finding the mean and max

Question

johnson saldanha 2018-11-21

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编辑： Andrei Bobrov 2018-11-21

suppose i have a matrix where the first column is x(:,1)=[ 1 1 1 2 2 1 1 3 1 ] and second column is x(:,2)=[ 2 3 2 4 6 9 7 8 9];

i want the output as y(:,1)=[ 1 2 3 ], y(:,2)=[ 9 6 8] and y(:,3)=[5.33 5 8];

the first column should be the values from first column of x reduced to a single value and in ascending order, the second column should be the highest number from the second column of x for a particular value in first input column. for ex. for all the 1's in first column of x should return one max value from the second column of x. the third column should contain the means of the second column of x for a particular value in first column of x. same example as given.

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Answer 1

Rik 2018-11-21

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编辑：Rik 2018-11-21

在 MATLAB Online 中打开

The first column you can achieve with unique, the second and third can be done with accumarray.

x=[ 1 1 1 2 2 1 1 3 1 ; 2 3 2 4 6 9 7 8 9]';
[col1,~,ind]=unique(x(:,1));
col2=accumarray(ind,x(:,2),[],@max);
col3=accumarray(ind,x(:,2),[],@mean);
y=[col1 col2 col3]

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Walter Roberson 2018-11-21

the third output of unique gives you information about the original order .

Rik 2018-11-21

在 MATLAB Online 中打开

This code produces the output like you describe. As Walter mentions (and as you can read in the doc of unique) the third output of the unique function returns the order so isequal(col1(ind),x(:,1)) returns true (barring any float rounding errors).

This means that this input will also work as intended:

x=[5 1 1 1 2 2 1 1 3 1 5;9 2 3 2 4 6 9 7 8 9 1]';

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Answer 2

Andrei Bobrov 2018-11-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/431127-finding-the-mean-and-max#answer_348155

编辑：Andrei Bobrov 2018-11-21

EDITED

>> XXX =[ 1 1 1 2 2 1 1 3 1 ; 2 3 2 4 6 9 7 8 9]'
XXX =
     1     2
     1     3
     1     2
     2     4
     2     6
     1     9
     1     7
     3     8
     1     9
>> T = varfun(@(x)[max(x),mean(x)],array2table(XXX),'G',1);
out = T{:,[1,3]}
out =
            1            9       5.3333
            2            6            5
            3            8            8
>> 