MATLAB: What does this ODE error mean

entries must strictly increase or decreaseode error

Hello,
I don't understand why I am getting the following error:
Error using odearguments (line 87)
The entries in tspan must strictly increase or decrease.
Error in ode15s (line 150)
    odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);
Error in trajecttest (line 39)
		[T,S] = ode15s(@bdipuniodefun, tspan, icv);
For this for loop:
spart = [0.05 0.05 0.];
vpart = [-1.7585E+7 -1.7585E+7 0.];
tstep = 1E-7; %Defining time step

tfin = 1.1E-6; %Defining final time

intspan = [0:tstep:tfin]; %Total time span

[introw,intcol] = size(intspan);
W = zeros((3*intcol),6); %Generates matrix of zeros that the trajectory solver will populate later
for t = 0:1:intcol/2-1
    
    x = spart(1);
	y = spart(2);
	z = spart(3);
	vx = vpart(1);
	vy = vpart(2);
	vz = vpart(3);
    
		icv = [x; y; z; vx; vy; vz];
		%Time span
        tspan = [intspan(2*t+1) intspan(2*t+2)/2 intspan(2*t+2)];
		[T,S] = ode15s(@bdipuniodefun, tspan, icv);
		[rownum,colnum] = size(S);
        W((1+3*t):(3+3*t),(1:6)) = S;
    
    vparts(1) = S(rownum,4);
	vparts(2) = S(rownum,5);
	vparts(3) = S(rownum,6);
	sparts(1) = S(rownum,1);
	sparts(2) = S(rownum,2);
	sparts(3) = S(rownum,3);
    
end
The ODE solver accepts the inputs if there is no for loop invovled like this:
x = 0.05;
y = 0.05;
z = 0.;
vx = -1.7585E+7;
vy = -1.7585E+7;
vz = 0.;
tstep = 0.5E-8; %Defining time step
tfin = 1E-8; %Defining final time
intspan = [0:tstep:tfin]; %Total time span
[introw,intcol] = size(intspan);
icv = [x; y; z; vx; vy; vz]; %Format: [x; y; z; vz; vy; vz]
%Time Span (sec)
tspan = [intspan];
[T,W] = ode15s(@bdipuniodefun, tspan, icv);
[rownum,colnum] = size(W);
So why is it that adding the for loop causes this error? I know that the numbers that the "tspan" array is taking in are exactly the same since I tested it in the command window.

Best Answer

Hi,
the message is clear. See what happens if you inspect some lines of your code - therefore lets assume t=1 which is the second step of your loop in:
for t = 0:1:intcol/2-1
So with t=1 what do we get as tspan?
t = 1
tstep = 1E-7;
tfin = 1.1E-6;
intspan = [0:tstep:tfin]; 
tspan = [intspan(2.*t+1) intspan(2.*t+2)/2 intspan(2.*t+2)]
And the result is:
tspan =
   1.0e-06 *
    0.2000    0.1500    0.3000
The error message tells us:
    Error using odearguments (line 87)
The entries in tspan must strictly increase or decrease.
Since your values first decrease and then increase you get this error.
Best regards
Stephan
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