File: bvpfcn.m Line: 1 Column: 23 Invalid use of operator.

Hi, I'm doing a project on chlorine decay in water distribution pipes, to solve this ODE with the following Matlab's BVP4C function . I have wrriten the code but I need to plot the profile C vs x. I have tried searching in MATLAB Answers pertaining to my topics of study, but to no avail. The MATLAB code and the error message are shown below.

At steady state, this can be described by: 𝐷*𝑑^2𝐶/𝑑𝑥^2 − 𝑈*𝑑𝐶/𝑑𝑥 − 𝐾𝐶 = 0 where, 𝐶 is the chlorine concentration (mg/L) at any location 𝑥 (m) along the pipe, 𝐷 is the diffusion coefficient (m2 /s), 𝑈 is the flow velocity in the pipe (m/s), and 𝐾 is the first order decay coefficient (s-1 ). To solve the aforementioned second order ODE, two boundary conditions must be known at the inlet and the outlet of the pipe (i.e., 𝐶 = 𝐶𝑖𝑛 @ 𝑥 = 0, and 𝐶 = 𝐶𝑜𝑢𝑡 @ 𝑥 = 𝐿, where L is the pipe length).I have also attached the code/data for your convenience. I appreciate your help with troubleshooting the problem.Thanks for considering my request.

function dydx = bvp4c(@ode,@bc,solinit); % equation to solve
plot(r.x,r.y(1,:),'--o');
title('chlorine decay')
xlabel('pipe length(m)');
ylabel('Concentration (mg/L)');
function dydx = ode(x,y)
D=0.1; % m2/s
U=1; % m/s
K = 1*10^-6; % 1/s
dydx = [y(2);(U*y(2)+k*y(1))/D];
end
    function res = bcfcn(ya,yb) %boundary conditions
res = [ya(1)-3 yb(1)-2];
    end
function g = guess (x) % initial guess for y and y'
guess = [2;1];
xmesh = linspace(0,1,5);
solinit = bvpinit(xmesh,guess);

Answer 1

[Steven Lord]

When you define a function you need to specify the input arguments as the names of the variables in which the user's input will be stored in the function. You cannot specify them as expressions.

When you call a function you need to pass in an expression. Specifying an unknown variable name as an input argument will not work.

You also don't want to call your function bvp4c since there's already a function by that name in MATLAB.

y = mytimestwo(1:10) % Calling the function I can pass in an expression

y = 1×10

2 4 6 8 10 12 14 16 18 20

y = mytimestwo(z) % Calling the function I cannot pass in an unknown name

Unrecognized function or variable 'z'.

% Defining the function I specify a variable name, in this case theInput
%
% When I called this function as y = mytimestwo(1:10) above MATLAB will
% assign 1:10 to the variable theInput inside that mytimestwo call.
function theOutput = mytimestwo(theInput)
theOutput = 2*theInput;
end

% Were I to define a function like below it would error for the same
% reason your function definition would error
%{
function theOutput = mytimestwo(1:10) % Expressions not allowed here
theOutput = 2*theInput;
end
%}

Answer 2

[Torsten]

In principle, this is a direct copy of the first example for bvp4c in the MATLAB documentation.

You only had to give different values to the variables.

function main
  xmesh = linspace(0,1,10);
  solinit = bvpinit(xmesh, @guess);
  sol = bvp4c(@bvpfcn, @bcfcn, solinit);
  plot(sol.x, sol.y, '-o')
end
function dydx = bvpfcn(x,y)
  D = 0.1; % m2/s
  U = 1; % m/s
  K = 1*10^-6; % 1/s
  dydx = [y(2);(U*y(2)+K*y(1))/D];
end
function res = bcfcn(ya,yb)
  res = [ya(1)-3
         yb(1)-2];
end
function g = guess(x)
  g = [3-x;-1];  
end

[Naveen Krish]

different value to the variables like how?

[Torsten]

In principle, this is a direct copy of the first example for bvp4c in the MATLAB documentation.

You only had to give different values to the variables.

It means if you had taken the time to study the first example from the documentation of MATLAB's bvp4c, it would have taken you 5 minutes to make the appropriate changes to turn it into your problem from above.

