Re: Counting list elements above/below a given value
- To: mathgroup at smc.vnet.net
- Subject: [mg17281] Re: [mg17202] Counting list elements above/below a given value
- From: "Andrzej Kozlowski" <andrzej at tuins.ac.jp>
- Date: Fri, 30 Apr 1999 23:22:20 -0400
- Sender: owner-wri-mathgroup at wolfram.com
There are two problems here. First, your usage of patterns in Count is
wrong. Here is how it works. Lets create a pretty big matrix:
In[1]:=
m=Array[Random[Integer,{1,100}]&,{200,200}];
Now
In[2]:=
Count[m,x_/;x>20,Infinity]//Timing
Out[2]=
{1.08333 Second, 32059}
That seems to be fast enough for me (on PowerBook G3, 233 mghz).
The second problem is that your original method is actually slightly faster:
In[3]:=
Length[Select[Flatten[m], 20< #1&]]//Timing
Out[3]=
{0.983333 Second, 32059}
So now my question: what do you consider slow in this case?
--
Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp
http://eri2.tuins.ac.jp
----------
>From: "W.W. Sampson" <w.sampson at umist.ac.uk>
To: mathgroup at smc.vnet.net
>To: mathgroup at smc.vnet.net
>Subject: [mg17281] [mg17202] Counting list elements above/below a given value
>Date: Mon, Apr 26, 1999, 2:20 PM
>
> I wish to count the number of elements within a list above
> a given value. I can do this using the following:
>
> Length[Select[Flatten[listname], value > #1&]]
>
> However, my list is a 280 x 280 matrix and consequently
> this takes a while to evaluate. My guess is that there
> must be something more efficient. I've tried:
>
> Count[listname, value > #1&]
>
> but the output given is 0, which I know to be incorrect.
>
> Any ideas appreciated.
>
> Bill