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how to compute separate line 2 dimensional points

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Michael sch
Michael sch 2014-6-17
回答： Michael sch 2014-6-17
hi all i have set of points and class + -1 i want to calculate the line i want to use 1. direct linear algebra (inverse matrix , cross product) 2. use gradient descent i dont want to use the svm function or quadratic programming
x=[ 1 2 ; 3 3 ; 4 5; 45 17 ; 33 20];
label=[ 1 ;1 ;1;-1 ;-1];
gscatter(x(:,1),x(:,2),label)
so how to calculate the line wx+b , i need w,b

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回答（6 个）

David Sanchez
David Sanchez 2014-6-17
Once you have the plot, go to tools -> Basic fitting
select the data (1 or -1 in your case), linear and click in the bottom arrow to get the values of your parameters. You will see you can save to workspace.
Michael sch
Michael sch 2014-6-17
but it fits to one of the set of points and not the seperate line !! , also i dont have the process for compute the line
Zikobrelli
Zikobrelli 2014-6-17
Given X and Y, this function will give you your 'w' and 'b'
function [a b] = droite_reg(X,Y)
% Linear fitting
% Y = a + bX
n = length (Y);
sumY = sum(Y);
sumX = sum(X);
X2 = X.^2;sumX2 = sum(X2);
Y2 = Y.^2; sumY2 = sum(Y2);
XY = X.*Y; sumXY = sum(XY);
%slope estimation
a = (sumXY - ((sumX * sumY)/n))/ (sumX2 - ((sumX^2)/n));
% b estimation
b = (sumY/n)-(a.*(sumX/n));
% r^2
r2 = ((sumXY - ((sumX.*sumY)/n)).^2)/((sumX2-((sumX.^2)/n)).*(sumY2 - ((sumY.^2)/n)));
end
  1 个评论
Zikobrelli
Zikobrelli 2014-6-17
To get your two sets of data differentiated, you can try something like:
plot(x(label==1,1),x(label==1,2))
[w,b]=droite_reg(x(label==1,1),x(label==1,2))
hold on
plot(x(label==-1,1),x(label==-1,2),'r')
[w1,b1]=droite_reg(x(label==-1,1),x(label==-1,2))

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Michael sch
Michael sch 2014-6-17
it seems you go in the correct answer but stil the line is not between alsow what is theory (article )
x=[ 1 2 ; 3 3 ; 4 5; 45 17 ; 33 20];
label=[ 1 ;1 ;1;-1 ;-1];
gscatter(x(:,1),x(:,2),label)
%w = label'*x;
[a b] = droite_reg(x(:,1),x(:,2))
xvec=[-1:0.5:30];
yvec = b*xvec+a;
hold on;plot(xvec,yvec,'k-.');
xvec=[-1:0.5:30];
yvec = a*xvec+b;
hold on;plot(xvec,yvec,'k-.');
Michael sch
Michael sch 2014-6-17
i need to seperate plane like in svm and not like in the figure
http://en.wikipedia.org/wiki/Support_vector_machine
Michael sch
Michael sch 2014-6-17
as you see i need the black line and not the fit line

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