Intersection point of yline

Hi,

I have tried some diffrent metods, but so far without any luck. Is there a way to show the x,y coordinates of the intersection point of yline and the f1?

x=[-150:5:0, 0:5:100]
hold on
z=50
yline((z),'--r');

f1=[5E-08 -3E-05 -0.0058 1.0479 177.56]
y1=polyval(f1,x)
plot(x,y1)
grid on 

[Star Strider]

Straightforward:

lv = find(y1 <= z, 1, 'last');
isx = interp1(y1(lv+[-1 1]),x(lv+[-1 1]),z)

producing:

isx =
   -99.9323

Q.E.D.!

[madhan ravi]

Right ;)

[Star Strider]

Of course!

[Star Strider]

Kenneth Bisgaard Cristensen —

With respect to my code, the (x,y) coordinates of the intercept are (isx,z).

[Kenneth Bisgaard Cristensen]

Yeah, got that. Thanks for the help :)

[Star Strider]

My pleasure!

Answer 1

[madhan ravi]

x=[-150:5:0, 5:100];
INTERSECTION = interp1(y1,x,z)

[Kenneth Bisgaard Cristensen]

Thank you so much, I have tried to apply it to this. But I get an error I don't undestand. It says:

Error using griddedInterpolant

The number of input coordinate arrays must match the dimensions of the sample values.

Error in interp2>makegriddedinterp (line 228)

F = griddedInterpolant(varargin{:});

Error in interp2 (line 112)

F = makegriddedinterp({X,Y},V,method,extrap);

Error in Impact_Temp (line 23)

p2=interp2(y2,x,z)

x=[-20:5:0, 5:80]

hold on

z=50
yline((z),'--r');

f1=[-9E-05 0.0016 1.2251 23.754]
f2=[-1E-06 5E-06 0.0141 0.1642 -3.5317]

y1=polyval(f1,x)
y2=polyval(f2,x)


y1(y1<0)=0;
y2(y2<0)=0;

plot(x,y1,'g')
plot(x,y2,'g')

p1=interp1(y1,x,z)
    plot(p1,z,'bx')
p2=interp2(y2,x,z)
    plot(p2,z,'bx')

grid on 
hold off

[madhan ravi]

 p2 = interp1(y2,x,z)

[Kenneth Bisgaard Cristensen]

It still gives me this error:

Error using matlab.internal.math.interp1

Sample points must be unique and sorted in ascending order.

Error in interp1 (line 154)

VqLite = matlab.internal.math.interp1(X,V,method,method,Xqcol);

Error in Impact_Temp (line 23)

p2 = interp1(y2,x,z)

Answer 2

[neil jerome]

the code below shows how to 'brute force' a numerical answer, down to a precision you can adjust, by iteratively 'zooming in' on the region. for an analytical answer, matlab probably isn't the right tool :)

good luck!

n.

%% converge by using smaller increments

thresh = 0.000000000001; % choose precision of stopping criterion for 'y1 = z'

increment = 5;
x = -150:increment:100;
f1 = [5E-08 -3E-05 -0.0058 1.0479 177.56]; 
z = 50;
    
iteration = 1;
residual = 1;

while residual > thresh
    y1 = polyval(f1,x);
    % find closest point in current x vector
    signal = y1-z;
    smallestDiff = min(abs(signal));
    closestPoint = find(abs(signal) == smallestDiff);
    closestCoord = [x(closestPoint) y1(closestPoint)];
    residual = smallestDiff;
    
    % plot to show progressive zooming in
    figure; hold on;
    yline(z, 'r--');
    plot(x, y1);
    plot(x(closestPoint), y1(closestPoint), 'ro');
    grid on;
    pause; % press any key to continue
    close;
    
    % show progress
    disp([ num2str(iteration) ' ' num2str(closestCoord(1)) ', ' num2str(closestCoord(2)) ', residual: ' num2str(residual)]);
    
    % iterate
    increment = increment/10;
    x = x(closestPoint - 10):increment:x(closestPoint+10);
    iteration = iteration + 1;
    
end
format long;
disp('converged at:');
closestCoord