how to select from identical rows based on their associated value in one column?

Question

Deepika Vatsa 2019-4-3

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454233-how-to-select-from-identical-rows-based-on-their-associated-value-in-one-column

评论： Star Strider 2019-4-4

I have a matrix of 2 columns and lots of rows. First column has numeric values that occurred twice and second column has probabilities associated with it.

for e.g., a = [1 , 0.6

2 , 0.9

1, 0.4

3 , 0.5

4 , 0.2

2 , 0.1

4 , 0.8

3 , 0.5]

I want choose rows which have elements with higher probability i.e.,

ans = [1 , 0.6

2 , 0.9

4 , 0.8]

I want ignore rows with same probability. How to do this in fastest manner possible?

2 个评论
显示无隐藏无

madhan ravi 2019-4-3

Which version of matlab?

Deepika Vatsa 2019-4-3

It's Matlab R2015a

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Star Strider 2019-4-3

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454233-how-to-select-from-identical-rows-based-on-their-associated-value-in-one-column#answer_368944

在 MATLAB Online 中打开

Another approach using accumarray:

a = [1 , 0.6 
     2 , 0.9
     1,  0.4
     3 , 0.5
     4 , 0.2
     2 , 0.1
     4 , 0.8
     3 , 0.5];
[Ua,~,ix] = unique(a(:,1));                                 % Unique Values & Indices
L = accumarray(ix, a(:,2), [], @(x)max(sum(x)~=2*x));       % Eliminate Duplicates (Returns Logical Vector)
R = accumarray(ix, a(:,2), [], @max);                       % Maximum Probabilities
Out = [Ua(L), R(L)]                                         % Report Only Maxima For Non-Duplicated Probabilities

producing:

Out =
            1          0.6
            2          0.9
            4          0.8

4 个评论
显示 2更早的评论隐藏 2更早的评论

Deepika Vatsa 2019-4-4

Hey! I have got it. Using intersect I got the indexes for rows with correct sign as well. Your solution is fastest. Many thanks.

Star Strider 2019-4-4

在 MATLAB Online 中打开

As always, my pleasure.

I had to think about that.

This works:

a = [1 , 0 , 0.6 
     1 , 1 , 0.4
     2 , 0 , 0.1
     2 , 1 , 0.9];
[Ua,~,ix] = unique(a(:,1));                                 % Unique Values
L = accumarray(ix, a(:,3), [], @(x)max(sum(x)~=2*x));       % Eliminate Duplicates (Returns Logical Vector)
R = accumarray(ix, a(:,3), [], @max);                       % Maximum Probabilities
S = accumarray(ix, a(:,3), [], @(x) a(find(x==max(x)),2));  % Get ‘sign’
Out = [Ua(L) S(L), R(L)]                                    % Report Only Maxima & Sign For Non-Duplicated Probabilities

producing:

Out =
            1            0          0.6
            2            1          0.9

请先登录，再进行评论。

Answer 2

Bob Thompson 2019-4-3

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/454233-how-to-select-from-identical-rows-based-on-their-associated-value-in-one-column#answer_368924

编辑：Bob Thompson 2019-4-3

There might be some fancy function I don't know, but it's certainly possible to do this with a loop. I cannot comment on the speed of this method.

a = [1 , 0.6 
     2 , 0.9 
     1, 0.4 
     3 , 0.5
     4 , 0.2
     2 , 0.1
     4 , 0.8
     3 , 0.5];
 
uni = unique(a(:,1));
b = [];
for i = 1:length(uni)
    b(i,:) = a(a(:,2) == max(a(a(:,1)==uni(i),:)),:);
end

EDIT* Changed from uniques in second column to uniques in first column. I misread the question.

4 个评论
显示 2更早的评论隐藏 2更早的评论

madhan ravi 2019-4-3

编辑：madhan ravi 2019-4-3

Bob,I am not able to run your code but according to what I understand from the question is that if the probabilities are identical OP wants to omit those rows and find the max of probabilities of each group (which belong to column 1). Haven’t tested with the timings though.

Deepika Vatsa 2019-4-4

Thank you Bob and Madhan for the solutions! I have got my solution by first eliminating same probability rows using 'unique' and then using 'varfun' as suggested by Madhan. Bob, my matrix has lots of rows(in millions), I don't think using a for loop would help me with computation time. So, I think using varfun is quicker. Thanks again.

请先登录，再进行评论。