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]
Related
I am a complete newbie to MATLAB and the first task I have is to create a volcano plot. I have been using the documentation to understand about it and get started.
I tried to run it on dummy values -
a=[1 2 3]
b=[4.6 2.7 4.5]
c=[0.05 0.33 0.45]
And then I ran -
SigStructure = mavolcanoplot(a, b, c)
My understanding is that a represents the gene expression values for condition 1, b for condition 2, and c is the list of p-values for the 3 data points in a and b.
However running this code gives me the error -
Index exceeds matrix dimensions.
Error in mavolcanoplot (line 127)
appdata.effect = X(paramStruct.goodVals) - Y(paramStruct.goodVals);
Error in volc (line 4)
SigStructure = mavolcanoplot(a, b, c)
Can anyone explain where I am going wrong?
You're encountering an issue because you are using row vectors.
Inside the mavolcanoplot function (you can see the file by typing edit mavolcanoplot in the command window) there is a local function for checking the inputs, called check_inputdata.
Your data passes all of the validation checks, and then encounters this section:
% Here, 'X' and 'Y' are the local names for your inputs 'a' and 'b'
% Below code is directly from mavolcanoplot.m:
% Handle the matrix input. Use its mean values per row
if size(X, 2) > 1
X = mean(X,2);
end
if size(Y, 2) > 1
Y = mean(Y,2);
end
This reduces your inputs to their mean. Later in the main function (line 127) you encounter the error as described, where paramStruct.goodVals is a 3 element array, which is now trying to index a 1 element array and fails!
This is basically a lesson in debugging and reading the documentation, which states
DataX, DataY: If a [...] matrix, each row is a gene, each column is a sample and an average expression value is calculated for each gene.
You should use one of these equivalent methods to create column vector inputs
a=[1 2 3].'; % Using transpose (.') to create a column vector from a row vector
b=[4.6; 2.7; 4.5]; % Creating a column vector using the semi-colon operator to end each row
c=[0.05
0.33
0.45]; % Using actual code layout to create a column vector
In Matlab R2016a, I have a large set of small X-vectors and Y-vectors which are paired (e.g. 10,000 1x3 X-vectors paired with 10,000 1x3 Y vectors). For each {X,Y} pair, I want to calculate a 2-scalar-argument function for every pairwise combination of the elements in X and Y, (so in my example I would get 10,000 3x3 matrices).
I thought I could use bsxfun to perform these calculations, but it doesn't work when I try to do some simple tests. bsxfun(#(x,y) x*y,[1 2],[1 2]') returns:
ans =
1 2
2 4
Which is what I would expect. However, bsxfun(#(x,y) 1,[1 2],[1 2]') returns:
Error using bsxfun
Specified function handle produces invalid output dimensions. The function handle
must be a binary elementwise function.
Which makes no sense. The function handle is a binary elementwise function that always returns the scalar 1, so bsxfun should give the same result as ones(2,2), unless I'm not understanding how bsxfun works.
The inputs to the function handle that are passed to bsxfun are not scalars. In versions prior to R2016b, the inputs are either scalar or they are the same size.
FUNC can also be a handle to any binary element-wise function not listed
above. A binary element-wise function in the form of C = FUNC(A,B)
accepts arrays A and B of arbitrary but equal size and returns output
of the same size. Each element in the output array C is the result
of an operation on the corresponding elements of A and B only. FUNC must
also support scalar expansion, such that if A or B is a scalar, C is the
result of applying the scalar to every element in the other input array.
In releases since R2016b, they do not have to be equal sizes, but should be compatible sizes
In the example you have shown, the first input to the function handle is a scalar and the second is a vector (y) and the function is evaluated for every element of x and the output is expected to be the size of y
In the case you've posted, the call to bsxfun is essentially the equivalent of:
x = [1 2];
y = [1 2].';
yourfunc = #(x,y)x * y;
for k = 1:numel(x)
output(:,k) = yourfunc(x(k), y)
end
If you want to return a 1 for every entry, you need to replace your function with something that yields the appropriately sized output.
bsxfun(#(x,y)ones(max(size(x), size(y))), [1 2], [1 2]')
How you formulate the function handle really depends upon your specific problem
I am having trouble using fminsearch: getting the error that there were not enough input arguments for my function.
f = #(x1,x2,x3) x1.^2 + 3.*x2.^2 + 4.*x3.^2 - 2.*x1.*x2 + 5.*x1 + 3.*x2 + 2.*x3;
[x, val] = fminsearch(f,0)
Is there something wrong with my function? I keep getting errors anytime I want to use it as an input function with any other command.
