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fast way of filing up a matrix with function calls

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Kaushik
Kaushik 2012-12-21
i have a 2-D matrix m(i,j). for each (i,j) some function f(i,j) has to be evaluated and that fills up the matrix m(i,j). presently i am using a loop for i=1:N for j=1:N mat(i,j) = f(i,j); to fill up the matrix. Is there a faster implementation of this piece of code.

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回答（2 个）

Matt J
Matt J 2012-12-21
编辑：Matt J 2012-12-21
If f() is element-wise vectorized, you could do this
[I,J]=ndgrid(1:N,1:N);
mat=f(I,J)
but more optimal approaches might still be possible depending on the form of f().
Azzi Abdelmalek
Azzi Abdelmalek 2012-12-21
编辑：Azzi Abdelmalek 2012-12-21
M=rand(4); % your matrix
f=@(x) sin(x)*x % your function
out=arrayfun(@(x) f(x),M)
Or you can use operation element by elemment (Faster)
out=sin(M).*M
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Kaushik
Kaushik 2012-12-21
thank you for your answer. well my grid is something like x_grid (has length M) y_grid (has length N). and my output matrix will have size M*N. and output(i,j) = f(x_grid(i),y_grid(j),other parameters) inside f(x_i,y_i,other param) { i do integration of some kind on splines}. so you see the function has to take in two different values x,y in from two different vector and compute.
Azzi Abdelmalek
Azzi Abdelmalek 2012-12-21
编辑：Azzi Abdelmalek 2012-12-21
x_grid=1:5;
y_grid=1:4;
[M1,M2]=meshgrid(x_grid,y_grid)
Then calculate using M1 and M2

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