I have a vector v and would like to define a function x using following formular:
Now I try to define this in Matlab, but for some reason I can't access an array within a symsum.
v = [1 2 3 4];
syms k
x = #(t) symsum(v(k) * exp(2*pi*i*k*t/size(v)), k, 1, size(x_n));
When I try to evaluate this Function x(4) I get an exception:
Error using sym/subsindex (line 796)
Invalid indexing or function definition. When defining a function, ensure that the arguments are
symbolic variables and the body of the function is a SYM expression. When indexing, the input
must be numeric, logical, or ':'.
Do you know, why? Do you have a solution or workaround?
I want to define a vector,X, with five elements:
syms a
X = zeros(1,5)
X(1) = 1;
for k=1:4
X(k+1)=X(k)+a^2;
end
Actually I need to have the vector X, that its elements should be based on variable a. But I face an error in Matlab, when I write the code above:
The following error occurred converting from sym to double:
Error using symengine (line 58)
DOUBLE cannot convert the input expression into a double array.
If the input expression contains a symbolic variable, use VPA.
Error in Code2 (line 5)
X(k+1)=X(k)*a^2;
How to fix this?
You are mixing between symbolic variables to doubles,
Doubles holds a value, you can use the loop that you wrote only if a is a double and already holds a value (the value can be the input of a function) .
for example :
function ret=testFunc(a)
X = zeros(1,5)
X(1) = 1;
for k=1:4
X(k+1)=X(k)+a^2;
end
ret=X
end
if you want to work with syms (for other symbolic analysis) you can define x as a symbol too, for example :
syms a x
x(1)=1;
for i=2:5
x(i)=x(i-1)+a.^2;
end
now, x is a function of a, and if you print x you will get :
[ 1, a^2 + 1, 2*a^2 + 1, 3*a^2 + 1, 4*a^2 + 1]
to evaluate the value of x, you will still need to input a value for a.
Matlab help suggest subs function for replacing a in wanted value as follows:
y = subs(x,a,4)
in that point y is still a symbol and you will need to make it a double by using
double(y)
I have a matrix of numeric data;
(let's say 4*4 )
how can I add a syms expression (e.g 3*s) in some elements of it???
I've tried to add it but I get this Error:
The following error occurred converting
from sym to double:
Error using mupadmex
Error in MuPAD command: DOUBLE cannot
convert the input expression into a
double array.
I need help figuring out how to code the following problem. Any help would be greatly appreciated!
Create a function that will take a vector/array input for x (1 by n) and a scalar input for a, and produce the output defined by the following equation:
y(x,a)=((xsin(ax-2))/(sqrt(1+(ax)^2)
-π ≤ x ≤ π
a={.5 1 1.5 2}
The equation must be vectorized in terms of x and the output from the function is the array y which has the same dimension as the array x.
Write a script that calls this function to compute y(x,a) for the range of x defined above and each value of the parameter a. Results should be stored in a solution matrix using a different row of the solution matrix for each value of a.
So far for my function I have:
function [y] = part1(a,x)
y=((x*sin(a*x-2))/(sqrt(1+(a*x).^2)));
end
I'm not sure how to output this into the solution matrix
For my script I have:
%%
clear,clc
a={0.5 1 1.5 2};
x=-pi:0.1:pi;
for
part1(x,a)
end
I'm getting the following errors when I run this now:
Undefined function 'mtimes' for input arguments of type 'cell'.
Error in part1 (line 4)
y=((x*sin(a*x-2))/(sqrt(1+(a*x).^2)));
Error in labtest2 (line 8)
y(i,:)=part1(x,a(i));
EDIT
I've made some changes and am still getting some errors that I cannot resolve.
