清空筛选条件
清空筛选条件

a simpler way of doing a nested loop

1 次查看（过去 30 天）
Mnr
Mnr 2015-10-31
评论： Stephen23 2015-11-2
Hello,
Is there a more efficient alternative way that does the same function as:
V=[1+1i +1-1i -1+1i -1-1i];
for m1=1:length(V)
a=V(m1);
for m2=1:length(V)
b=V(m2);
for m3=1:length(V)
c=V(m3);
for m4=1:length(V)
d=V(m4);
h=[.3 .2]*[a b].'+[.5 .8]*[c d].';
h1(m1,m2,m3,m4)=h;
end
end
end
end
Thanks.

请先登录，再回答此问题。

采纳的回答

Walter Roberson
Walter Roberson 2015-10-31
V=[1+1i +1-1i -1+1i -1-1i];
[A,B,C,D] = ndgrid(V);
h1 = 0.3*A + 0.2*B + 0.5*C + 0.8*D;
  5 个评论
显示 3更早的评论
Mnr
Mnr 2015-11-1
Is there is a way of writing this code in a more general form such that it works for different sizes of V? Thank you.
Stephen23
Stephen23 2015-11-2
编辑：Stephen23 2015-11-2
@Mnr: try this:
V = [1+1i,+1-1i,-1+1i,-1-1i];
W = [0.3,0.2,0.5,0.8];
N = numel(V);
X = cell(1,N);
[X{:}] = ndgrid(V);
Y = cat(N+1,X{:});
S = [ones(1,N),N];
Z = sum(bsxfun(@times,reshape(W,S),Y),N+1);

请先登录，再进行评论。

更多回答（1 个）

Jan
Jan 2015-10-31
编辑：Jan 2015-11-1
I prefer Walter's solution, because it is nice. But it is worth to mention, that a pre-allocation improves the loop version. Replacing the dot-product by direct algebra and avoiding repeated calculations improves the speed also:
V = [1+1i, 1-1i, -1+1i, -1-1i];
n = length(V);
h1 = zeros(n, n, n, n);
for m1 = 1:n
c = 0.3 * V(m1);
for m2 = 1:n
c = c + 0.2 * V(m2);
for m3 = 1:n
c = c + 0.5 * V(m3);
for m4 = 1:n
h1(m1,m2,m3,m4) = c + 0.8 * V(m4);
end
end
end
end
Timings for 5000 repetitions, Matlab R2011b/64/Win7:
Original: 8.90 sec
Walters ndgrid: 1.47 sec
Loop, pre-allocation, inlined dot products: 0.34 sec
  2 个评论
Mnr
Mnr 2015-11-1
Is there is a way of writing this code in a more general form such that it works for different sizes of V? Thank you.

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Common Operations 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by