Index exceeds matrix dimensions.

Hello,

I am working in a programme to solve cauchy's problem. But I had this error "Index exceeds matrix dimensions." in pb_cauchy (line 3) g1=y(2)

The function pb_cauchy:
function g=pb_cauchy(y,A1,A2,B1,B2)
%fonction representing X'=AxX
 g1=y(2);
 g2=y(3);
 g3=A2.*y(2)+A1.*y(4);
 g4=y(5);
 g5=y(6);
 g6=y(7);
 g7=B2.*y(2)-B1.*y(4);
 g=[g1;g2;g3;g4;g5;g6;g7];
 end

Thnks

Answer 1

[Guillaume]

It sounds like y is just a scalar. For your function to work y must have at least 7 elements.

[Nouhayla EL GHANI]

Yes, I saw that but I don't know how to change it. I call pb_cauchy in the function main by the syntax:

y0=[y0(1) y0(2) y0(3) y0(4) y0(5) y0(6) y0(7)];         
[x,ysol]=ode45('pb_cauchy',x,y0);

[Adam]

y0=[y0(1) y0(2) y0(3) y0(4) y0(5) y0(6) y0(7)];

either does nothing at all if y0 already has just 7 elements or could be replaced with simply putting

y0(1:7)

into the od45 function call.

That won't solve the problem you are having though, just make your code a little less redundant.

The bug you are getting would probably be found trivially using the debugger with a breakpoint in your pb_cauchy function.

Answer 2

[Walter Roberson]

A1 = randn() * 5; A2 = randn() * 15;   %I had to invent _some_ value for them
B1 = randn() * 7; B2 = randn() * 21;

y0 = rand(1,7);                        %appropriate non-zero initial condition

[x, ysol] = ode45(@(t,y) pb_cauchy(t,y,A1,A2,B1,B2),y0);

with

function g=pb_cauchy(t, y, A1, A2, B1, B2)
%function representing X'=AxX
 g1=y(2);
 g2=y(3);
 g3=A2.*y(2)+A1.*y(4);
 g4=y(5);
 g5=y(6);
 g6=y(7);
 g7=B2.*y(2)-B1.*y(4);
 g=[g1;g2;g3;g4;g5;g6;g7];
 end

Notice the "t" before the "y".

[Nouhayla EL GHANI]

Thank you. When I added u re modification i get this errors

Error using odearguments (line 19)
When the first argument to ode45 is a function handle, the tspan
and y0 arguments must be supplied.

Error in ode45 (line 114)
[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs,
odeFcn, ...

Error in main_NR (line 82)
         [x,ysol]=ode45(@(t,y)pb_cauchy(t,y,A1,A2,B1,B2),y0);

[Torsten]

Then just do what the error message says:

A1 = randn() * 5; 
A2 = randn() * 15;   %I had to invent _some_ value for them
B1 = randn() * 7; 
B2 = randn() * 21;
y0 = rand(1,7);      %appropriate non-zero initial condition

tspan = [0 1]; 
[x,ysol] = ode45(@(t,y) pb_cauchy(t,y,A1,A2,B1,B2),tspan,y0);

with

function g=pb_cauchy(t, y, A1, A2, B1, B2)
%function representing X'=AxX
 g1=y(2);
 g2=y(3);
 g3=A2.*y(2)+A1.*y(4);
 g4=y(5);
 g5=y(6);
 g6=y(7);
 g7=B2.*y(2)-B1.*y(4);
 g=[g1;g2;g3;g4;g5;g6;g7];
 end

Best wishes

Torsten.

[Nouhayla EL GHANI]

Thnks a lot :)