Vectors seen as a combination of other vector's elements

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Hi everyone.
I have a problem that I think to be quite easy, but I can't succeed in solving it.
I have four vectors of different length (a, b, c, d). I'd like to have a final vector (V) which contains all the combinations of their elements.
eg, if I had a = [1 2 3], b = [4 5], c = [6], V should be [1 4 6; 1 5 6; 2 4 6; 2 5 6; 3 4 6; 3 5 6].
Now I'm using four nested for cycle, but I'd like to find a faster solution, without using the cycle but maybe some logical solutions.
Thank you in advance. Luca
P.S. I leave the portion of my code:
for n = 1:length(comb_n)
for i = 1:length(comb_i)
for c = 1:length(comb_c)
for a = 1:length(comb_a)
combinations(index,1:number_surrogates) = [comb_n(n,:), comb_i(i,:), comb_c(c,:), comb_a(a,:)];
index = index+1;
end
end
end
end

Risposte (2)

madhan ravi
madhan ravi il 12 Apr 2019
[X,Y,Z]=meshgrid(a,b,c);
[X(:),Y(:),Z(:)]
  3 Commenti
Luca Freilino
Luca Freilino il 12 Apr 2019
I tried with meshgrid, but it doesn't work since I've always four vectors. I forgot to say that these vectors could be even matrices (eg the second one could be a (N,2) size). Could you explain me how to use the ndgrid? I know the function, but I have no idea which conditions should I use. Thanks, Luca

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Guillaume
Guillaume il 12 Apr 2019
This is the generic version of Madhan's answer. Works for any (reasonable!) number of inputs.
in = {[1, 2, 3], [4, 5], 6}; %cell array of vectors.
combs = cell(size(in));
[combs{:}] = ndgrid(in{:});
combs = reshape(cat(numel(combs) + 1, combs{:}), [], numel(combs));
You could also download AllComb from the filexchange, which does the same.
  6 Commenti
madhan ravi
madhan ravi il 12 Apr 2019
Same thought as Torsten's but your question is most likely solved by Guillaume's method.
Luca Freilino
Luca Freilino il 12 Apr 2019
Modificato: Luca Freilino il 12 Apr 2019
It's not a good solution or, at least, this is not good for my problem. I forward the code, it shoud be easier to explain:
v = [1 2 2 1];
a = 1:12;
b = 13:29;
c = 30:39;
d = 40:47;
A = nchoosek(a,v(1));
B = nchoosek(b,v(2));
C = nchoosek(c,v(3));
D = nchoosek(d,v(4));
What I need is the family of combination, like:
1 13 14 30 31 40;
1 13 14 30 31 41;
.........................
1 13 14 30 32 40;
.........................
.........................
.........................
12 28 29 38 39 47

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