My Program Is Ignoring my ELSE statement...How To Fix...Projectile Motion

Question

Taylor McMillan 2018-5-31

0
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https://ww2.mathworks.cn/matlabcentral/answers/403456-my-program-is-ignoring-my-else-statement-how-to-fix-projectile-motion

回答： Walter Roberson 2018-5-31

I AM WANTING TO STOP PLOTTING ONCE Y < 0.

%{
This function was created by Taylor McMillan.
PURPOSE: Model simple projectile motion. Introduce students to Matlab.
ENTER THE FOLLOWING INTO THE COMMAND WINDOW:
ProjectileMotion(0.1, 0, 0, 50, pi/4, 30)
%}
%%START PROGRAM %%
function ProjectileMotion(time_step, xic, yic, vic, angic, max_time)
%%Create A Time Series %%
h = time_step;
N = max_time/h;
t = 0:h:max_time - h;
%%Initial Conditions / Parameters %%
x(1) = xic;     % Initial x-position (meters)
y(1) = yic;     % Initial y-position (meters)
v(1) = vic;     % Initial speed (meters/second)
ang(1) = angic; % Initial launch angle (radians)
g = 9.81;       % Acceleration due to gravity (meters/second/second)
%%Projectile Equations %%
if y >=0 && x >= 0
      x = x(1) + v(1)*cos(ang(1))*t;
      y = y(1) + v(1)*sin(ang(1))*t - 0.5*g*t.^2;
%%Plot The Motion %%
else 
      figure();
      plot(x, y)
      xlabel('x - position (m)', 'FontSize', 20)
      ylabel('y - position (m)', 'FontSize', 20)
      title('Trajectory', 'FontSize', 26)
end

END PROGRAM %%

end

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Answer 1

Walter Roberson 2018-5-31

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/403456-my-program-is-ignoring-my-else-statement-how-to-fix-projectile-motion#answer_322696

Your initial x and y are 0 . 0 >=0 && 0 >= 0 so the code in the body if the "if" will be executed, and the "else" will not be executed. And then you get to the end of the function and return. You do not have a loop.

Your plot() assumes that x and y are vectors, consistent with you having initialized x(1) and y(1). But the code y >=0 && x >= 0 will fail as soon as y becomes a vector, because you cannot use && with vector values. You could try switching to & instead of && but remember that y >=0 & x >= 0 means the same as all(y >=0 & x >= 0)