Plotting a piecewise function

I have a function m-file, goes like this:

function y=periodic(x);
if x<=1
    y=0;
end
  if x > 1 & x < 2
    y=x-1;
  end
   if x>=2 & x < 3
    y=3-x;
   end
    if x>=3 
        y=0;
    end

At prompt, when I type

    x=0:0.1:4;
    plot(x,periodic(x))

MATLAB returns 'Output argument "y" (and maybe others) not assigned during call to..' and points out that 'if x<=1' is in error. What is the problem here?

Answer 1

[Walter Roberson]

 x=0:0.1:4;

That is a vector.

plot(x,periodic(x))

You are passing the entire vector to the function.

if x<=1
    y=0;
end

You are comparing the entire vector to 1, getting a vector of true and false values. You are then using if with that vector. In MATLAB, when you use if or while with a non-scalar object, then the test is considered true only if all of the values being checked are non-zero. So if all of the values in the vector are <= 1 then you would set y to be a scalar 0.

Likewise, your other if test all of the vector, and if all of the elements satisfy the condition, then y is set to a scalar value in the final case (though a vector value in the middle two cases.)

You need to take one of three approaches:

1. loop so that you apply periodic to only one element of x at a time, such as arrayfun(@periodic, x); OR
2. loop inside periodic() over the elements of what was passed in, using appropriate indexing to set appropriate elements of y; OR
3. learn how to use logical indexing.