I have a function that takes in any number of arguments into a cell array
function sumThese(varargin)
subtotals = cellfun(#sum, varargin);
total = sum(subtotals);
disp(total)
end
This works for arrays and numbers, except as soon as I have a square matrix it doesn't. It'll tell me:
Non-scalar in Uniform output, set 'UniformOutput' to false.
However if I set 'uniformoutput' to false, I get this error now:
Undefined function or method 'sum' for input arguments of type 'cell
How to approach this?
Change the function #sum in the cellfun
subtotals = cellfun( #(x) sum(x(:)), varargin );
Why?
becuase the output of sum, when applied to a matrix is no longer a scalar what turns subtotals into a cell array of scalars and vectors, instead of a 1D vector.
Use debugger to see the difference.
PS,
Did you know that cellfun is not always better than a simple loop.
EDIT:
A solution using for loop:
total = 0;
for ii = 1:numel(varargin)
total = total + sum( varargin{ii}(:) );
end
Related
How can I access both arguments of ismember when it is used inside splitapply?
slitapply only returns scalar values for each group, so in order to compute nonscalar values for each group (as returned by the first argument of ismemebr), one has to enclose the anonymous function (in this case ismember) inside curly brackets {} to return a cell array.
But now, when I provide two output arguments to splitapply, I get an error:
Output argument "varargout{2}" (and maybe others) not assigned during call to
"#(x,y) {ismember(x,y)}"
ADD 1
I can create another function, say, ismember2cell which would apply ismember and turn outputs into cell arrays:
function [a, b] = ismember2cell(x,y)
[a,b] = ismember(x,y);
a = {a};
b = {b};
end
but maybe there is a solution which doesn't require this workaround.
One potentially faster option is to just do what splitapply is already doing under the hood by splitting your data into cell arrays (using functions like mat2cell or accumarray) and then using cellfun to apply your function across them. Using cellfun will allow you to easily capture multiple outputs (such as from ismember). For example:
% Sample data:
A = [1 2 3 4 5];
B = [1 2 1 5 5];
G = [1 1 1 2 2]; % Group index
% Group data into cell arrays:
cellA = accumarray(G(:), A(:), [], #(x) {x(:).'}); % See note below about (:).' syntax
cellB = accumarray(G(:), B(:), [], #(x) {x(:).'});
% Apply function:
[Lia, Locb] = cellfun(#ismember, cellA, cellB, 'UniformOutput', false);
NOTE: My sample data are row vectors, but I had to use the colon operator to reshape them into column vectors when passing them to accumarray (it wants columns). Once distributed into a cell array, each piece of the vector would still be a column vector, and I simply wanted to keep them as row vectors to match the original sample data. The syntax (:).' is a colon reshaping followed by a nonconjugate transpose, ensuring a row vector as a result no matter the shape of x. In this case I probably could have just used .', but I've gotten into the habit of never assuming what the shape of a variable is.
I cannot find a global solution, but the accepted answer of this post helps me to define a helper function for your problem:
function varargout = out2cell(varargin)
[x{1:nargout}]=feval(varargin{:});
varargout = num2cell(x);
I think that you may succeed in calling
splitapply(#(x,y) out2cell(#ismember, x, y), A, B);
For example I want to have a function which let me delete rows of my matrix where the highest value is 1. Thus I wrote:
% A is an input matrix
% pict2 suppose to be output cleared matrix
function pict2 = clear_(A)
B=A
n = size(A,1)
for i=1:n
if(max(B(i,:))==1)
B(i,:)=[]
end
end
But after I call:
pict2=clear_(pict) the Matlab resposes:
"warning: clear_: some elements in list of return values are undefined
warning: called from
clear_ at line 5 column 1 pict2 = "
I'm not sure which elements were left undefined?
The variable name of your output argument must match the variable that you want to be returned. So you'll want to change the first line to the following so that your modifications to B are saved and returned.
function B = clear_(A)
As far as your algorithm is concerned, it's not going to work because you are modifying B while trying to loop through it. Instead, you can replace your entire function with the following expression which computes the maximum value of each row, then determines if this value is equal to 1 and removes the rows where this is the case.
B(max(B, [], 2) == 1, :) == [];
I believe that, alternatively to the suggestions you already recieved, you might want to try the following. Using logicals is probably one of the best options for such a problem, since you don't need to use for-loops:
function out = clear_matr(A)
% ind is true for all the rows of A, where the highest value is not equal to 1
ind = ~(max(A, [], 2) == 1);
% filter A accordingly
out = A(ind, :);
end
I have a 1x28 cell array called magV with each element containing a 246x247 matrix containing mostly NaNs.
I am trying to set up a for loop to go through each of these matrices and calculate a mean. The attempt so far:
mean_speeds = cell(1,28);
for x = 1 : 28
mean_speeds{x} = mean(magV{x});
end
This doesn't work; it just outputs another 1x28 cell array, with each element containing a 1x28 row of NaNs
What am I doing wrong?
Mean of anything containing NaN is a NaN. Remove . . .
mean(magV{x(~isnan(x))});
The mean function does not support NaN arguments. You can add a logic step to remove the invalid numbers then calculate the mean of the resulting array.
Or, you can use nanmean: see the nanmean Help Page
You can use cellfun to get rid of the loops.
If you want to ignore nan's
noNaN = cellfun(#(x) mean(x(~isnan(x))), magV, 'uni', 0);
If you want to treat them as zeros
zeroNaN = cellfun(#(x) sum(x(~isnan(x)))/numel(x), magV, 'uni', 0);
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 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]