Is there a function that returns a vector or array or list of values obtained by applying a function to margins of an array or matrix?
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For example, assume x is a vector of length n, and y is a vector of length m. I would like to construct the matrix the (n x m) matrix M(i,j) = f(x(i),y(j)). Of course I do not want to use loops!
Btw, in R, there is such a function (called apply).
采纳的回答
Image Analyst
2015-7-9
Use meshgrid()
[X, Y] = meshgrid(x,y);
M = f(X, Y);
You might need to use reshape() after that to make M be 2D
M = reshape(M, m, n);
3 个评论
Bruno Remillard
2015-7-9
编辑：Bruno Remillard
2015-7-9
James Tursa
2015-7-9
To avoid the transpose, simply reverse the initial indexing. E.g.,
[Y, X] = meshgrid(y,x);
Image Analyst
2015-7-10
You need to be careful. meshgrid() works on x and y, while other functions, like reshape() work on rows and columns. Note, you used x and y and said x had n columns and y had m rows. However you said that M was nxm, meaning n rows and m columns, which is the reverse. So you either made the very common mistake of thinking (rows,columns) was the same as (x,y) or you intentionally thought that your "f" function would do a transpose. Either way, you just have to be careful.
更多回答（2 个）
James Tursa
2015-7-9
编辑：James Tursa
2015-7-9
A bit clunky, but this works and makes no assumptions about f being vectorized:
n = numel(x);
m = numel(y);
M = arrayfun(@f,repmat(x(:),1,m),repmat(y(:)',n,1));
John D'Errico
2015-7-9
I'm surprised nobody mentioned this.
fun = @(x,y) sin(x+y);
x = 0:5;
y = (0:2:6)';
M = bsxfun(fun,x,y)
M =
0 0.84147 0.9093 0.14112 -0.7568 -0.95892
0.9093 0.14112 -0.7568 -0.95892 -0.27942 0.65699
-0.7568 -0.95892 -0.27942 0.65699 0.98936 0.41212
-0.27942 0.65699 0.98936 0.41212 -0.54402 -0.99999
3 个评论
James Tursa
2015-7-9
Good as long as fun works with column vector inputs.
John D'Errico
2015-7-10
Easy enough to ensure that X and Y are always the proper shapes.
M = bsxfun(fun,x(:).',y(:));
James Tursa
2015-7-10
编辑：James Tursa
2015-7-10
I guess I wasn't clear. bsxfun passes column vectors to the function, so whatever the function is, it must be able to deal with column vector inputs (i.e., it must be vectorized to this extent). That was the only point I was trying to make. It is similar to the f(X,Y) used in IA's solution ... f must be vectorized for this to work.
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