must return a column vector

function dy=lodosact (t,y,So,V,Qo,U,Ks,Kd,Qr,Y,B)
global So V Qo U Ks B Kd Qr Y
V=17e6
Qr=0.65e10
Xo=100
Qo=2.08e6
B=4
So=200
Ks=100
U=0.5
Kd=0.0021
Y=0.6
dy(2)=(((So-y(2))./(V.*Qo))-((y(1).*y(2).*U)./(Y.*(Ks+y(2)))))
dy(1)=(((y(1)./(V.*Qo)).*((B.*(Qr.*Qo))-1-(Qr.*Qo))+((y(1).*y(2).*U)./((Ks+y(2))+(Kd.*y(1))))))
end

[t,y]= ode45('lodosact',[0 10],[100,200]);
plot (t,y)
title ('Actividad 20')
xlabel('T')
ylabel('ds/dx')
grid on 
hold off 

[Stephen23]

Moved from tags:

error using odearguments (line 93)
lodosact must return a column vector.
error in ode45 (line 115) odearguments(fcnhandlesused,solver_name,ode,tspan,y0,options,varargin)
error in correrlodosactivados (line 1)
[t,y]= ode45(@lodosact,[0 10],[100,200])

Answer 1

[madhan ravi]

[t,y]= ode45(@lodosact,[0,2],[100,200]);
plot(t,y)
title('Actividad 20')
xlabel('T')
ylabel('ds/dx')
grid on

function dy=lodosact(t,y)
% no need for Global!!
V=17e6;
Qr=0.65e10;
Xo=100; % used nowhere
Qo=2.08e6;
B=4;
So=200;
Ks=100;
U=0.5;
Kd=0.0021;
Y=0.6;
dy = zeros(2,1); % have a look here
dy(1)=(((y(1)./(V.*Qo)).*((B.*(Qr.*Qo))-1-(Qr.*Qo))+((y(1).*y(2).*U)./((Ks+y(2))+(Kd.*y(1))))));
dy(2)=(((So-y(2))./(V.*Qo))-((y(1).*y(2).*U)./(Y.*(Ks+y(2)))));
end

Answer 2

[Walter Roberson]

When you use a single subscript to assign to an undefined variable, then MATLAB creates a row vector. For example,

xyz(2) = 42;

creates xyz as a 1 x 2 variable containing [0 42]

You assign to dy(2) and dy(1) without having initialized dy, so this rule applies, and you are creating dy as a 1 x 2 variable.

However, the output from an ode function must be a column vector.

Easiest fix is to assign to dy(2,1) and dy(1,1)