how to solve " Undefined function 'lt' for input arguments of type 'tf'. Error in ftfs (line 5) while (t < tf) ?"

the function is :

function []= ftfs()
j = 1;
t = 0;

while (t < tf)
    u(1,j+1) = u(1,j) - (lambda*a).*(u(2,j)-u(1,j));
    u(N,j+1)= u(1,j+1);
    u(2:(N-1),j+1) = u(2:(N-1),j) - (lambda*a).*(u(3:N,j)-u(2:(N-1),j));
    t = t + dt;
    j = j + 1;
end

plot(x,u(:,i),'m-..-')
end

I am calling this function in this program :

a = input('Insert the value of a: ');
CFL = input('Insert the value of CFL: ');
tf = input('Insert final time : '); %final time
dx = input('Insert the value of dx: ');
dt = (CFL * dx)/ abs(a);
lambda = (dt/dx)
N = input('Insert the value of N: ');
x = linspace(-1,1,N);
%choose test case:
tc= input('choose test case number : ');
if tc == 1 
    u(:,1)= -sin(pi.*x)

end

plot(x,u(:,1),'k')
hold on
ftfs();
hold on

Answer 1

[OCDER]

The problem lies in understanding the scope of each variable used in Matlab. Here's a read on that: http://matlab.izmiran.ru/help/techdoc/matlab_prog/ch12_nd5.html

Each function is like a new workspace, where it does NOT see the variables made OUTSIDE the function, unless passed as an input, used by a nested function, or used as global variables.

You did not specify what "tf" is in ftfs. So this while loop has no idea what the conditions are. This is what the function is seeing:

while (t < tf) translates to:
while t is less than an undefined function called tf (or a transfer function with no inputs)

To fix this, you need ftfs to take inputs.

function [] = ftfs(a, CFL, tf, dt, lambda, N, x, u)

And then you need to pass the variables into the function when you call ftfs.

hold on
ftfs(a, CFL, tf, dt, lambda, N, x, u);
hold on

Note that you will get an error if tc is unequal to 1, since then u is undefined. Also, your ftfs function does not know what i is, so you'll get another error.

[guetty abboud]

is there any way that i can input all the variables once ?

( i mean i did ask for them at the beginning of the program. so is there a way where i may not enter them in the function manually ?) and you're right i did encountered the errors you noted about.

can you help me and show me how to solve them ?

[guetty abboud]

using :

function [] = ftfs(a, CFL, tf, dt, lambda, N, x, u)

it gave me the following error:

Error using ftfs
Too many input arguments.

[OCDER]

You can place ftfs as a nested function in the main function. Otherwise, you have to just type out the full input. It shouldn't be too bad as you only have to write it out once as I've shown you.

The nested function method:

function mainFunc

a = input('Insert the value of a: ');
CFL = input('Insert the value of CFL: ');
tf = input('Insert final time : '); %final time
dx = input('Insert the value of dx: ');
dt = (CFL * dx)/ abs(a);
lambda = (dt/dx)
N = input('Insert the value of N: ');
x = linspace(-1,1,N);
%choose test case:
tc= input('choose test case number : ');
if tc == 1 
    u(:,1)= -sin(pi.*x)
end
plot(x,u(:,1),'k')
hold on
ftfs();
hold on

    %NESTED FUNCTION ---------------------------------------------
    function []= ftfs()
    j = 1;
    t = 0;
    while (t < tf)
        u(1,j+1) = u(1,j) - (lambda*a).*(u(2,j)-u(1,j));
        u(N,j+1)= u(1,j+1);
        u(2:(N-1),j+1) = u(2:(N-1),j) - (lambda*a).*(u(3:N,j)-u(2:(N-1),j));
        t = t + dt;
        j = j + 1;
    end
    plot(x,u(:,1),'m-..-')
    end  %End of nested function ---------------------------------

end

[OCDER]

For the too many input argument problem, are you modifying line 1 of ftfs?

function []= ftfs(a, tf, dt, lambda, N, x, u)
j = 1;
t = 0;
while (t < tf)
    u(1,j+1) = u(1,j) - (lambda*a).*(u(2,j)-u(1,j));
    u(N,j+1)= u(1,j+1);
    u(2:(N-1),j+1) = u(2:(N-1),j) - (lambda*a).*(u(3:N,j)-u(2:(N-1),j));
    t = t + dt;
    j = j + 1;
end
plot(x,u(:,i),'m-..-')
end

Your script is then:

a = input('Insert the value of a: ');
CFL = input('Insert the value of CFL: ');
tf = input('Insert final time : '); %final time
dx = input('Insert the value of dx: ');
dt = (CFL * dx)/ abs(a);
lambda = (dt/dx);
N = input('Insert the value of N: ');
x = linspace(-1,1,N);
%choose test case:
tc= input('choose test case number : ');
if tc == 1 
    u(:,1)= -sin(pi.*x);
else
    u = sin(pi.*x); %WHAT should be the default???
end
plot(x,u(:,1),'k')
hold on
ftfs(a, tf, dt, lambda, N, x, u);
hold on

[guetty abboud]

thanks it worked very well

[OCDER]

You're welcome!