How to get the combinations of elements of two arrays?

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Hemming
Hemming 2018년 12월 3일
편집: Stephen23 2023년 10월 27일
Hi! I need to generate the combinations of elements of two arrays with different lengths. For example, if
A = [1,2,3]; B = [4,5];
I wish to get all combinations of elements from two arrays as
C = [1 4;1 5;2 4;2 5;3 4;3 5];
What comes to my mind is
[m,n] = meshgrid(A,B');
[C(:,1),C(:,2)] = deal(reshape(m,[],1),reshape(n,[],1));
Is there any more straight way to accomplish this?
And further, if I have three or more arrays to combine, what should I do?

채택된 답변

Stephen23
Stephen23 2018년 12월 3일
편집: Stephen23 2023년 10월 27일
A = [1,2,3];
B = [4,5];
[n,m] = ndgrid(B,A);
out = [m(:),n(:)]
out = 6×2
1 4 1 5 2 4 2 5 3 4 3 5
"And further, if I have three or more arrays to combine, what should I do?"
Put them all in a cell array and use two comma-separated lists:
C = {A,B};
[C{end:-1:1}] = ndgrid(C{end:-1:1});
EDIT: using CAT & RESHAPE is probably the most efficient approach:
n = numel(C);
E = reshape(cat(n,C{:}),[],n)
E = 6×2
1 4 1 5 2 4 2 5 3 4 3 5
This approach is from Dyuman Joshi's comment here:
  댓글 수: 4
Stephen23
Stephen23 2023년 10월 26일
@Dyuman Joshi: thank you, that is a very good approach! I copied it into my answer so that it does not get lost in the comments.
Dyuman Joshi
Dyuman Joshi 2023년 10월 26일
You are welcome!
Note that the usage of the indices in reverse order is to obtain the output in lexicographical manner (in the order of the values provided in the inputs).

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추가 답변 (3개)

Mike Croucher
Mike Croucher 2023년 4월 4일
편집: Mike Croucher 2023년 4월 4일
>> A = [1,2,3];B=[4,5];
>> C = combinations(A,B)
C =
6×2 table
A B
_ _
1 4
1 5
2 4
2 5
3 4
3 5
The result is a table. When all data types are compatible (as is the case here) you can get the matrix like this
>> C.Variables
ans =
1 4
1 5
2 4
2 5
3 4
3 5

Jeff Miller
Jeff Miller 2018년 12월 3일
You might like allcomb on File Exchange:
For example:
>> A = [1,2,3]; B = [4,5];
>> C=allcomb(A,B)
C =
1 4
1 5
2 4
2 5
3 4
3 5
  댓글 수: 1
Hemming
Hemming 2018년 12월 11일
편집: Hemming 2018년 12월 11일
Nice choice! And this code performs faster than the above one (MATLAB R2016a) although both adopt the similar method. Thanks a lot!

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ROHIT KHATRI
ROHIT KHATRI 2023년 10월 26일
In MATLAB R2017b allcomb and combinations codes not working. So please tell me what can i do?

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