Ok, so let's start off with a quick example of an SSCCE:
function [Z,Y] = khan
options = odeset('RelTol',1e-7,'AbsTol',1e-7);
[Z,Y] = ode45(@momentum,[0 12],[0 0],options);
end
function Dy = momentum(z,y)
Dy = [0 0]';
Dy(1) = 3*y(1) + 2* y(2) - 2;
Dy(2) = y(1) - y(2);
Ve = Dy(1)+ y(2);
assignin('base','Ve_step',Ve);
evalin('base','Ve_out(end+1) = Ve_step;');
assignin('base','T_step',z);
evalin('base','T_out(end+1) = T_step;');
end
By running [Z,Y] = khan
as the command line, I get a complete functional code that demonstrates your problem, without all the headaches associated. My patience for this has been exhausted: live and learn.
This seems to work, however, the size of the array does not match the size of the solution vector acquired from ode45
Note that I added two lines to your code which extracts time variable. From the command prompt, one simply has to run the following to understand what's going on:
Ve_out = [];
T_out = [];
[Z,Y] = khan;
size (Z)
size (T_out)
size (Ve_out)
plot (diff(T_out))
ans =
109 1
ans =
1 163
ans =
1 163
Basically ode45 is an iterative algorithm, which means it will regularly course correct (that's why you regularly see diff(T) = 0). You can't force the algorithm to do what you want, you have to live with it.
So your options are
1. Use a fixed step algorithm
2. Have a function call that reproduces what you want after the ode45 algorithm has done its dirty work. (am304's solution)
3. Collects the data with the time function, then have an algorithm parse through everything to removes the extra data.