In Octave 4.0.2 I have defined a function S as follows:
S = #(x) (Y(k)+((Y(k+1)-Y(k))/h(k)-(2*M(k)+M(k+1))*h(k)/6)*(x-X(k))+M(k)*(k-X(k))^2/2+(M(k+1)-M(k))*(x-X(k))^3/(6*h(k)))
When I call it to evaluate a number in the interval [X(k), X(k+1)] I get the result I expect, but when I try plotting it with the command:
fplot(S, [X(k), X(k+1)]); hold on;
I get the error "error: for A^b, A must be a square matrix. Use .^ for elementwise power."
What is going on?
Alright, I figured out that when passing a function to fplot it must be able to take a vector of inputs and return a vector of outputs.
Related
I use this code and i don't know what it needs to work for my problem:
syms x k t
for t=0:10
num=((-1)^k)/k
t1=sin(8*3.1415*k*t)
S1=symsum((num*t1),k,1,2);
x=0.5-((1/3.1415)*S1);
end
Plot(x)
On the x axis I show time and on the y axis I show the function over four periods.
When I try to run the code I get the following error:
Undefined function 'symsum' for input arguments of type 'double'.
Maybe I can't use symsum with my argument type, but is there another function I can use? Sum also didn't work:
Error using sum Dimension argument must be a positive integer scalar within indexing range.
Since you want to plot x(t), you need to use plot(t,x) where t and x are vectors.
Instead of using for t=0:10, just let t=0:10 and calculate the corresponding x.
Also, the symbolic variable is just k.
syms k
t=0:10;
num=((-1)^k)/k;
t1=sin(8*3.1415*k*t);
S1=symsum((num*t1),k,1,2);
x=0.5-((1/3.1415)*S1);
plot(t,x)
It is noted that if you let t=0:10, then the sin(8*k*pi*t) will always be 0 since t is a vector of the integer from 0 to 10. The result of x(t) will be 5:
Output when t=0:10:
As you can see, the value of x(t) is very close to each other. Theoretically, they should all be 5. But there is some numerical approximation which leads to the small error.
You probably want non-integer t. Here is a output when t=0:0.1:10
I'm trying to ezplot this function(f(x)= e^-2t cost t=[-20,20]), and I guess i'm missing the syntax or something.
t=[-20:20]
x= e^-2*t,cos(t)
ezplot(t,x)
but it bringing out an error
you're mixing between x-y plot and function plot, plus you have several syntax errors.
First, t is a 41 elements vector between -21 and 21.
Second, unless you predefined a variable e then e^(-2*t) will give you Undefined function or variable 'e'. error. for exponential function just use exp(-2*t).
Third, assume you fixed the syntax errors such that x = exp(-2*t).*cos(t);, then x is also a 41 elements vector, and therefore you can simply plot it using plot(t,x). ezplot (or fplot in newer versions) is used for plotting functions (rather than vectors). If you want to plot the function in the [-21,21] interval do something like:
f = #(t) exp(-2*t).*cos(t); % this is a function handle
ezplot(f,[-20 20]) % use ezplot with function handle and t interval
I am using nonlinearfit tool in matlab.
I keep getting the following error:
Error using nlinfit (line 210)
MODELFUN must be a function that returns a vector of fitted values the same size as Y (1-by-100). The model function you provided returned a result that was 1-by-2.
One common reason for a size mismatch is using matrix operators (, /, ^) in your function instead of the corresponding element-wise operators (., ./, .^).
I found this question very similar to mine, but still I get the same error. I have tried calculating myfun on the console while using a vector as an input, which gives me output of correct dimension. It will be ton of help if anybody can point out the mistake.
