Don't know how to fix error preallocating for speed

Keep getting an error for preallocating for speed, but I don't know how to fix it

% March/2023
% 
% Solve f(x) = exp(x)-x.^2+3*x-2 using false-position method.
%
% The function is defined in func(x); 
%
% Input
tol = 1.e-5;
a = 0.0; 
b = 1.0;
nmax = 100;
% Initialization 
itcount = 0;
error = 1.0;
% Graph of the function
xval = linspace(a,b,100);
for i=1:10
     fval(i) = func(xval(i));
end

Unrecognized function or variable 'func'.

plot(xval,fval);
grid on; 
hold on; 
% iteration begins here
while (itcount <= nmax && error >= tol)
   itcount = itcount + 1;
   xold = x;
   % Generate and save iteratres
  
   fa = func(a); 
   fb = func(b);
  
   x = a - ((b - a)/(fb - fa))*fa;
   z(itcount) = x;
  
   fx = func(x);
   
   error = abs(fx);
%  error = abs(x - xold);
   
   if (error < tol)
      x_final = x;
   else
      if (fa*fx < 0)
      % root is between a and x
         b = x;
      else
      % root is between x and b
         a = x;
      end
   end
   
   plot(z(1:itcount),zeros(itcount,1),'r+');
   pause(2)   
end
 
if (itcount < nmax);
  val = func(x);
  
  fprintf(1,'Converged solution after %5d iterations',itcount);
  fprintf(1,' is %15.7e, %e \n',x_final, val);
else
  fprintf(1,'Not converged after %3d iterations',nmax);
end

[the cyclist]

Can you include the definition of func(), as well, so that we can run your code?

Which line gives the error? What's the complete error message you see?

Also, is it really an error (that causes your code to halt execution), or just a warning that is suggesting you preallocate memory?

[Kameron]

The function is exp(x)-x.^2+3*x-2.

The error says:

Unrecognized function or variable 'func'.

Error in hw3q4 (line 18)

fval(i) = func(xval(i));

[the cyclist]

OK. So, you haven't actually defined that in your code. So, you could put a line like

func = @(x) exp(x)-x.^2+3*x-2

Then, you'll get an error because in this code

for i=1:10
     fval(i) = func(xval(i));
end

you only define values for 10 values of xval, but you need all 100 for

plot(xval,fval);

to work. So, you need

for i=1:100
     fval(i) = func(xval(i));
end

instead.

Then, you'll get an error on this line

   xold = x;

because x doesn't exist, in order to assign its value to xold.

I did not debug that error, because it started to feel like I was doing your homework for you.

[Walter Roberson]

The function is exp(x)-x.^2+3*x-2.

Okay. Did you create an anonymous function named func ? Or did you create a func.m file? Or did you include a function func block at the end of the code you posted?

[Kameron]

That must be the issue, how could I include a function block at the end?

[the cyclist]

I already showed you how to do this. Include this line before the first time you use func:

func = @(x) exp(x)-x.^2+3*x-2

Answer 1

[Star Strider]

It’s likely not an error, simply a suggestion that preallocating for speed would be advisable. (A preallocated loop generally requires about 20% less time compared to not preallocating.) It is most likely to appear with respect to a loop that stores values. The only one that I can see where that would work is:

for i=1:10

fval(i) = func(xval(i));

end

because it has a fixed number of iterations.

It could also work in the while loop because you are storing ‘z’, however unless you know about how many iterations that loop would require, preallocating would simply be a guess. One way to do that in the while loop would be to preallocate with perhaps:

z = NaN(1,1000);

and when the loop finishes, use the rmmissing function to remove any remaining NaN values.

x = [1 2 3 4 5 NaN NaN]

x = 1×7

1 2 3 4 5 NaN NaN

x = rmmissing(x)

x = 1×5

1 2 3 4 5

This assumes that the calculation of ‘x’ never produces a NaN value.

The preallocation statements would be placed just before their respective loops.

.