Error "Not enough Input Arguments" in predator-prey model

I'm trying to understand the working of ODE45 to solve multiple variable equations. I copied a code from internet but when I run this code, matlab gives error

Not enough input arguments.

Error in predPrey (line 2)
f=[-x(1)+3*x(3);-x(2)+2*x(3);x(1)^2-2*x(3)];

function f=predPrey(t,x);

the code is given here:

        f=[-x(1)+3*x(3);-x(2)+2*x(3);x(1)^2-2*x(3)];
          [t,xa] = ode45(@predPrey,[0 1.5],[0 1/2 3]);
          plot(t,xa(:,2))
          title('y(t)')
          xlabel('t'), ylabel('y')

can someone please help me in this error so I can continue modifying model for my system.

Answer 1

[Star Strider]

You did not copy it correctly or implement it correctly.

Try this:

predPrey =  @(t,x) [-x(1)+3*x(3);-x(2)+2*x(3);x(1)^2-2*x(3)];
[t,xa] = ode45(predPrey,[0 1.5],[0 1/2 3]);
plot(t,xa(:,2))
title('y(t)')
xlabel('t'), ylabel('y')

[summyia qamar]

thankyou. what if I want to see separate graphs of each equation? the results are a single line.

[Walter Roberson]

plot(t, xa)

[Star Strider]

@summyia qamar — My pleasure.

@Walter — Thank you.

Answer 2

[Walter Roberson]

The way you give your code makes it appear as if all of your code is within predPrey.m and that your call to ode45 is on line 3 of predPrey.m and that you try to run the ode by executing the predPrey file. If so then you need to break out the line starting from [t,xa] into a different file. Or, if you are running R2016b or later, you could use one file that is not named prepPrey.m that contains

          [t,xa] = ode45(@predPrey,[0 1.5],[0 1/2 3]);
          plot(t,xa(:,2))
          title('y(t)')
          xlabel('t'), ylabel('y')

          function f=predPrey(t,x)
          f=[-x(1)+3*x(3);-x(2)+2*x(3);x(1)^2-2*x(3)];

The alternative to all of this is to use just one file containing:

          prePrey = @(t, x) [-x(1)+3*x(3);-x(2)+2*x(3);x(1)^2-2*x(3)];
          [t,xa] = ode45(predPrey,[0 1.5],[0 1/2 3]);
          plot(t,xa(:,2))
          title('y(t)')
          xlabel('t'), ylabel('y')