How to find the position of a given vector A in a matrix M when the size of vector A is smaller than the columns of that matrix?

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GANESH R
GANESH R 2022-5-7
评论： Voss 2022-5-8
I trying to find the (row) postion of A = [15 2] in a matrix M.
M = [46 58 50
46 41 49
15 2 1
16 2 15
21 16 15
14 16 20
16 23 20]
The solution is pos = [3 4] as the vector A are in 3 and 4 rows of M.

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Voss
Voss 2022-5-7
Ran in:
Here's one way, assuming you want to match A or its reverse and that the elements have to be contiguous:
M = [46 58 50
46 41 49
15 2 1
16 2 15
21 16 15
14 16 20
16 23 20];
A = [15 2];
pos = [];
for ii = 1:size(M,1)
if ~isempty(strfind(M(ii,:),A)) ... % check if A is in row ii of M
|| ~isempty(strfind(M(ii,:),A(end:-1:1))) % or the reverse of A is in row ii of M
pos(end+1) = ii;
end
end
disp(pos)
3 4
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GANESH R
GANESH R 2022-5-8
I found the answer.
M =
14 5 4
16 4 3
14 4 16
33 29 26
35 38 40
2 16 3
>> A
A =
16 3
>> pos = find(sum(ismember(M,A),2)==2)
pos =
2
6
Voss
Voss 2022-5-8
Ran in:
Ah, ok. I wasn't sure if elements from A that are separated in a row of M should be counted as a match, which is why I said "that the elements have to be contiguous" in my answer.
Your solution will work if the elements in a given row of M are distinct, but consider the following situation, where I've modified row 3 of M:
M = [14 5 4
16 4 3
16 4 16
33 29 26
35 38 40
2 16 3]
M = 6×3
14 5 4 16 4 3 16 4 16 33 29 26 35 38 40 2 16 3
A = [16 3]
A = 1×2
16 3
pos = find(sum(ismember(M,A),2)==2)
pos = 3×1
2 3 6
Now row 3 is counted because it has two 16's. To handle that situation correctly, you can do this instead:
pos = [];
for ii = 1:size(M,1)
if all(ismember(A,M(ii,:)))
pos(end+1) = ii;
end
end
disp(pos);
2 6
This counts a row of M if all elements of A are found in that row. It doesn't take into account the order of the elements of A in the row of M, as my initial answer did.

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