[Naveen Krish]

function dydx = ode(y, x)

D = 0.1; % m2/s

U = 1; % m/s

K = 1e-6; % 1/s

dydx = [y(2)(U * y(2) + K * y(1)) / D]; % EDITED, parentheses added

end

function dydx = bvp4c(@ode,@bc,solinit); % equation to solve

plot(r.x,r.y(1,:),'--o');

title('chlorine decay')

xlabel('pipe length(m)');

ylabel('Concentration (mg/L)');

function dydx = ode(x,y)

D=0.1; % m2/s

U=1; % m/s

K = 1*e-6; % 1/s

dydx = [y(2);(U*y(2)+k*y(1))/D];

end

function res = bcfcn(ya,yb) %boundary conditions

res = [ya(1)-3 yb(1)-2];

end

function g = guess (x) % initial guess for y and y'

guess = [2;1];

xmesh = linspace(0,1,5);

solinit = bvpinit(xmesh,guess);

[Torsten]

Yes, that's the problem I gave you the corrected code for.

[Naveen Krish]

I'm baffled here, I'm not sure what to do next can you help me out?

File: bvpfcn.m Line: 7 Column: 23

Invalid use of operator.

[Naveen Krish]

Error using bvp4c

Too many input arguments.

Error in bvpfun (line 4)

sol = bvp4c(@bvpfcn, @bcfcn, solinit);

[Torsten]

Then you have to put your function with the same name into a different directory.

[Naveen Krish]

I put my function in the same name into a different category still the error shows like this

>> bvpfun

Error using bvp4c

Too many input arguments.

Error in bvpfun (line 4)

sol = bvp4c(@bvpfcn, @bcfcn, solinit);

[Walter Roberson]

In an earlier posting, you showed that you had defined your own function named bvp4c . Make sure that bvp4c refers to the Mathworks function, not to your own function.

which -all bvp4c

[Naveen Krish]

I'm only using the corrected code given by torsten above. I haven't defined any function now

[Naveen Krish]

Can you able to rectify it in the above code what am i missing?

[Walter Roberson]

What shows up for

which -all bpv4c

[Torsten]

Name the file main.m, start MATLAB anew, change to a directory with only this file main.m in, load main.m and execute it.

[Naveen Krish]

I did like you told @Torsten but still getting the same error

main

Error using bvp4c

Too many input arguments.

Error in main (line 4)

sol = bvp4c(@bvpfcn, @bcfcn, solinit);

[Naveen Krish]

Did you tried this code and got the plot?

[Torsten]

No, I didn't. But it should produce no errors, as far as I can see.

Try a code from the MATLAB examples for bvp4c.

[Naveen Krish]

Can you please check and see

[Torsten]

I have MATLAB not available. Check a problem from the examples.

[Walter Roberson]

The first line of your code of this very post is

function dydx = bvp4c(@ode,@bc,solinit); % equation to solve

which tries to define your own bvp4c . You cannot do that: you need to rely on MATLAB's bvp4c .

Again, we need you to tell us what you see when you command

which -all bvp4c

[Naveen Krish]

how to do that with Matlab's bvp4c?

[Naveen Krish]

Can you rectify in my code? what you trying to say?

[Naveen Krish]

@Walter Roberson

I asked already about this to you before,

Unrecognized function or variable 'solinit'.

Error in bvp4c (line 1)

r = bvp4c(@ode,@bc,solinit);

r = bvp4c(@ode,@bc,solinit);

solinit = bvpinit(xmesh,guess);

guess = [2;1];

xmesh = linspace(0,1,5);

plot(r.x,r.y(1,:),'--o');

title('chlorine decay')

xlabel('pipe length(m)');

ylabel('Concentration (mg/L)');

function dydx = ode(x,y)

D=0.1; % m2/s

U=1; % m/s

K = 1*10^-6; % 1/s

dydx = [y(2)*(U*y(2)+K*y(1))/D];

end

function res = bc(ya,yb)

res = [ya(1)-3 yb(1)-2];

end

[Naveen Krish]

Can you please rectify where i did wrong? and kindly plot C vs x. I already spend too much time on this problem

[Walter Roberson]

You have to assign to solinit before you use it.

solinit = bvpinit(xmesh,guess);
r = bvp4c(@ode,@bc,solinit);