Error using dde23 : Derivative and history vectors have different lengths.

I am trying to solve a second order DE and I am not able to resolve this error. Please help and thanks in advance

sol = dde23(@ddex1de,[0.04],@ddex1hist,[0, 0.7]);


t = linspace(0.04,0.7,1000);
y = deval(sol,t);
ylag = deval(sol,t - 0.04);
F = ((K_f*a)/m)*(h - ylag + y)
plot(t,F);
xlabel('t');
ylabel('F');

% --------------------------------------------------------------------------


function s = ddex1hist(t)
    % Constant history function for DDEX1.
    s = zeros(2,1);
end

% --------------------------------------------------------------------------

function dydt = ddex1de(t,y,Z)
   global m c n k K_f T h a


   % Differential equations function for DDEX1.
    ylag1 = Z(:,1);
    dydt = [ y(2) ; (((K_f)*(a))/(m))*((h)-ylag1(1)+y(1))-(c/m)*y(2)-((k)/m)*y(1) ];
end

[Atharva Modi]

I have converted the second order DE into two first order DEs

Answer 1

[Stephan]

You did not define your constants - i took some fake values for them. n and T are unused. Also for good reasons, try to avoid global variables:

%% constants

m = 1;
c = 1;
n = 1;
k = 1;
K_f = 1;
T = 1;
h = 1;
a = 1;

%%  code
sol = dde23(@(t,y,Z)ddex1de(t,y,Z,m,c,n,k,K_f,T,h,a),[0.04],@ddex1hist,[0, 0.7]);

t = linspace(0.04,0.7,1000);
y = deval(sol,t);
ylag = deval(sol,t - 0.04);
F = ((K_f*a)/m)*(h - ylag + y);
plot(t,F);
xlabel('t');
ylabel('F');

%% --------------------------------------------------------------------------

function s = ddex1hist(~)
    % Constant history function for DDEX1.
    s = zeros(2,1);
end

%% --------------------------------------------------------------------------

function dydt = ddex1de(~,y,Z,m,c,~,k,K_f,~,h,a)
    % Differential equations function for DDEX1.
    ylag1 = Z(:,1);
    dydt = [ y(2) ; (((K_f)*(a))/(m))*((h)-ylag1(1)+y(1))-(c/m)*y(2)-((k)/m)*y(1) ];
end