How to calculate the predicted ellipse area and draw the graph?

I use this code but there is an error in execution

chisquare = chi2inv(0.95,2); %inverse of the chi-square cumulative distribution function with 2 degrees of freedom at P = 0.95 
[vec,val] = eig(cov(x,y)); %calculation of eigenvalues 
pea = pi*chisquare*prod(sqrt(svd(val))) %area calculation
% End of script part of m-file

%====================================================================================
% Now define a function.
function a = ComputePEA(x,y,P)
%
%a = PEA(x,y,P): plots a prediction ellipse of the center of 
%pressure (COP) data separated into x and y components with probability 
%value P and calculates the prediction ellipse area (PEA)
%
%inputs: 
% x,y: data (column vectors)
% P  : a value of the interval (0,1)
%
%output:
% a  : represents the area of the ellipse
%
%type PEA without inputs to plot 95% prediction ellipse of exemplary data

%%
%exemplary data illustration
if nargin==0 %case of no input arguments 
    x = [2,4,6,2,3,4,2,3,3,4,5,5,0,8,3,7]; %exemplary x component
    y = [1,2,3,4,3,3,1,1,2,1,4,5,0,0,1,1]; %exemplary y component
    a = ComputePEA(x,y,0.95);
    axis([-4 11 -3 7])
    title('95% PEA of exemplary data')
    text(-3,6,'blue points = data')
    text(5.5,2,'major axis')
    text(4.3,0,'minor axis')
    text(7,-2,['PEA: ' num2str(a)])
    return %end of function
end 
%%

%begin of function

chisquare = chi2inv(P,2); %inverse of the chi-square cumulative distribution function with 2 degrees of freedom at P
x = x(isfinite(x));
y = y(isfinite(y));
mx = mean(x);
my = mean(y);
[vec,val] = eig(cov(x,y)); %calculation of eigenvalues
a = pi*chisquare*prod(sqrt(svd(val))); %area calculation

hold on

%COP data
plot(x,y,'b.'); 

%ellipse
N = 100; %fixed value (the higher the smoother the ellipse)
t = linspace(0,2*pi,N);
elip = sqrt(chisquare)*vec*sqrt(val)*[cos(t); sin(t)] + repmat([mx; my],1,N);
elip = elip';
line(elip(:,1),elip(:,2),'Color', [0 0 0], 'LineWidth', 1);

%major and minor axes
ax1 = sqrt(chisquare)*vec*sqrt(val)*[-1,1; 0,0] + repmat([mx; my],1,2);
ax2 = sqrt(chisquare)*vec*sqrt(val)*[0,0; -1,1] + repmat([mx; my],1,2);
ax_dat = [ax1'; NaN,NaN; ax2'];
line(ax_dat(:,1),ax_dat(:,2),'Color',[0 0 0],'LineWidth', 1);

axis equal

end % end of function

and then

Unrecognized function or variable 'x'.

Error in pea_test (line 2)
[vec,val] = eig(cov(x,y)); %calculation of eigenvalues

[Matt J]

Neither x or y is defined in the code you've posted, so the error message seems to make a lot of sense.

[Dyuman Joshi]

The error is quite clear. You have not defined "x" (nor "y" as well).

You are asking a function to calculate the output but you have not defined the input.

Answer 1

[VBBV]

chisquare = chi2inv(0.95,2); %inverse of the chi-square cumulative distribution function with 2 degrees of freedom at P = 0.95 
x = [2,4,6,2,3,4,2,3,3,4,5,5,0,8,3,7]; %exemplary x component
y = [1,2,3,4,3,3,1,1,2,1,4,5,0,0,1,1]; %exemplary y component
[vec,val] = eig(cov(x,y)); %calculation of eigenvalues 
pea = pi*chisquare*prod(sqrt(svd(val))) %area calculation

pea = 57.4753

% End of script part of m-file

%====================================================================================
% Now define a function.
function a = ComputePEA(x,y,P)
%
%a = PEA(x,y,P): plots a prediction ellipse of the center of 
%pressure (COP) data separated into x and y components with probability 
%value P and calculates the prediction ellipse area (PEA)
%
%inputs: 
% x,y: data (column vectors)
% P  : a value of the interval (0,1)
%
%output:
% a  : represents the area of the ellipse
%
%type PEA without inputs to plot 95% prediction ellipse of exemplary data

%%
%exemplary data illustration
if nargin==0 %case of no input arguments 
    x = [2,4,6,2,3,4,2,3,3,4,5,5,0,8,3,7]; %exemplary x component
    y = [1,2,3,4,3,3,1,1,2,1,4,5,0,0,1,1]; %exemplary y component
    a = ComputePEA(x,y,0.95);
    axis([-4 11 -3 7])
    title('95% PEA of exemplary data')
    text(-3,6,'blue points = data')
    text(5.5,2,'major axis')
    text(4.3,0,'minor axis')
    text(7,-2,['PEA: ' num2str(a)])
    return %end of function
end 
%%

%begin of function

chisquare = chi2inv(P,2); %inverse of the chi-square cumulative distribution function with 2 degrees of freedom at P
x = x(isfinite(x));
y = y(isfinite(y));
mx = mean(x);
my = mean(y);
[vec,val] = eig(cov(x,y)); %calculation of eigenvalues
a = pi*chisquare*prod(sqrt(svd(val))); %area calculation

hold on

%COP data
plot(x,y,'b.'); 

%ellipse
N = 100; %fixed value (the higher the smoother the ellipse)
t = linspace(0,2*pi,N);
elip = sqrt(chisquare)*vec*sqrt(val)*[cos(t); sin(t)] + repmat([mx; my],1,N);
elip = elip';
line(elip(:,1),elip(:,2),'Color', [0 0 0], 'LineWidth', 1);

%major and minor axes
ax1 = sqrt(chisquare)*vec*sqrt(val)*[-1,1; 0,0] + repmat([mx; my],1,2);
ax2 = sqrt(chisquare)*vec*sqrt(val)*[0,0; -1,1] + repmat([mx; my],1,2);
ax_dat = [ax1'; NaN,NaN; ax2'];
line(ax_dat(:,1),ax_dat(:,2),'Color',[0 0 0],'LineWidth', 1);

axis equal

end % end of function

[VBBV]

As the error states, Unrecognized function or variable 'x'.

define the example values for x, y before using them in any function

[Tsu Ming Chang]

thanks! it's working but how do I output the predicted ellipse area graph?