CODEWITHSUNDEEP
matlabsearchindexingrow
user2037922
user2037922

Reputation: 33

MATLAB search row index of matrix with values of another matrix

In MATLAB I have a very large matrix (matrix A). Now I would like to find the row-index of the row which contain certain values in the second column. These values - which I'm looking for in Matrix A - are stored in anonther Matrix (Matrix B) with consists out of a row (800 numbers).

Besides I would like to redo this calculation for the same matrix A, but for ten different matrices, with different sizes (which contain the values I'm looking for in different columns of matrix A).

Because of the sizes of the matrix I think i need a loop to extract the row in matrix A which contain te value of Matrix B. How can I do this?

regards,

V

Thanks for the quick response! Indeed the problem is maybe a bit complex to answer without an example, and indeed duplicate entries cause some problems. Therefore hereby an example

For example I have a -simplified- matrix A:

1 2 3 4

9 9 9 9

4 3 2 1

And a -simplified- matrix (row) B: [9 3]

And a -simplified- matrix (row) C: [9 2]

Then I would like to get matrix D and matrix E.These matrices should contain in the first column the numbers from the original matrix D(or E) and in the second column the corresponding row-location of this value in matrix A.

So, matrix D =

9 2

3 3

matrix E =

9 2

2 3

As represented in this example matrix B and matrix C can contain data which is present in several column of matrix A (like the nine). However, martix B should "search" in column 2 of matrix A. Likewise, should matrix C "search" in column 3 of Matrix A, resulting in matrix D and E as given in the example.

Upvotes: 1

Views: 3060

Answers (2)

Dennis Jaheruddin
Dennis Jaheruddin

Reputation: 21563

Assuming you have two vectors, largeDataIndex (the second column of your matrix) and interestingIndex (your b) and you want the following:

For each value of interestingIndex , find the position in largeDataIndex

Then an easy method would be this:

result = zeros(size(interestingIndex))
for i = 1:length(result)
    result(i) = find(interestingIndex(i) == largeDataIndex)
end

Note that this assumes there is always just one entry that matches, otherwise you should define result as a cell array rather than a vector.

Upvotes: 0

Aabaz
Aabaz

Reputation: 3116

As mentionned by Shai in his comment, your question is quite vague and a lot of special case could arise (duplicate entries, relative size of A and B, etc.). But in all generality I tried a small piece of code that seems to do what you want. There are certainly quicker ways of doing it, and certainly more information on your problem could help optimize this.

colA=2;
% Example
nmax=10;
nA=5;
A=randi(nmax,[nA nA]);
nB=3;
B=randi(nmax,[1 nB]);
% Find rows
rows=cell(size(B));
for i=1:numel(B)
    rows(i)={find(A(:,colA)==B(i))};
end

The input / output was:

A =
     3     7     8     5     4
     9     7     3     7     5
     8     2     9     9     8
     9     5     9     7     9
     3     3     4     6     8

B =
     1     7     5

rows =
     [0x1 double]    [1;2]    [4]

Upvotes: 0

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