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could anyone help me how to update the array values with respect to the following arrays

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jaah navi
jaah navi 2019-10-14
关闭： MATLAB Answer Bot 2021-8-20
A= [1.0119 1.6739;
2.8012 1.6381;
0.0752 0.1222;
1.3143 0.8189;
0.7956 0.8750;
1.8512 1.1499]
B =[0.9512 0.4314;
0.2490 0.8309;
0.3864 0.8246]
C= [ 3;
1;
2;
1;
2;
1]
I am having the above three matrices,I want to update B by considering the average values of A with respect to C.
For example,
In C i have 2nd,4th and 6th rows to be 1 which means i need to add the mentioned rows in A and find the average of it and it needs to be updated in first row of B.
in the same way,i need to add 3rd and 5th rows in A and it needs to get updated in second rows of B.
On doing this the updated B should be [1.9889 1.2023;
0.4354 0.4986;
0.3864 0.8246]
could anyone please help me on this.

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回答（2 个）

Alaster Meehan
Alaster Meehan 2019-10-14
B(1,:) = mean(A(C == 1,:));
B(2,:) = mean(A(C == 2,:));
B(3,:) = mean(A(C == 3,:));
Or you can put it in a for loop.
for ii = 1:3
B(ii,:) = mean(A(C == ii,:));
end
Cheers Alaster
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Alaster Meehan
Alaster Meehan 2019-10-14
Sorry should have specified dimension for mean().
Also add in the max value for C, to generalise the loop.
If thre is an index not present in C then this will result in a row of NaNs.
for ii = 1:max(C)
B(ii,:) = mean(A(C == ii,:),1)
end
Andrei Bobrov
Andrei Bobrov 2019-10-14
编辑：Andrei Bobrov 2019-10-15
a = (1:max(C))';
b = accumarray(C,1);
i = a(accumarray(C,1) > 1);
[lo,ii] = ismember(C,i);
AA = A(lo,:);
[iii,j] = ndgrid(ii(lo),1:size(B,2));
B(i,:) = accumarray([iii(:),j(:)],AA(:),[],@mean);
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jaah navi
jaah navi 2019-10-15
I tried your code with respect to the following matrices
A=[1.0119 1.6739
2.8012 1.6381
0.0752 0.1222
1.3143 0.8189
0.7956 0.8750
1.8512 1.1499];
B=[0.3788 0.6150;
0.7567 0.4021;
0.5070 0.1980]
C =[1;
2;
3;
2;
2;
2]
a = unique(C)
b=accumarray(C,1)
i = a(accumarray(C,1) > 1)
lo = ismember(C,i)
AA = A(lo,:)
[ii,j] = ndgrid(C(lo),1:size(B,2))
B(i,:) = accumarray([ii(:),j(:)],AA(:),[],@mean)
When i run the code i am getting error stating Subscripted assignment dimension mismatch.
Error in line
B(i,:) = accumarray([ii(:),j(:)],AA(:),[],@mean)
Could you please help me on this.

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