Generate matrix with for-loop in matlab

Question

Say I have two functions f(x), g(x), and a vector:

xval=1:0.01:2

For each of these individual x values, I want to define a vector of y-values, covering the y-interval bounded by the two functions (or possibly a matrix where columns are x-values, and rows are y-values).

How would I go about creating a loop that would handle this for me? I have absolutely no idea myself, but I'm sure some of you have something right up your sleeve. I've been sweating over this problem for a few hours by now.

Thanks in advance.

s.bandara · Accepted Answer · 2013-10-26 18:12:21Z

3

Since you wish to generate a matrix, I assume the number of values between f(x) and g(x) should be the same for every xval. Let's call that number of values n_pt. Then, we also know what the dimensions of your result matrix rng will be.

n_pt = 10;
xval = 1 : 0.01 : 2;
rng = zeros(n_pt, length(xval));

Now, into the loop. Once we know what the y-values returned by f(x) and g(x) are, we can use linspace to give us n_pt equally spaced points between them.

for n = 1 : length(xval)
  y_f = f(xval(n))
  y_g = g(xval(n))
  rng(:, n) = linspace(y_f, y_g, n_pt)';
end

This is nice because with linspace you don't need to worry about whether y_f > y_g, y_f == y_g or y_f < y_g. That's all taken care of already.

For demsonstration, I run this example for xval = 1 : 0.1 : 2 and the two sinusoids f = @(x) sin(2 * x) and g = @(x) sin(x) * 2. The points are plotted using plot(xval, rng, '*k');. Example

edited Oct 26, 2013 at 18:12

answered Oct 26, 2013 at 18:01

s.bandara

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1 Comment

bertoxxulous Over a year ago

That is absolutely brilliant, that is exactly what I was looking for. It is very much appreciated, I don't think I'd ever figure that out myself. Thanks again.

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