Extracting values from a 3D surface plot for each iteration

I have a 3D surface plot (f_plot) of a propagating wave which has 25 time steps. I created a time loop and generated the wave and I want to extract the z value corresponding the x,y values (20,20) in each time step. I want to create a variable (var_z) with these z values for the 20 iterations.

The code i wrote to extract the Z value at each iteration only provides the value for the final iteration. The code is below.

h=surf(f_plot);
var_z = zeros(100,1);
Ts = 25
for n = 1:Ts
    m=find((h.XData==20)&(h.YData==20));
    Var_z(n)=(h.ZData(m));
    
end

Answer 1

[Walter Roberson]

You do not need to do the surf() in order to find the value.

And you should probably not be assuming that the exact X and Y coordinates exist. Typically you should use interp2() instead.

However, in the context of surf() in which you are only passing in the Z data, then the easiest approach is to skip the surf() and just index the data you would have passed in:

X=20; Y = 20;
f_plot(Y, X)    %notice that Y is ROWS not columns !

Anyhow, the problem with your code is that you are not updating the plot between iterations. Your surf() would have to be inside your for n loop to make it worth doing.

[Melecia Senior]

Even when i put the surf () within the for n loop it still only returns one value Var_z

[Walter Roberson]

It seems to work when I experiment.

Ts = 4;
X=20; Y = 20;
var_z = zeros(Ts,1);
for n = 1:Ts
   f_plot = randi(9, 25, 30);
   h = surf(f_plot);
   fprintf('for the record, iteration #%d, the pixel is %g\n', n, f_plot(Y, X));
   m = find((h.XData==X)&(h.YData==Y));
   Var_z(n) = (h.ZData(m));
end

for the record, iteration #1, the pixel is 7
for the record, iteration #2, the pixel is 4
for the record, iteration #3, the pixel is 1

for the record, iteration #4, the pixel is 2

fprintf('\nAfterwards:\n');

Afterwards:

Var_z

Var_z = 1×4

7 4 1 2