% Defining the function
myfun = #(t,b)exp(t.*b(1)+b(2));
[y_a] = arrayfun(myfun,x_a);
% Using nonlinear least square minimization
beta0 = [1 1];
nlinfit(x,y,myfun, beta0)
Thanks in advance...:)
Edit: Found this to be working.
g = fittype('exp(k*x + a)');
[fit1,gof,fitinfo] = fit(x',y',g,'StartPoint',[1 1]);
The function used in nlinfit takes the parameter vector as its first argument, then the independent data vector. You want,
myfun = #(b,t)exp(t.*b(1)+b(2));
Note that you could just use * rather than .* in this instance too.
i am trying to plot two function in matlab, the first one is of kinf symfun:
p = symfun(0, [m]);
p(m) = p(m)+Ck(k-3)*exp(m*(k-3)*complex(0, 2*pi/25));
here Ck is another symfun and k is a variable i pre-defined.
i want to plot it in the same graph with a function i created using the function mode:
function [x1] = xt_otot_q3( t)...
i cant make the xt_otot_q3 function a symfun because it involves if statements.
- i tried to create 2 vectors sampling the two functions and plotting them together with the plot function but for some reason the 'p' function vectors gets preatty grotesque giving me wierd output...
- i tried plotting them both using ezplot function but for some reason the sampled vector i got form xt_otot_q3 shows only as a straight line at 0.
any ideas how i should plot them together? to plot the xt_otot_q3 function i must create a vector if i try to plot it directly using ezplot it gives me the following eror:
>> ezplot(xt_otot_q3, [-10 10])
Error using xt_otot_q3 (line 2)
Not enough input arguments.
thanks in advance.
If I am understanding it properly, you have two functions p, and xt_otot_q3. You want to plot them together.
syms t;
func1 = xt_otot_q3(t);
ezplot(func1, [-10 10]);
# retain current graph, for new graph
hold on;
# symbolic function p
ezplot(p, [-10 10]);
I hope it helps.
I am getting this error while running my code:
Error using ==> mldivide Matrix dimensions must agree.
Here is my code :
%make the plots of phase and group velocity vs discreteness of the grid
c=1;
a=input('Please enter the ratio cdt/dx : ')
figure(1)
R=2:40;
plot(R,phase_vel(R,a)/c)
xlabel('R=l/dx')
ylabel('u_phase/c')
%figure(2)
%plot(group_vel(R,a),R,0,40)
%xlabel('R=l/dx')
%ylabel('u_group/c')
and here are my functions :
function phase_velocity = phase_vel(R,a)
%numerical phase velocity of the discrete wave
c=1;
phase_velocity=(2*pi*c)/(R*knum(R,a));
end
function group_velocity =group_vel(R,a )
%numerical group velocity of the discrete wave
c=1;
group_velocity=(a*sin(knum(R,a)))/(sin(2*pi*a/R))
end
function knumber = knum(R,a)
%This is the k wave number
knumber=acos((1/a)^2*(cos(2*pi*a/R)-1)+1);
end
How can I resolve this error?
EDIT: I used . operator in every equation and i changed the limits of R=4:40
If your goal is to apply your formulas to each individual value in the vector R then you should be performing all of your computations using the element-wise arithmetic operators .*, ./, and .^ instead of the matrix operators *, /, and ^.
Your error is probably occurring in the first call to your function knum, specifically when you try to compute 2*pi*a/R. Since 2*pi*a is a single scalar value, you get an error when trying to perform matrix right division / using the row vector R. The really weird thing is the error message:
??? Error using ==> mldivide
Matrix dimensions must agree.
which implies you are using the matrix left division operator \, which you clearly aren't. I tested this in MATLAB R2010b and I get the same incorrect function name appearing in my message. I think this may just be a typo in the error message, and I've dropped a note to the MATLAB folks to take a look at it and clear it up.
I don't have the Symbolic Math Toolbox, but your problem seems to be that you are using plot, a function which can deal with arrays of numbers, and feeding it the result of a symbolic calculation. Have a look at the Matlab Help, where the Topic Creating Plots of Symbolic Functions suggests using ezplot(). Alternatively you need to evaluate your symbolic expression for certain input values to create an array of numbers that plot can deal with - but you can't use double() for that since it wouldn't know what numbers to plug into your variables.