Forward differencing, error when plotting u(x_i)!

Question

Andrew Sanderson 2011-10-23

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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.

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the cyclist 2011-10-23

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

the cyclist 2011-10-23

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.

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Answer 1

Jan 2011-10-23

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在 MATLAB Online 中打开

 ??? 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.

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Andrew Sanderson 2011-10-23

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 2011-10-23

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.

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Answer 2

Andrew Sanderson 2011-10-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/19091-forward-differencing-error-when-plotting-u-x_i#answer_25507

在 MATLAB Online 中打开

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