I have the following function that I wish to solve using fzero:
f = lambda* exp(lambda^2)* erfc(lambda) - frac {C (T_m - T_i)}/{L_f*sqrt(pi)}
Here, C, T_m, T_i, and L_f are all input by the user.
On trying to solve using fzero, MATLAB gives the following error.
Undefined function or variable 'X'.
(where X are the variables stated above)
This error is understandable. But is there a way around it? How do I solve this?
This is answered to the best of my understanding after reading your question as it's not really clear what you are exactly trying and what you want exactly.
Posting the exact lines of code helps a big deal in understanding(as clean as possible, remove clutter). If then the output that matlab gives is added it becomes a whole lot easier to make sure we answer your question properly and it allows us to try it out. Usually it's a good idea to give some example values for data that is to be entered by the user anyway.
First of to make it a function it either needs a handle.
Or if you have it saved it as a matlab file you generally do not want other inputs in your m file then the variable.
So,
function [out]=yourfun(in)
constants=your values; %you can set a input or inputdlg to get a value from the user
out= something something, your lambda thingy probably; %this is the equation/function you're solving for
end
Now since that is not all that convenient I suggest the following
%declare or get your constants here, above the function makes it easier
syms lambda
f = lambda* exp(lambda^2)* erfc(lambda) - frac {C (T_m - T_i)}/{L_f*sqrt(pi)};
hf=matlabFunction(f); %this way matlab automatically converts it to a function handle, alternatively put #(lambda) in front
fzero(hf,x0)
Also this matlab page might help you as well ;)
Related
The image shows my problem (link)
I want to use that to do a Jacobian for a Multivariable method of aproximation. So There is a way to use that easily? or I have to put manually the partial derivatives like new functions?
I'm sorry for my english.
Firstly, you dont have to use images here, you can easily insert code pieces. As for your problem, i didnt understand it. Im sure you already know that you can define a Jacobian matrix function of 2 variables:
j2_fun(f1,f2):=[[derivative(f1,x),derivative(f1,y)][derivative(f2,x),derivative(f2,y)]]
... and call it later with specific arguments, and evaluate for some x,y:
g1:=x^2+y^2
g2:=x^2-y^2
d2:=j2_fun(g1,g2)
d2|x=3 and y=1
So staring from here, please define more clearly what would you like to do with this.
So Im trying to calculate how to compute Newtons Forward Difference method for a unit in uni, and its the last question on this group assignment I need to complete.
It's hard to explain really what it does without writing a couple paragraphs, but it generally interpolates polynomial functions, and returns a function, in other words you provide some X and Y coordinates you know this function should pass through, and it will return some function that really closely approximates the actual function, and does pass through those points. So really useful stuff.
Anyway...
I have a massive function that this all gets plugged in to, but what Im having trouble with is returning a function from a product series.
The series is something like...
s(s-1)(s-2)...(s-n+1) %Not coded obviously
Where,
s = #(x) (x - X(1))/h;
So s is a function of x, and Im trying to get the function of that series in terms of x. Any help would be highly appreciated.
My attempt is this:
func = #(x) Y(1) + symprod(s*(s-n+1)) ./ factorial(n);
Includes a few extra things I havn't mentioned, but it still involves my attempt at getting a function in terms of s for the product series...
n is just an integer, known value.
Even after using isolate function as suggested below, still no result. Not really sure what to do, someone hellpp
Managed to figure this out, so I never actually ended up getting the above method to work, wherein I was trying to get a function from a bunch of variables, and an infinite product series, I ended up just using a for loop to simulate the product infinite series, got that value, and then when I wanted a function I simply wrote f = #(x) value_from_function(x).
For an example of some working code, I uploaded it on my GitHub...
So, in that example I would do
f = #(x) interpolating_polynomial(X, Y, x);
I want to make symbolic functions theta1(t), theta2(t), theta3(t),...,thetaN(t) where N is some parameter I can define in MATLAB. I know that I can use something like sym('theta',[1 N]) to get [theta1, theta2, theta3,..., thetaN]. However, how can I do the same thing with theta being a function of t? The way to hard-code it would be like syms theta1(t) theta2(t) theta3(t) ... thetaN(t), but I want to make this general.
I do not want to directly use the sym command here because "support of character vectors that are not valid variable names and do not define a number will be removed in a future release", meaning something like sym('theta1(t)') would not be valid in future releases.
Any suggestions?
Figured part of it out. I could do something like the following
for i = 1:N
syms(strcat('theta',num2str(i),'(t)'))
end
However, if I want to assign a variable that contains all the symbolic expressions I'm still stuck. If I try
for i = 1:N
my_array(i) = syms(strcat('theta',num2str(i),'(t)'))
end
I get Error using syms (line 133). Using input and output arguments simultaneously is not supported. It works if I use sym instead of syms, but this leads to the warning I mentioned in my original post.
