == b
Out[6]= True
In[7]:= $Version
Out[7]= 5.2 for Mac OS X (June 20, 2005)