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Re-arranging / selecting of a data table very slow in For loop [duplicate]

3 answers

This seems like the sort of thing where Mathematica can give a very simple solution.

I have a list of points {x,y}:

{{1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4}, {3, 5}}

From this list, I want to generate a new list of pairs {x,ymean}, where ymean is the average of all y’s in the original list which were paired to the same value of x:

{{1, 3}, {2, 7/2}, {3, 5}}

What’s the simplest, most elegant way to accomplish this in Mathematica?

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2 Answers

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Assuming you have your 2D list in a variable named lis, then the following should work:

Mean /@ GatherBy[lis, First]

+1 I knew my solution was too complicated…

– becko

May 15 ’14 at 18:32

Replace[GatherBy[data, First], d_List :> {First@First@d, Mean[d[[All, 2]]]}, {1}]

where data is the original list of points.