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Error using +. Matrix dimensions must agree

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Oman Wisni
Oman Wisni on 11 Nov 2018
Commented: Oman Wisni on 12 Nov 2018
Hi, I'm trying to create neural network for train my data. But I get error. I though its right but which one should I fix. Here my code
close all;
% Multilayer Perceptron (MLP) neural networks.
N = 220; %sum of data
D = 25 ; %dimension
K = 11; %target
h = 500; %sum of neuron
load datatrain1.mat
Train1= cell2mat(Train);
load classtrain1.mat
CTrain1 = cell2mat(TTrain);
b1 = randn(1,h);%bias from input to hidden
W1 = randn(D,h);%weight from input to hidden
b2 = randn(1,K);%bias from hidden to output
W2 = randn(h,K);%weight from hidden to output
for efoch = 1 : 1000
H = Train1*W1+b1;
Y = H*W2+b2;
I get error in variable H, like this Error using +. Matrix dimensions must agree.
Any help for fix my code will must appreciated. Thanks


Oman Wisni
Oman Wisni on 11 Nov 2018
yes sir, I already attached my file .mat. But I'm not sure it can open in matlab, cause before I ever upload and download, when I download the format of file is different. And when I upload I no get reply.

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Accepted Answer

Guillaume on 11 Nov 2018
Edited: Guillaume on 11 Nov 2018
The error message is very clear, you're trying to add two matrices of different size, which is never possible. Since b1 is a 1xh vector, Train1*W1 must be a 1xh vector, and since W1 is a Dxh matrix, Train1 must be a 1xD vector for the product to be 1xh.
Therefore, your train1 is not 1xD. There is nothing we can do about that. Either use a train1 matrix that is 1xD or change your D to reflect the size of train1.
Note that it is never a good idea to hardcode the size of matrices. It is always safer to ask matlab for the actually size:
D = size(train1, 2);
would ensure it always match train1. Of course, train1 must only have one row.
edit: got confused between D and h but see comment below.


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Oman Wisni
Oman Wisni on 12 Nov 2018
Sir, what code is right ?
H = bsxfun(@sum, train1*W1, b1);
H = bsxfun(@plus, Train1*W1, b1);
using @sum I get error, but when I using @plus I get result. My question is, if I using @plus there is nothing change in the result? thanks
Guillaume on 12 Nov 2018
Sorry, it should have been @plus indeed.

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