Here is my full code for function followed by full code for script:
Function
function [y] = part1(x,a)
nx=numel(x);
na=numel(a);
y=((x.*sin(a.*x-2))./(sqrt(1+(a.*x).^2)));
size(y)=[nx na]
end
Script
%%
clear,clc
a={0.5 1 1.5 2};
x=-pi:0.1:pi;
for i = 1:length(a)
y(i,:)=part1(x,a(i));
end
Errors
Undefined function 'times' for input arguments of type 'cell'.
Error in part1 (line 6)
y=((x.*sin(a.*x-2))./(sqrt(1+(a.*x).^2)));
Error in labtest2 (line 8)
y(i,:)=part1(x,a(i));
The reason you're getting Undefined function 'times' for input arguments of type 'cell' is because your variable a is a cell array. You need to change your assignment of a from
a={0.5 1 1.5 2};
to
a=[0.5 1 1.5 2];
which will make it just a normal array. Alternatively, you need to reference it with cell array notation: a{i} instead of a(i).
You're almost there. Note that you've written
function [y] = part1(a,x)
but you call it in your script as
part1(x,a)
so you should probably correct that.
A few things jump out at me:
You never assign the output of part1(x,a) to anything. You're told that
Results should be stored in a solution matrix using a different row of the solution matrix for each value of a.
What I take this to mean is that the 1st row corresponds to part1() evaluated for the 1st element of a. Since we're operating on x which is a vector, that row will have multiple columns. Your output is indeed a matrix. In your case, length(-pi:0.1:pi) == 63, therefore size(y) == [4 63], where y is your output matrix.
Your for loop is backwards. You're told to accept scalar a and vector x. Therefore, your script should be something like:
a = 0.5:0.5:2;
x = -pi:0.1:pi;
for i = 1:length(a)
y(i,:) = part1(x, a(i));
end
Note the use of length and the : operator. I want to iterate between 1 to length(a) (in this case, length(a) == 4) so that I can use the current a(i) value as an index into my output matrix, y. The : operator in y(i,:) signifies "The ith row and all columns of y will take the value output by part1(x,a(i))."
Your function needs to be changed up for element-by-element operations. Notice that, for instance, x*sin(a*x-2) works for scalar x but not vectors. This is because x is a vector and sin(a*x-2) is also a vector (since the sin call will operate element-by-element and a is a scalar). Trying to multiply two vectors together will result in errors since MATLAB will try to perform a matrix multiplication. Resolve this by replacing * with .*. This way it is unambiguous that you are going to multiply these two vectors element-by-element. You'll also need to change / to ./.
On another note, thank you for attempting to do your homework before asking SO for help. We've been getting a huge influx of questions from students that have made no attempt to do their own work before dumping it on us, so it's refreshing that we regulars of the MATLAB tag get to actual help out instead of telling people to do their own work.
Finally got the whole thing worked out.
Function
function [y] = part1(x,a)
y=((x.*sin(a.*x - 2))./(sqrt(1 + (a.*x).^2)));
end
Script
%%
clear all;
clc;
close all;
x=[-pi:.1:pi];
a=[.5:.5:2];
for i=1:length(a)
y(i,:)=part1(x,a(i));
plot(x,y)
end
Sol=[y]
I want to insert 'double' type values in a matrix. For that I am creating a matrix with following lines of Matlab code:
dpitchcnt=(N/256); %N is total number of byte
pitchvec(1:int64(dpitchcnt)); %creating a matrix 'pitchvec' with 1 row and int64(dpitchcnt)' columns
size(pitchvec) %Trying to display the size.
I am getting the following error while carrying out the above operation:
Undefined function or method '_colonobj' for input arguments of type
'int64'. Error in ==> sample at 31 pitchevec(1:int64(dpitchcnt));
What am I doing wrong?
The syntax varName(1:10) will get the first 10 values of varName, not create the variable varName;
To create a matrix you can use
pitchvec = zeros(1,int64(dpitchcnt)); %A zero-matrix
matrixSize = size(pitchvec);
You can also use ones(n,m);%Create a n times m matrix with 1 all over.