how to determine coordinate from graph

i plotted a graph.

>> x = 0:1:20; 
>> y = (668.061./x).*[1-exp(-0.1468.*x)]-40;
>> plot(x,y) 

And i wanna determine the coordinate of x when y=0. (i want it be labelled on the graph). wht code should i put in?

Answer 1

[Star Strider]

Try this:

x = 0:1:20; 
y = (668.061./x).*[1-exp(-0.1468.*x)]-40;

Try this:

x = 0:1:20; 
y = (668.061./x).*[1-exp(-0.1468.*x)]-40;

Lv = ~isnan(y);                                     % Avoid ‘NaN’ Values
yq = 0;
xq = interp1(y(Lv), x(Lv), yq);                     % Interpolate To Find 'x' Value 'xq' Corresponding to 'yq'

figure
plot(x,y)
hold on
plot(xq, 0, 'r+')
hold off

It works here because ‘y’ is monotonic (not oscillating). Other approaches would be necessary otherwise.

[Hao Ming Low]

hi, what if i want to find value of y when x is given? how may i change the code?

[Star Strider]

To find the value of ‘y’ for a given ‘x’, create an anonymous function for ‘y’:

yfcn = @(x) (668.061./x).*(1-exp(-0.1468.*x))-40;

so for example:

xq = pi;
y = yfcn(xq)

produces:

y =
  38.566778862611955

This also introduces a new way to find ‘xq’:

yq = 0;
xq = fzero(@(v)yfcn(v)-yq, 1)

producing:

xq =
  14.799462199459661