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Need help graphing a step function within Matlab

  1. Aug 29, 2011 #1
    So I'm not going to lie, I'm pretty new with this software. I was assign a homework assignment, but I'm not sure how you graph a step function.

    Given this waveform use the step function to find an equation:

    http://img163.imageshack.us/img163/2545/stepfunction.jpg" [Broken]

    I calculated the step function to be:

    x(t) = (t^2)[u(t)-u(t-2)]+2(t-4)[u(t-2)-u(t-4)] , which I'm pretty positive is right.

    Now graphing in Matlab is my biggest problem. I've looked over my textbook at sample graphing techniques, this is what I have attempted.

    Code (Text):
    >> x = inline('(t^2)(u(t)-u(t-2))+2(t-4)(u(t-2)-u(t-4))', 't')

    g =

         Inline function:
         g(t) = (t^2)(u(t)-u(t-2))+2(t-4)(u(t-2)-u(t-4))

    >> t = (-1:5);
    >> plot(t,g(t));
    [COLOR="Red"]??? Error using ==> inlineeval at 15
    Error in inline expression ==>
    (t^2)(u(t)-u(t-2))+2(t-4)(u(t-2)-u(t-4))
     Error: Unbalanced or unexpected
     parenthesis or bracket.

    Error in ==> inline.subsref at 27
        INLINE_OUT_ =
        inlineeval(INLINE_INPUTS_,
        INLINE_OBJ_.inputExpr,
        INLINE_OBJ_.expr);[/COLOR]
    Any tips or suggestions would be helpful, thank you.
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Aug 29, 2011 #2
    Matlab does not perform multiplication in general when one enters things such as a(b + c).

    You need to specify the array product (.*) operator for multiplication, as you want the function evaluated for each value in the vector t, and do not wish to do matrix multiplication using the entire t vector.

    There is no function u provided with MatLab. You may create one with the following code.

    Code (Text):

    u = @(x)(x>=0);
     
     
  4. Aug 29, 2011 #3
    What do you mean by no function u? Do you mean I have no declared one? Shouldn't I enter this as u, since it is the unit step function?

    Code (Text):
    >> u = inline('(t>=0)','t')
     
  5. Aug 30, 2011 #4
    I meant no function named "u" is included with MatLab.

    You may name it that if you wish.
     
  6. Aug 30, 2011 #5
    Oohhhhh I got it, dumb mistake :(
    So what I have now is:

    Code (Text):
    >> u = inline('(t>=0)','t')       //this defines u also known as the step function
    u = Inline function:             //this is returned by Matlab
         u(t) = t(>=0)
    p = inline('(t>=0&(t<4))','t')  //this defines p and lets the t boundaries go from 0 to 4 just like the one in the problem
    t = (-1:5);                           // sets t for the values from -1 to 5 just to be able to graph the graph
    plot(t,p(t));                //plots the graph
    xlabel('t'); ylabel('p(t) = (t.*t)(u(t)-u(t-2))+2.*(t-4).*(u(t-2)-u(t-4))     //sets the function
    axis([-.1 5 -5 5]);      // sets the axis to be able to see the graph
    But there is a problem after I put in:

    Code (Text):
    xlabel('t'); ylabel('p(t) = (t.*t)(u(t)-u(t-2))+2.*(t-4).*(u(t-2)-u(t-4))
    [COLOR="Red"]Error: A MATLAB string constant is not
    terminated properly.[/COLOR]
    Any ideas? :cry:
     
  7. Aug 30, 2011 #6

    jhae2.718

    User Avatar
    Gold Member

    Code (Text):
    xlabel('t'); ylabel('p(t) = (t.*t)(u(t)-u(t-2))+2.*(t-4).*(u(t-2)-u(t-4)[B][COLOR="Red"][SIZE="6"]'[/SIZE][/COLOR][/B])
    Check your syntax.
     
  8. Sep 5, 2011 #7
    So I scraped all my old code and just started a new simple approach using an if statement. Although there is a problem When I graph it, all I get is a linear straight line.

    Code (Text):

    x = -1:.01:5;         //Sets limits of x
    steps = size(x,2);   // I'm not exactly sure what these do, but I was told to do this because
    y = zeros(steps);   // I needed to define y
     if (x <= 2)            
    y = x^2;
    elseif (x >= 2)
    y = 2.*x-8;
    plot(x,y)
    Any tips?
     
  9. Sep 5, 2011 #8
    The expression "x <= 2" evaluates to a "logical array" with the same dimensions as x. Each value in this "logical array" would correspond an element of x, and would indicate if that particular value was less than or equal to 2.

    The situation is similar for the expression "x >= 2".


    According to the the http://www.mathworks.com/help/techdoc/ref/if.html", "An evaluated expression is true when the result is nonempty and contains all nonzero elements (logical or real numeric). Otherwise, the expression is false."

    So the lines
    Code (Text):
    y = x^2;
     
    and
    Code (Text):
    y = 2.*x-8;
    would never be executed, because the "logical arrays" would not contain all nonzero elements. Even if they did, it would not achieve the desired result.

    The following links may be worth looking at.

    http://www.mathworks.com/help/techdoc/matlab_prog/f0-40063.html#f0-38145" [Broken]

    http://www.mathworks.com/help/techdoc/math/f1-85462.html#bq7egb6-1"

    You can use a logical array produced with a relational operator such as "<=" to index an array. You can use this technique to assign different parts of "y" separately.
     
    Last edited by a moderator: May 5, 2017
  10. Sep 5, 2011 #9

    jhae2.718

    User Avatar
    Gold Member

    1. Use a for loop to calculate the values individually.
    2. Avoid the if statement by using relational operators inline.
     
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