# Pendulum code error. How do I resolve unrecognized function or variable when it is clearly stated?

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Nicholas Davis on 27 Sep 2020
Commented: Walter Roberson on 27 Sep 2020
I have a code written out to simulate a pendulum, but there seems to be an error after the loop, and I can't seem to resolve it on my own. I would appreciate any advice on how to solve this issue. My professor can't seem to solve this issue either, and as a result, has told me that I must solve it without further assistance. I have everything written out with comment lines to help the user. I have also included the error message I get. Thanks!
Unrecognized function or variable 'period'.
Error in pendul (line 59)
AvePeriod = mean(period);
%pendul - Program to compute the motion of a simple pendulum
%using either the Euler or Verlet methods
clear all; help pendul %Clear the memory and print header
% Select the numerical method to use: Euler or Verlet
NumericalMethod = menu('Choose a numerical method:','Euler','Verlet');
%Set initial position and velocity of pendulum
theta0 = input('Enter initial angle (in degrees): ');
theta = theta0*pi/180; %Convert angle to radians
omega = 0; %Set the initial velocity
%Set the physical constants and other variables
g_over_L = 1; %The constant g/L
time = 0; %Initial time
irev = 0; %Used to count the number of reversals
tau = input('Enter time step: ');
%Take one backward step to start Verlet
accel = -g_over_L*sin(theta); %Gravitational Acceleration
theta_old = theta - omega*tau + 0.5*tau^2*accel;
%Loop over desired number of steps with given time step and numerical method
nstep = input('Enter number of time steps: ');
for istep=1:nstep
%Record angle and time for plotting
t_plot(istep) = time;
th_plot(istep) = theta*180/pi; %Convert angle to degrees
time = time + tau;
%Compute new position and velocity using Euler or Verlet method
accel = -g_over_L*sin(theta); %Gravitational Acceleration
if(NumericalMethod == 1)
theta_old = theta; %Save Previous Angle
theta = theta + tau*omega; %Euler Method
omega = omega + tau*accel;
else
theta_new = 2*theta - theta_old + tau^2*accel;
theta_old = theta; %Verlet Method
theta = theta_new;
end
%Test if the pendulum has passed through theta = 0;
% if yes, use time to estimate period
if( theta*theta_old < 0) %Test position for sign change
fprintf('Turning point at time t= %f \n',time);
if(irev == 0) %If this is the first change, just record the time
time_old = time;
else
period(irev) = 2*(time - time_old);
time_old = time;
end
irev = irev + 1; %Increment the number of reversals
end
end
%Estimate period of oscillations, including error bar
AvePeriod = mean(period);
ErrorBar = std(period)/sqrt(irev);
fprintf('Average period = %g +/- %g\n', AvePeriod,ErrorBar);
%Graph the oscillations as theta versus time
clf; figure(gcf); %Clear and forward figure window
plot(t_plot,th_plot, '+');
xlabel('Time'); ylabel('\theta (degrees)');

Walter Roberson on 27 Sep 2020
for istep=1:nstep
if nstep is less than 1 then the loop will not be executed at all and period will not be assigned to.
If nstep is at least 1 then we need to investigate whether we can be certain that period is assigned to.
if( theta*theta_old < 0)
If we have not executed enough steps for the angle to have crossed zero then period would not have been assigned to.
if(irev == 0)
The first time we cross 0, we do not assign to period.
Putting these together: period will not be assigned to until you have executed enough steps to have crossed zero twice.
if( theta*theta_old < 0)
No crossing will be detected if theta or theta_old are exactly 0 because then their multiple would be exactly 0 rather than less than 0.
You need to be careful about this case so that you do not accidentally detect the same crossing twice.

Nicholas Davis on 27 Sep 2020
Thank you for the prompt reply, Walter. Your insight is incredibly valuable. I've tried to alter things given your thoughts, but it seems I still have this error. What kind of addition should I include to the code to actually make it run enough to get a period? Thanks.
Walter Roberson on 27 Sep 2020
Increase the number of steps you allow. Each step only goes a fraction of a cycle.
But also before you output check that irev is at least 2, because if not then you have not found the period yet.

R2020b

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