Forward differencing, error when plotting u(x_i)!

Guys,

I know this must be a reallllly trivial error, but I seriously cannot see what is going on. I am purely plotting 100 elements against another 100 elements and I am getting the following error:

??? Subscript indices must either be real positive integers or logicals.

Error in ==> Test at 22 plot(x,u);

The code I am using is below (would really appreciate some help in this because it's driving me crazy; usually i am good at debugging!):

function Test(n,f,length_check,plot)

% Create grid:
x_min = 1;
x_max = 11;
dx = (x_max - x_min)/(n-1);
x(1) = x_min;
for i = 1:(n-1); 
    x(i+1) = x(i) + dx; 
end
x(n) = [];

% Compute u(x):
i = 1:(n-1);
u(i)= f(x(i));
if (length(x)==length(u)); 
    'Vector lengths agree' 
end
%figure;
length(x)
length(u)
plot(x,u);
axis([0,12,-0.1,0.4]);
xlabel('x'); ylabel('u');
grid on;

CHEERS FOR THE HELP GUYS!

Andrew.

[the cyclist]

It would be helpful if you used the "Code" button to format your code into a more readable form.

[the cyclist]

Also, it looks like you haven't defined "n" or the function "f" for us, so I don't think we can run your code to look for errors.

Answer 1

[Jan]

 ??? Subscript indices must either be real positive integers or logicals.
 Error in ==> Test at 22 plot(x,u);

The you have defined a variable called "plot". You can check this by using the debugger:

dbstop if error

Then start your program until it stops at the error. Now user this to check the type of "plot":

which plot
whos plot

Solution: Do not use names of builtin functions for variables.

[Andrew Sanderson]

OH! Of course... Thanks so much for this help. And for the other guys; sorry for the unreadable code, this was my first ever post on the matlab forums :)

[Jan]

You do not have to apologize. Simple edit the original question, mark the code and press the "{} code" button. I've done this for you this time.

Answer 2

[Andrew Sanderson]

function Development_Task_1(n,f,length_check,plot_check)

% Create grid:
x_min = 1;
x_max = 11;
dx = (x_max - x_min)/(n-1);
x(1) = x_min;
for i = 1:(n-1); 
    x(i+1) = x(i) + dx; 
end
x(n) = [];

% Compute u(x):
i = 1:(n-1);
u(i)= f(x(i));
if (length(x)==length(u)); 
    'Vector lengths agree' 
end
%figure;
length(x)
length(u)
plot(x,u);
axis([0,12,-0.1,0.4]);
xlabel('x'); ylabel('u');
grid on;

% We must compute:
%     
%      du     u(i+1) - u(i)
%      --  =  -------------
%      dx          dx
%
% Using the method of forward differencing;

i = 2 : (n-1);
dudx(i) = (u(i) - u(i-1)) / dx;
dudx(n-1) = [];
figure;
plot(x,dudx);

if (plot_check == 1)

if (length_check == 1)
    fig_ = figure;
    lengths_ = [length(x), length(u), length(dudx)];
    names_ = {'x','u(x)','u''(x)'};
    uitable(fig_, 'Data', lengths_, 'ColumnName', names_);
end
end

Sorry after changing the input to plot_check I am still getting the same error.

The inputs I have been using are;

n = 100; f = @(x) exp(-x);

Thanks,

Andrew.

[Andrew Sanderson]

EDIT: Sorry, I am having an absolute nightmare with matlab today. The above answer did work, apologies for the stupidity and cheers for all the help!