Anonymous Function

[EDIT: 20110623 22:10 CDT - reformat - WDR]

I am having considerable difficulty with the program below. My goal is to be able to enter or change the Current Density (J) in the integral as an anonymous function. When j = 2*cos(theta)/r^3, the result should be about 31.4. Are there any other methods that would allow me to enter J inot the integral?

When I run I get the following error message:

??? Undefined function or method 'mtimes' for input arguments of type 'function_handle'.

When I swap the "%" on the operator inside the loops I get a 31.4, a good result.

clc
clear  
close
r = 0.2;
dt = .1;
dp = .1;
jinput =  input('Enter an expression for the Current Density (J)... > ','s') 
jfactor = eval(['@(r,theta,phi)' jinput]);
% set initial total sum to zero
sum1 = 0; 
% outer integral loop
for theta = 0:dt:pi/2
    % inner integral loop 
    for phi = 0:dp:2*pi
% add the partial sums to the total sum

sum1 = sum1 + jfactor*r^2*sin(theta)*dt*dp; 
%sum1 = sum1 + 2*cos(theta)/r^3*r^2*sin(theta)*dt*dp; 
    end 
end 
% display the output  
fprintf('>The total current through the defined spherical shell is %g  A.\n', sum1)

[Steven Lord]

%{
jfactor = eval(['@(r,theta,phi)' jinput]);
%}

Don't use eval. Use str2func. [I've commented out the code above so the code below can run.] Since the input function isn't supported when running code in MATLAB Answers I've hard-coded the body of the function below.

jinput = 'r+theta.^phi';
jfactor = str2func(['@(r, theta, phi) ', jinput])

jfactor = function_handle with value:
    @(r,theta,phi)r+theta.^phi

% check
jfactor(1, 2, 3) % 1+2^3 = 9

ans = 9

Answer 1

[Walter Roberson]

jfactor becomes a function, and that function needs to be passed arguments.

sum1 = sum1 + jfactor(r,theta,phi)*r^2*sin(theta)*dt*dp;

Answer 2

[Jerry Walker]

Thanks, I made the change and the program works as expected.

Answer 3

[Siphumelele Vatsha]

sum1 = sum1 + jfactor(r,theta,phi)*r^2*sin(theta)*dt*dp;