Using Accumarray with @maxk instead of @max?

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Ayman Al-Sukhon 2019-12-16

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评论： Ayman Al-Sukhon 2019-12-17

采纳的回答： Walter Roberson

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Hi,

Say you have three vectors:

a = [1;2;3;4;5;1;3;1;4];
b = [100;200;300;400;500;400;300;200;100];
c = [123;456;221;111;800;1000;10;25;150];

And you use accumarray so that:

maxval = sparse(accumarray(a(:,1),max(b,c),[],@max))

You get:

maxval =
   (1,1)           1000
   (2,1)            456
   (3,1)            300
   (4,1)            400
   (5,1)            800

Now lets, say I have a whole lot of variables for each subs (something like 2000 each), and I want the average of the top 3 values using the same method. How can I accomplish this? For example, in the same problem, I want my output to be:

The top three values with subs 1 are: 100,400 and 200, so their average is 233.33 and the first row in my sparse matrix is:

maxval =
   (1,1)           1000

and so on.

Is it maybe possible to use maxk as a function handle?

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采纳的回答

Walter Roberson 2019-12-16

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accumarray(a(:,1), max(b,c), [], @(v) mean(maxk(v,3)), 0, true)   %final parameter is sparse flag

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dpb 2019-12-17

@() is the preamble to define an anonymous function. The v is the dummy argument variable name Walter chose; you'll see it reflected in the argument to maxk().

All the details about anonymous functions is in the documentation under the general subject of functions.

Ayman Al-Sukhon 2019-12-17

I see, thank you very much dpb!

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dpb 2019-12-16

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编辑：dpb 2019-12-16

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Don't see anyway around with accumarray because the VAL parameter must be 1:1 with rows of SUBS; use findgroups/splitapply or varfun (altho latter must be table or timetable).

g=findgroups(a);
mnk=splitapply(@(x) mean(maxk(x,3)),b,g);

yields

>> mnk =
3333
0000
0000
0000
0000
>> 

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dpb 2019-12-16

What it looked like to me, too, Walter.

Your's works for the specific instance (or anywhere sum is the needed intemediary); however in the followup here I was thinking of the more general cases where the function needs the elements rather than a single result.

Ayman Al-Sukhon 2019-12-17