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Re: More Efficient Method

• To: mathgroup at smc.vnet.net
• Subject: [mg105126] Re: [mg105076] More Efficient Method
• From: "David Park" <djmpark at comcast.net>
• Date: Sat, 21 Nov 2009 03:37:22 -0500 (EST)
• References: <15484002.1258719328902.JavaMail.root@n11>

One possibility is:

datalist = Range[11, 3411, 10];

If[40 <= # <= 1500 \[Or] 1600 <= # <= 3300, 
   Unevaluated[Sequence[]], #] & /@ datalist

Sequence[] is dropped from a sequence of items. It has to be Unevaluated in
the If statement so it's not dropped there!


David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/  


From: blamm64 [mailto:blamm64 at charter.net] 

I have a couple of functions designed to poke a single hole, and to
poke multiple holes, in a one-level list:

We define a function which, given the imported pressure data, finds
the subset of that pressure data excluding the pressure data points
between "targetL " and "targetU".

In[5]:= findsubset[data_?VectorQ,targetL_?NumericQ,targetU_?
NumericQ] := Select[data,(#<=targetL || #>=targetU &)]

This function will pluck out multiple holes in the data list.

In[6]:= subsets[data_?VectorQ,tarList_?ListQ]:=Module[{tmp,tmp1},
tmp=data;
Do[tmp1=findsubset[tmp,tarList[[i,1]],tarList[[i,2]]];tmp=tmp1,
{i,Dimensions[tarList][[1]]}];
tmp
]

The following works fine (big holes chosen not to give large result):

In[7]:= datalist=Range[11,3411,10];

In[12]:= targetlist={{40, 1500},{1600,3300}};

In[13]:= resultdata=subsets[datalist,targetlist]

Out[13]=
{11,21,31,1501,1511,1521,1531,1541,1551,1561,1571,1581,1591,3301,3311,3321,3
331,3341,3351,3361,3371,3381,3391,3401,3411}

But if "datalist" happens to be very large, surely there is a (much)
more efficient method?

I tried unsuccessfully to use pure functions with Select, but have a
somewhat nebulous feeling there's a pure function way of doing this
effectively much more efficiently.

I know, I know: the above have no consistency checking.  I also know
"subsets" could be used in place of "findsubset" just by replacing the
call of "findsubset" with the code of "findsubset" in "subsets".

>From what I've seen on this forum there are some really experienced
people who might provide an efficient way of implementing the above.

-Brian L.