Not enough input arguments Error in bvpfcn (line 5) dydx = [y(2)*(U*y​(2)+K*y(1)​)/D];

Error in bvpfcn (line 5) dydx = [y(2)*(U*y(2)+K*y(1))/D];

[Jan]

Please post the relevant part of the code and a copy of the complete error message.

[Naveen Krish]

r = bvp4c(ode,bc,solinit);

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

function g = guess(x)

guess = [2;1];

xmesh = linspace(0,1,5);

solinit = bvpinit(xmesh,guess);

end

[Naveen Krish]

Not enough input arguments.

Error in bvpfcn>ode (line 10)

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

Error in bvpfcn (line 1)

r = bvp4c(ode,bc,solinit);

[Naveen Krish]

function dydx = ode(y,x) % equation to solve
y1 = y(1);
y2= y(2);
dy1_dx = y(2);
dy2_dx = [U*y2+K*y1/D];
dy_dx = [dy_dx; dy2_dx];
end

[Torsten]

dy_dx = [dy1_dx; dy2_dx]

instead of

dy_dx = [dy_dx; dy2_dx]

Answer 1

[Jan]

r = bvp4c(ode,bc,solinit);
%         ^^^

This is a call of the function ode() without input arguments. You want to provide a function handle instead:

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

This will fail also:

function dydx = ode(y,x) % equation to solve
y1 = y(1);
y2= y(2);
dy1_dx = y(2);
dy2_dx = [U*y2+K*y1/D];
dy_dx = [dy_dx; dy2_dx];
end

The function replies "dydx", but you define "dy_dx". Easier:

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

Remember that 1*10^-6 is a multiplication and an expensive power operation, while 1e-6 is a cheap constant.

[Naveen Krish]

How to provide a function handle instead?

[Naveen Krish]

what is the input arguments for ode function for this derivative D* d^2C/dx^2 - U* dC/dx - kC =0

[Torsten]

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

Note the brackets around U*y(2)+k*y(1) !

[Jan]

@Naveen Krish: "How to provide a function handle instead?" - did you read my answer? I've posted the code to show, where the @ has to be inserted and marked it in addition with the ^ characters.

"what is the input arguments for ode function for this derivative D* d^2C/dx^2 - U* dC/dx - kC =0" - the question is not meaningful. You can use what ever you want as input argument. The initial value can be chosen freely.

[Naveen Krish]

>> bvpfcn

Not enough input arguments.

Error in bvpfcn>bc (line 14)

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

Error in bvpfcn (line 1)

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

[Walter Roberson]

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

[Naveen Krish]

Unrecognized function or variable 'solinit'.

Error in bvpfcn (line 1)

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

[Walter Roberson]

You do not appear to have assigned anything to solinit, such as by using https://www.mathworks.com/help/matlab/ref/bvpinit.html