How can i optimise weights for BPN using genetic algorithm?

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I am working on prediction of rainfall using BPN. I have read that in order to get better convergence, optimisation method like Genetic Algorithm can be applied. How do we use the tool box especially when the weights are to be optimised?

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Kelvin 2013-3-21

编辑：Kelvin 2013-3-21

在 MATLAB Online 中打开

m=5; %number of nodes in input layer

n=10; %number of nodes in hidden layer

o=1; %number of nodes in output layer

xy=load('2010-2011-2012.txt'); %loading data

mx=max(xy) %normalising the data

x=xy/mx;

a=0;

b=1;

w1=a+(b-a)*rand(m,n) %weight from input to hidden layer

w2=a+(b-a)*rand(n,o) %weight from hidden to output layer

w1ini=w1;

w2ini=w2;

theta1=a+(b-a)*rand(1,n) %bias

theta2=a+(b-a)*rand(1,o)

theta1ini=theta1;

theta2ini=theta2;

deltheta1=a+(b-a)*rand(1,n);

deltheta2=a+(b-a)*rand(1,o);

yh=zeros(1,n);

yo=zeros(1,o);

deltah=zeros(1,o);

deltao=zeros(1,n);

delw1=a+(b-a)*rand(m,n);

delw2=a+(b-a)*rand(n,o);

alpha=0.7; %learning rate

etta=0.95; %momentum factor

z=0;

t=[];

er=[];

while (z<1500)

    c=0;
    while(c<1091)
        for j=1:n
            sum=0;
            for i=1:m
                sum=sum+x(i+c)*w1(i,j);
            end
            yh(j)=logsig(sum+theta1(j));    %output at hidden layer
        end
        for k=1:o
            sum=0;
            for j=1:n
                sum=sum+yh(j)*w2(j,k);
            end
            yo(k)=logsig(sum+theta2(k));  %output at output layer
        end
    err=x(c+m+1)-yo(o);
    t(c+1)=err;
    oldw1=w1;
    oldw2=w2;
    %using gradient descent 
    deltao(o)=err*(1-yo(o))*yo(o);
    for k=1:o
        for j=1:n
            delw2(j,k)=alpha*yh(j)*deltao(k)+etta*(w2(j,k)-oldw2(j,k));
            deltheta2=alpha*1*deltao(o);
        end
    end
    for j=1:n
        sum=0;
        for k=1:o
            sum=sum+deltao(k)*w2(j,k);
        end   
        deltah(j)=yh(j)*(1-yh(j))*sum;
    end
    for j=1:n
        for i=1:m
            delw1(i,j)=alpha*deltah(j)*x(i+c)+etta*(w1(i,j)-oldw1(i,j));
            deltheta1=alpha*1*deltah(j);
        end
    end
    w1new=delw1+w1;
    w2new=delw2+w2;
    theta1new=deltheta1+theta1;
    theta2new=deltheta2+theta2;
    w1=w1new;
    w2=w2new;
    theta1=theta1new;
    theta2=theta2new;
    c=c+1;
    end
    sum=0;
    for i=1:1091
        sum=sum+t(i)*t(i);
    end
    mse=sum/1091
    er(z+1)=mse;
    z=z+1;
end

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Answer 1

Greg Heath 2013-3-21

在 MATLAB Online 中打开

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The value MSE = 0.0054 means absolutely nothing to me because I don't know the scale of the target data.

On the other hand, the normalized values NMSEtrn, NMSEval and NMSEtst instantly tell me whether or not I think the design is acceptable.

ytrn00 = mean(ttrn,2) is the output of the NAIVE CONSTANT OUTPUT MODEL. Using that output for an arbitrary input from the same probability distribution yields the reference values

 MSEtrn00 = mse(ttrn-ytrn00) = mean(var(ttrn',1))
 MSEval00 = mse(tval-ytrn00)
 MSEtst00 = mse(ttst-ytrn00)

the normalized values and corresponding Rsquared (R^2, coefficient of determination ... See Wikipedia) values are

 NMSEtrn = MSEtrn/MSEtrn00
 NMSEval = MSEval/MSEval00
 NMSEtst = MSEtst/MSEtst00

and

 R2trn = 1 - NMSEtrn
 R2val = 1 - NMSEval
 R2tst = 1 - NMSEtst

R^2 can be interpreted as the fraction of target variance that is modeled (AKA "explained") by the net.

Exactly what gradient-descent disadvantages do you proclaim to have overcome? Did you obtain any gd designs for comparison (especially training times)?

Hope this helps

Greg

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Kelvin 2013-3-22

The value ranges from 0 to 143 and I have normalised it between 0 and 1. While training I get an MSE of 0.0054 and while testing 0.0153.

Greg Heath 2013-3-23

The value of what? Normalized it how?

What are R2trn, R2val and R2tst?

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Answer 2

Greg Heath 2013-3-21

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I have read that ... Where? ... The internet?

Please clarify. Is your prediction problem static (simultaneous input and output)or dynamic ( input and/or feedback delays are involved).

The genetic algorithm is most useful in determining optimum nonstandard network node topology. As examples, nonuniform (sporadic?) input, feedback, skiplayer and output connections. However, it is too slow for estimating weights given a fixed topology like those represented by the various MATLAB functions.

What MATLAB design functions have you tried for comparison ?

Greg

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Kelvin 2013-3-21

http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=6160092&tag=1 In the paper attached its said that genetic algorithm and simulated annealing can be used to overcome the disadvantages of BPN. The prediction that I am using is static. I was able to train the network with the past data and also test it obtaining an mse of 0.0054. I want to bring down the error and also increase the convergence rate.

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