How to set an array identifier within code
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Hi,
I am trying to write a snippet that takes an input array of any size (I will refer to it as A with N dimensions) and a column or row vector v, that returns an array B having N+1 dimensions, which elements along the (N+1)th dimension are the products A*v(i) (where v(i) are the individual scalar elements in v).
If A is a 2D matrix, it can be done this way:
A=rand(4,5);
v=rand(1,3);
B=zeros([size(A) length(v)]);
for k=1:length(v)
B(:,:,k)=v(k)*A;
end
This works fine but since I need to write as many ' : ' as there are dimensions in A, this is not very versatile.
So I am looking for a way to change this argument as the code executes to adapt to the number of dimensions in A. I have tried the following:
A=rand(4,5);
v=rand(1,3);
bit=sprintf(':,'); %Define the string element to be repeated N times in the argument
arg=bit; %Initiate the argument string
for c=1:(length(size(A))-1)
arg=strcat(arg,bit) %Concatenate as many times as necessary
end
arg=strcat(arg,'k') %Append with the letter k
%In this example, arg is ':,:,k'
B=zeros([size(A) length(v)]);
for k=1:length(v)
B(arg)=v(k)*A;
end
But it doesn't work...
采纳的回答
Trying to build the indices as text is never going to be efficient. What you want can be trivially obtained:
%A: an N-d matrix
%v: a vector
%B: an (N+1)-d vector where B(..., k) = B(...) .* v(k). Dimensions 1:N have same size as A, dimension N+1 is numel(v)
B = A .* reshape(v, [ones(1, ndims(A)), numel(v)]); %move v in the N+1 dimension and multiply
4 个评论
Julien Lopez
2019-3-29
madhan ravi
2019-3-29
B = bsxfun(@times,A,reshape(v, [ones(1, ndims(A)), numel(v)]));
Guillaume
2019-3-29
Up there, in the right corner of the webpage there's a field for you to enter the version of matlab that you're using. It's important to fill out if you're on an old version.
My guess is that you're using a version prior to R2016b which does not have implicit expansion. In that case,
B = bsxfun(@times, A, reshape(v, [ones(1, ndims(A)), numel(v)])));
The code in my answer is guaranteed to never generate a Matrix dimensions must agree error in R2016b and later.
Julien Lopez
2019-3-29
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