I am having trouble using fminsearch [...]
Stop right there and think some more about the function you're trying to minimize.
Numerical optimization (which is what fminsearch does) is unnecessary, here. Your function is a quadratic function of vector x; in other words, its value at x can be expressed as
x^T A x + b^T x
where matrix A and vector b are defined as follows (using MATLAB notation):
A = [ 1 -1 0;
-1 3 0;
0 0 4]
and
b = [5 3 2].'
Because A is positive definite, your function has one and only one minimum, which can be computed in MATLAB with
x_sol = -0.5 * A \ b;
Now, if you're curious about the cause of the error you ran into, have a look at fuesika's answer; but do without fminsearch whenever you can.
It is exactly what Matlab is telling you: your function expects three arguments. You are passing only one.
Instead of
[x, val] = fminsearch(f,0)
you should call it like
[x, val] = fminsearch(f,[0,0,0])
since you define the function f to accept a three dimensional vector as input only.
You can read more about the specification of fminsearch in the online documentation at http://mathworks.com/help/matlab/ref/fminsearch.html:
x = fminsearch(fun,x0) starts at the point x0 and returns a value x
that is a local minimizer of the function described in fun. x0 can be
a scalar, vector, or matrix. fun is a function_handle.
So I'm trying to create a matlab function that takes two inputs, a matrix and a value, and returns the sum of all values in the matrix except for all instances of the given value. So far this is the code I have written:
function [total] = sumAllExcept(matrix, except)
total = 0;
for i = 1:size(matrix, 1)
for k = 1:size(matrix, 2)
if(matrix(i, k) ~= except)
total = total + matrix(i,k);
end
end
end
end
The error message that I am receiving when trying to run the program is:
" Undefined function 'sumAllExcept' for input arguments of type 'double'. "
I would greatly appreciate it if you would show me whats wrong with this and fix anything that you can. Thank you!
Sum the array after filtering out except using logical indexing:
total = sum(matrix( matrix ~= except ));
The result of using the logical index matrix ~= except on matrix returns a column vector, so only one sum is required.
The error "Undefined function 'sumAllExcept' for input arguments of type 'double'." is likely due to the function not being on MATLAB's path or the function name sumAllExcept not matching the filename (i.e., sumAllExcept.m).
I've been searching the net for a couple of mornings and found nothing, hope you can help.
I have an anonymous function like this
f = #(x,y) [sin(2*pi*x).*cos(2*pi*y), cos(2*pi*x).*sin(2*pi*y)];
that needs to be evaluated on an array of points, something like
x = 0:0.1:1;
y = 0:0.1:1;
w = f(x',y');
Now, in the above example everything works fine, the result w is a 11x2 matrix with in each row the correct value f(x(i), y(i)).
The problem comes when I change my function to have constant values:
f = #(x,y) [0, 1];
Now, even with array inputs like before, I only get out a 1x2 array like w = [0,1];
while of course I want to have the same structure as before, i.e. a 11x2 matrix.
I have no idea why Matlab is doing this...
EDIT 1
Sorry, I thought it was pretty clear from what I wrote in the original question, but I see some of you asking, so here is a clarification: what I want is to have again a 11x2 matrix, since I am feeding the function with arrays with 11 elements.
This means I expect to have an output exactly like in the first example, just with changed values in it: a matrix with 11 rows and 2 columns, with only values 0 in the first column and only values 1 in the second, since for all x(i) and y(i) the answer should be the vector [0,1].
It means I expect to have:
w = [0 1
0 1
0 1
...
0 1]
seems pretty natural to me...
You are defining a function f = #(x,y) [0, 1]; which has the input parameters x,y and the output [0,1]. What else do you expect to happen?
Update:
This should match your description:
g=#(x,y)[zeros(size(x)),ones(size(y))]
g(x',y')
Defining an anonymous function f as
f = #(x,y) [0,1];
naturally returns [0,1] for any inputs x and y regardless of the length of those vectors.
This behavior puzzled me also until I realized that I expected f(a,b) to loop over a and b as if I had written
for inc = 1:length(a)
f(a(inc), b(inc))
end
However, f(a,b) does not loop over the length of its inputs, so it merely returns [0,1] regardless of the length of a and b.
The desired behavior can be obtained by defining f as
g=#(x,y)[zeros(size(x)),ones(size(y))]
as Daniel stated in his answer.