# Vectorization: incompatible sizes of arrays

2 views (last 30 days)
Milan Lstiburek on 18 Sep 2022
Commented: Torsten on 19 Sep 2022
O is a matrix with n_p x n_o rows; P is a matrix with n_p rows. Both matrices have three columns.The second column in O should be filled with the average of two values in the second column of P: the first value at the row ip and the second value at randomly selected row.
counter = 0;
for ip = 1:n_p
for io = 1:n_o
counter = counter + 1;
O(counter,2) = (P(ip,2) + P(randi(n_p),2))/2;
end
end
I attempted to vectorize the code as:
ip = 1:n_p;
io = 1:n_o;
O(ip(:)+io(:),2) = (P(ip(:),2)+P(randi(n_p),2))/2;
I am receiving the following message: „Arrays have incompatible sizes for this operation“. Thank you for any suggestions.

Torsten on 18 Sep 2022
Edited: Torsten on 18 Sep 2022
n_p = 3;
n_o = 3;
P = rand(n_p,2);
R = randi(n_p,n_p*n_o,1)
R = 9×1
3 2 2 2 3 1 3 2 1
IP = (cell2mat(arrayfun(@(i)vertcat(repmat(i,1,n_o)),1:n_p,'UniformOutput',0))).'
IP = 9×1
1 1 1 2 2 2 3 3 3
O(:,2) = (P(IP,2)+P(R,2))/2
O = 9×2
0 0.8950 0 0.5663 0 0.5663 0 0.2625 0 0.5912 0 0.5663 0 0.9199 0 0.5912 0 0.8950
Torsten on 19 Sep 2022
Or:
IP = reshape(repmat(1:n_p,n_o,1),n_p*n_o,1)
But yours is shorter :-)

### More Answers (2)

Bruno Luong on 19 Sep 2022
Edited: Bruno Luong on 19 Sep 2022
Simpler one liner
O(:,2) = P(randi(end,n_o,end),2) + repelem(P(:,2),n_o) / 2;

Bruno Luong on 18 Sep 2022
Edited: Bruno Luong on 18 Sep 2022
O(:,2) = reshape(reshape(P(randi(end,n_o,end),2),n_o,[]) + reshape(P(:,2),1,[]), [],1) / 2;
##### 2 CommentsShowHide 1 older comment
Bruno Luong on 19 Sep 2022
Good.
It is not easy to check the correctness due to randomness.

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