How do I perform statements on the dependent variable of a graph in MATLAB?

Question

I would like to know how to grab a specific number from an interval to test it and later be able to built a different functions under one graph. For example (in this case the 'x' variable),

 x 0:.5:5;

 Ids=ones(x);
 figure;hold on;

 for n = 1:5
     if(x < 3.0) %problem here
         Ids(n) = plot(x,x.^x);
     else 
         if (x > 4.0)  %and here
            Ids(n) = plot(x,-x.^x);
         end
     end
 end

EDIT

What I really want to do in MATLAB is to be able to do the following piecewise function:

y(x) = {  0                   (t - 5) < 0
       { (t - 5)*(t - x)      x < (t - 5)
       { (t + x^2)            x >= (t - 5)

I don't seem to understand how to graph this function since x = 0:.5:10 and t = 0:.1:10. I know how to do this without the t, but I get lost when the t is included and has different intervals compared to the x.

Y_Y, can you clarify what you want to do? As gnovice wrote, it is unclear from your code what you are trying to do. Can you write in 'words' what you want to do? Is the function f(x) given in gnovice's answer what you're looking for? — Y.T., Dec 06 '10 at 04:52

score 1 · Answer 1 · answered Dec 05 '10 at 09:04

You may be looking for piecewise polynomials: http://www.mathworks.com/help/techdoc/ref/mkpp.html

Otherwise, I would suggest making two vectors, "x" and "y", so to speak, and filling y by iterating through x and applying your conditions and results, then plot y against x. This will avoid the need to hold the plot.

If you want to animate the drawing, add plot() to the for loop followed by "drawnow". It's been a while since I had to animate plots though so I would suggest tutorials for drawnow and animation.

How can I do it using piecewise function 'mkpp'? – Y_Y Dec 05 '10 at 09:26 — Y_Y, Dec 05 '10 at 09:26

gnovice · Accepted Answer · 2010-12-05T22:20:19.800

It's a little unclear from your code what you are trying to do, but it appears that you want to create and plot a function f(x) that has the following form:

f(x) = [ x     for 3 <= x <= 4
       [ x^x   for x < 3
       [ -x^x  for x > 4

If this is what you want to do, you can do the following using logical indexing:

x = 0:0.5:5;  %# 11 points spaced from 0 to 5 in steps of 0.5
y = x;        %# Initialize y
index = x < 3;                   %# Get a logical index of points less than 3
y(index) = x(index).^x(index);   %# Change the indexed points
index = x > 4;                   %# Get a logical index of points greater then 4
y(index) = -x(index).^x(index);  %# Change the indexed points
plot(x,y);                       %# Plot y versus x

How do I perform statements on the dependent variable of a graph in MATLAB?

2 Answers2

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