I would like to sum a 2 dim array group by value in second column

there are a lot of ways to do this, here's a simple solution. i would recommend you to understand every line and run it line by line. try writing this code vectorize, i mean without for loop :)

X = [rand(6,1),randi(4,6,1)]
X = 6×2
    0.2612    2.0000
    0.6470    3.0000
    0.4749    4.0000
    0.6048    3.0000
    0.6482    1.0000
    0.1296    2.0000
Y = unique(X(:,2));
Y(end,2)=0;
for i=1:size(X,1)
    ind = X(i,2)==Y(:,1);
    Y(ind,2) = Y(ind,2) + X(i,1);
end
Y = flip(Y,2)
Y = 4×2
    0.6482    1.0000
    0.3909    2.0000
    1.2518    3.0000
    0.4749    4.0000

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simon lumsdon 2022-6-18

Many thanks, it works perfectly. Wow, had rather hoped there was a single line solution to this type of problem, some form of sum, group by approch, hadnt expected to have to tackle it long hand. Not sure I have the skills to turn that into one line of code though?

Abolfazl Chaman Motlagh 2022-6-18

在 MATLAB Online 中打开

Here's a one line solution :)

X = [rand(6,1),randi(4,6,1)]
X = 6×2
    0.8407    1.0000
    0.1779    4.0000
    0.2698    1.0000
    0.7353    4.0000
    0.8105    1.0000
    0.1059    3.0000
Y = [sum((X(:,2) == unique(X(:,2))') .* repmat(X(:,1),1,numel(unique(X(:,2)))))' , unique(X(:,2))]
Y = 3×2
    1.9210    1.0000
    0.1059    3.0000
    0.9132    4.0000