a simpler way of doing a nested loop
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Mnr
am 31 Okt. 2015
Kommentiert: Stephen23
am 2 Nov. 2015
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.
Akzeptierte Antwort
Walter Roberson
am 31 Okt. 2015
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;
Weitere Antworten (1)
Jan
am 31 Okt. 2015
Bearbeitet: Jan
am 1 Nov. 2015
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
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