Undefined function 'piecewise' for input arguments of type 'double'.

I am trying to loop through data and find the max profit. Please help!

qh=@(p) piecewise(p<=100, 50, (100<=p)&&(p<=200), 100-(p/2),p>=200, 0);
qa=@(p) piecewise(p<=150,50-(p/3),p>=150,0);
revenue=@(p) p*(qh(p)+qa(p));
cost=@(p) -2000-(20*(qh(p)+qa(p)));

z=FindMaxPrice;

function maxPro=FindMaxPrice(qh,qa)

maxPro=0;
for p=0:1:202
  profit=@(p) (p - 20)*qh(p) + (p - 20)*qa(p) - 2000;
  if profit(p)>maxPro
      maxPro=profit(p);
  end
end
maxPro
end

[Stephen23]

The function piecewise is part of the symbolic toolbox: is p symbolic?

Why are you not doing this with basic numeric operations?

[Haley Palmer]

Yes p is symbolic. I am not doing it with basic operations because I am trying to substite p in in the function to find the max profit value. Inside my function p is defined

[Haley Palmer]

Unless that is not what you meant

[Haley Palmer]

I changed the code to look like this:

z=FindMaxPrice();


function maxPro=FindMaxPrice()
syms p
qh=piecewise(p<=100, 50, (100<=p<=200), 100-(p/2),p>=200, 0);
syms p
qa=piecewise(p<=150,50-(p/3),p>=150,0);
maxPro=0;
for p=0:202
  profit=(p - 20)*qh + (p - 20)*qa - 2000;
  if profit>maxPro
      maxPro=profit(p);
  end
end
maxPro
end

[Stephen23]

"Unless that is not what you meant"

If you are not sure if p is symbolic, then it probably isn't. A variable is symbolic if it is declared with sym or syms, and allows the use of the functions and methods provided in the Symbolic Toolbox:

https://www.mathworks.com/products/symbolic.html

As I wrote in my earlier comment, the function piecewise is part of that toolbox. You cannot simply use piecewise on some numeric variable that you have defined (the piecewise documentation makes it very clear that at a bare minimum its conditions must be symbolic).

"I am not doing it with basic operations because I am trying to substite p in in the function to find the max profit value"

I don't see why that requires symbolic variables: basic MATLAB numeric operations, functions, and/or indexing would probably do everything you need. You should read about logical indexing, and probably do the intrductory tutorials:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Haley Palmer]

Okay! Thank you!

Answer 1

[Star Strider]

Note that you can easily convert your piecewise symbollic expressions to appropriate numerical expressions.

So ‘qh’ and ‘qa’ become respectively:

qh=@(p) (p<=100).*50 + ((100<=p)&(p<=200)).*100-(p/2) + (p>=200).*0;
qa=@(p) (p<=150).*50-(p/3) + (p>=150).*0;

The element-wise operation (.*) isn’t always necessary. I use it in expressions such as these because it’s better to do it than not, in the event the sub-expression is a function of the argument variable (here ‘p’).

The easiest way to check to be certain these do what you want them to is to plot them:

g = linspace(0, 300, 100);
figure
plot(g, qh(g),    g, qa(g))
grid

.