Plotting a piecewise function

Question

Ali Kiral 2021-6-25

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/864960-plotting-a-piecewise-function

回答： Walter Roberson 2021-6-25

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?

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Answer 1

Walter Roberson 2021-6-25

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/864960-plotting-a-piecewise-function#answer_733095

在 MATLAB Online 中打开

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:

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