why do I get "Undefined function or variable 'tspan'."

I'm trying to solve for pendulum and everytime I try to run the file I get

Undefined function or variable 'tspan'.

function [z_ydot] = pendulum(t,z_y,k,d,m,L,g)
z_ydot = zeros(4,1);
z_ydot(1) = z_y(2);
z_ydot(2) = (k.*d^2.*(z_y(3)-z_y(1))./(m.*L^2) - ((g.*z_y(1))./L);
z_ydot(3) = z_y(4);
z_ydot(4) = (k.*d^2.*(z_y(1)-z_y(3))./(m.*L^2) - ((g.*z_y(3))./L);

clear; clc; close all
g=9.81;  % acceleration due to gravity
m=2;     % mass in kilograms
L=0.75;  % length in metres
d=0.5;   % distance to the spring in metres
k=130;   % spring stiffness in N/m
timespan = [0 20];  % timespan in seconds
% z1=theta; z2=thetadot
y_z0 = [0 0 13 0];
[t,z_y] = ode45(@(t,z_y) pendulum(t,z_y,k,d,m,L,g), tspan, y_z0);
% time history of theta1
figure
plot(t,z_y(:,1))
title('Theta 1');
xlabel('time')
ylabel('theta')
grind on
% time history of theta2
figure
plot(t,z_y(:,3))
title('Theta 2');
xlabel('time')
ylabel('thetadot')
grind on

Answer 1

[Star Strider]

The code defines that variable as ‘timespan’. The argument names and the varialble names must always match.

Try this:

[t,z_y] = ode45(@(t,z_y) pendulum(t,z_y,k,d,m,L,g), timespan, y_z0);

[Raad Alsulaiti]

I get another error

" Error in Untitled3>@(t,z)pendulum(t,z,k,d,m,L,g) "

I dont know whats wrong is it from the matlab it self or not

[Walter Roberson]

z_ydot(2) = (k.*d^2.*(z_y(3)-z_y(1))./(m.*L^2) - ((g.*z_y(1))./L);
      1 0   1        2   3 2    3 21  2      1   23      4 32   1
      

The numbers indicate the number of active bracket nestings after the character they are below. You can see that at the end of the line, there is one active bracket nesting -- so you are missing one )

[Star Strider]

@Walter — Thank you!

[Raad Alsulaiti]

finally it worked, thank you guys

[Star Strider]

Our pleasure!