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Having difficult time trying to plot a simple trajecory

  1. May 30, 2014 #1
    Hello all, I am trying to plot a simple trajectory in MATlab. Here is the code I wrote,

    v0 = 100;
    theta = pi/4;
    %teacher gone missing, again, ARRRGGHHH!!!!!!
    title('Ballistic Trajectory of a Projectile'), xlabel('horizontal distance, m'), ylabel('vertical distance, m')
    plot (t,horizontal(t))

    I am getting this error,

    "Subscript indices must either be real positive integers or logicals."

    If I input individual values for 't' it produces the correct results up until the vertical distance reaches it's 'maximum' but do not know why?
  2. jcsd
  3. May 30, 2014 #2


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    If 'horizontal (t)' and 'vertical (t)' are supposed to be arrays rather than functions of 't', then the error message would make sense.

    To keep Matlab happy, why don't you use a dummy array index variable 'i' instead of t. Start with i = 1 and t = 0.0, and then t = (i - 1)*0.1
  4. May 30, 2014 #3


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    Just don't make the all too common error of redefining ##i## if your code uses complex numbers. :biggrin:
  5. May 30, 2014 #4
    I'm new to Matlab so I am not sure what you mean by 'dummy array index variable' (this is not in my book). I do believe I see what you are getting at but we have been using,
    and the lecturer is a jerk so anything outside what we go over, "...will result in a zero!!!" (has it plastered everywhere on the assignment).

    Anyway, it isn't working for this plot so... :P
    Last edited: May 30, 2014
  6. May 30, 2014 #5
    Got it, :P
    This is a fickle program, grrrrrr
  7. Jun 17, 2014 #6
    I take that back, this is a wonderful program.
  8. Jun 17, 2014 #7
    That is perhaps because so many students use the internet to get their programming assignments done for them :)
  9. Jun 18, 2014 #8
    That could be the reasoning behind these criteria although it seems most students in this class genuinely want to know how to write code.

    A big part of the problem seems to be a Professor buried under departmental expectations that are unreasonable for a 1 credit hour course so with this new insight in mind I rescind my earlier comment about the lecturer; it is the department heads who are the jerks :)
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