I am trying to run code similar to the following, I replaced the function I had with one much smaller, to provide a minimum working example:
clear
syms k m
n=2;
symsum(symsum(k*m,m,0,min(k,n-k)),k,0,n)
I receive the following error message:
"Error using sym/min (line 86)
Input arguments must be convertible to floating-point numbers."
I think this means that the min function cannot be used with symbolic arguments. However, I was hoping that MATLAB would be substituting in actual numbers through its iterations of k=0:n.
Is there a way to get this to work? Any help much appreciated. So far I the most relevant page I found was here, but I am somewhat hesitant as I find it difficult to understand what this function does.
EDIT following #horchler, I messed around putting it in various places to try and make it work, and this one did:
clear
syms k m
n=2;
symsum(symsum(k*m,m,0,feval(symengine, 'min', k,n-k)),k,0,n)
Because I do not really understand this feval function, I was curious to whether there was a better, perhaps more commonly-used solution. Although it is a different function, there are many pieces online advising against the eval function, for example. I thought perhaps this one may also carry issues.
I agree that Matlab should be able to solve this as you expect, even though the documentation is clear that it won't.
Why the issue occurs
The problem is due the inner symbolic summation, and the min function itself, being evaluated first:
symsum(k*m,m,0,min(k,n-k))
In this case, the input arguments to sym/min are not "convertible to floating-point numbers" as k is a symbolic variable. It is only after you wrap the above in another symbolic summation that k becomes clearly defined and could conceivably be reduced to numbers, but the inner expression has already generated an error so it's too late.
I think that it's a poor choice for sym/min to return an error. Rather, it should just return itself. This is what the sym/int function does when it can't evaluate an integral symbolically or numerically. MuPAD (see below) and Mathematica 10 also do something like this as well for their min functions.
About the workaround
This directly calls a MuPAD's min function. Calling MuPAD functions from Matlab is discussed in more detail in this article from The MathWorks.
If you like, you can wrap it in a function or an anonymous function to make calling it cleaner, e.g.:
symmin = #(x,y)feval(symengine,'min',x,y);
Then, you code would simply be:
syms k m
n = 2;
symsum(symsum(k*m,m,0,symmin(k,n-k)),k,0,n)
If you look at the code for sym/min in the Symbolic Math toolbox (type edit sym/min in your Command Window), you'll see that it's based on a different function: symobj::maxmin. I don't know why it doesn't just call MuPAD's min, other than performance reasons perhaps. You might consider filing a service request with The MathWorks to ask about this issue.
I am not a very hardcore coder in MATLAB, i have learned every thing from youtube and books. This might be a very simple question but i do not know what search for answer.
In MATLAB i am trying to do something like this.
>>[a,b,c] = [1,2,3]
and i want output like this.
>> a = 1
b = 2
c = 3
So Bsically question is that : - User will define the matrix of variables([a,b,c]) in staring of code and during process of the code similar matrix will be displayed and as input a matrix will be asked([1,2,3]). I dont know how do this without writing a loop code in which i will take every single variable from variable matrix and save the value in that variable by eval function.
well above written code is wrong and i know, i can do this with "for" loop and "eval" function.
but problem is that no. of variables(a,b,c) will never be constant and i want know if there exist any in built function or method in MATLAB which will work better than a for loop.
As i told earlier i don't know what to search for such a problem and either this is a very common question.
Either way i will be happy if you can at least tell me what to search or redirect me to a related question.
Please do write if you want any more information or for any correction.
Thank you.
The deal function can do this for a fixed number of inputs:
[A,B,C]=deal(1,2,3)
If you don't know how many inputs you will get beforehand, you have to do some fooling around. This is what I've come up with:
V=[1,2,3,4,5,6,7]
if length(V)>1
for i=1:length(V)
S{i}=['A' num2str(i)];
G{i}=['V(' num2str(i) ')'];
end
T=[S{1} ','];
R=[G{1} ','];
for i=2:length(V)-1
T=[T S{i} ','];
R=[R G{i} ','];
end
T=[T S{length(V)}];
R=[R G{length(V)}];
eval(['[' T ']=deal(' R ')'])
else
A1=V
end
But then dealing with A1, ... , An when you don't know how many there are will be a pain!
This is somehow known as "tuple unpacking" (at least it's what I would search in python!). I could find this thread which explains that you could do this in Octave (I checked and it works in Matlab also). You have to transform the vector into a cell array before:
values = num2cell([1,2,3])
[a,b,c] = values{:}