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Re: Combining data from indexed lists efficiently

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106228] Re: Combining data from indexed lists efficiently
  • From: Bill Rowe <readnews at sbcglobal.net>
  • Date: Tue, 5 Jan 2010 01:48:52 -0500 (EST)

On 1/4/10 at 5:59 AM, steve at take5.org (Steve W. Brewer) wrote:

>I have several lists of the format:

>{ {index1, value}, {index2, value}, ... {indexN, value} }

>For example:

>list1 = { {"A", 1}, {"B",  2}, {"C",  3}, {"D", 4} }
>list2 = { {"A", 5}, {"B",  6}, {"D",  7}, {"E", 8} }
>list3 = { {"A", 9}, {"B", 10}, {"C", 11} }

>The indexes are not necessarily strings; they may be any expression.
>(In the specific case I'm addressing now, each index is a list
>representing a date/time in the format returned by DateList[].)  The
>lists are not necessarily the same length.  Also, while most of the
>indexes appear in all lists, there are some holes (missing data).

>I want to combine the lists into a single list of the format:

>{ { index1, {value1, value2, ... valueN} },
>{ index2, {value1, value2, ... valueN} },
>...
>{ indexN, {value1, value2, ... valueN} } }

>Only the data points with indexes appearing in all lists should be
>included; the rest should be dropped.  Also, I want to include some
>derived values along with the original data values.

>Using the sample data above, let's say I want to include two derived
>values from the functions:
>
>f1[list1Data_, list2Data_] := list1Data + list2Data f2[list2Data_,
>list3Data_] := list2Data + list3Data
>
>The result would be:

>combinedList = { { "A", {1, 5,  9, 6, 14} },
>{ "B", {2, 6, 10, 8, 16} } }

>I have a solution that works fine on "small" data sets. However,
>it's impractically slow on the "large" data sets I really need to
>run it on (over 100k elements in each list).

>Here's what I'm doing now:

<details snipped>

If you are using version 7, then what you want can be achieve
with a lot less code by doing:

In[7]:= list1 = {{"A", 1}, {"B", 2}, {"C", 3}, {"D", 4}};
list2 = {{"A", 5}, {"B", 6}, {"D", 7}, {"E", 8}};
list3 = {{"A", 9}, {"B", 10}, {"C", 11}};

In[10]:= {#[[1, 1]], #[[All, 2]]} & /@
  GatherBy[Join[list1, list2, list3], First]

Out[10]= {{"A", {1, 5, 9}}, {"B", {2, 6, 10}}, {"C", {3,
    11}}, {"D", {4, 7}}, {"E", {8}}}

I don't know how well this will scale to 100K + items per list.
My guess is this will perform better than what you indicated you
had tried.



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