Error using figure First argument must be a figure object or a positive Integer

Hi, I keep getting the error message "Error using figure, First argument must be a figure object or a positive Integer". I'm trying to plot vertical acceleration of a projectile with no drag, which should be easy but I keep getting the same error message. The issue seems rather simple but has been frustrating me. Here is all related code to the error that I'm having

v0 = 50; %initial speed(m/s)
theta = 25; %launch angle (degrees)
N = 250; % Number of timesteps
a = zeros(1,N);
a(1) = -9.8;
a(2) = -9.8;

Tmax = 2*v0*sind(theta)/g; %values created elsewhere in code, no issue
t = linspace(0,2.5*Tmax,N); % seconds: Time axis, 

for n = 3:N
     a(n) = -9.8;
end

 figure 4
  plot(t,a)

  

Answer 1

[Peter O]

Figure is a function. Try:

figure(4)

This should activate a figure with ID number 4.

[Steven Lord]

Note that figure(4) is not the same as figure 4. That is actually equivalent to figure('4') which passes a character into figure. While figure can accept some char vector inputs (pairs of parameter names and parameter values) it can't accept the char vector '4'.

[Peter O]

Great point!