Finding similar elements in a single matrix

I have one matrix,

x = [
    2	0	0
    3	3	0
    4	4	0
    5	5	5
    6	6	6]

How can I find the common elements between columns in the matrix and count them? for example, here 3 and 4 repeated two times.

[Walter Roberson]

SUppose that row 3 column 3 were 2. Should that be considered a match to row 1 column 1? Or is it only a match if at least two columns of the same row are equal?

Should the 0 be counted as matches in row 1?

What should the output be? If there were another row at the end that was [3 0 3] for example, then should the overall output summarize that 3 occurred 4 times? But if so then should the 5 5 5 count as 3 because there are 3 elements? Should it count as 3 because columns (1,2) -> 1 match, columns (1,3) -> 1 match, colums (2,3) -> 1 match, total 3? So if there were 4 rows all identical items the match count for the row should be 6?

[Md. Nazrul Islam Siddique]

Yes. I need to match one item at a time. Here, 3 and 4 repeated 2 times and 5 and 6 repeated 3 times. Number 2 is repeated only 1. So the total count is 11. I don't want to count the number 0.

[Walter Roberson]

So you just need to know the total number of non-zero elements? You can use nnz for that.

[Md. Nazrul Islam Siddique]

That's easy. what about the non zero elements between two metrices?

Answer 1

[Star Strider]

One approach using accumarray —

x = [2	0	0
     3	3	0
     4	4	0
     5	5	5
     6	6	6];

[Aunique,~,idx] = unique(x(:));
Counts = accumarray(idx, 1);

Result = table(Aunique, Counts)

Result = 6×2 table
    Aunique    Counts
    _______    ______

       0         4   
       2         1   
       3         2   
       4         2   
       5         3   
       6         3   

.

[Md. Nazrul Islam Siddique]

If I don't want to count the number 0?

[Walter Roberson]

x = [2	0	0
     3	3	0
     4	4	0
     5	5	5
     6	6	6];
xnz = x(x~=0);
[Aunique,~,idx] = unique(xnz);
Counts = accumarray(idx, 1);
Result = table(Aunique, Counts)

Result = 5×2 table
    Aunique    Counts
    _______    ______

       2         1   
       3         2   
       4         2   
       5         3   
       6         3   

[Walter Roberson]

But do you want to find matching elements, or do you want to find rows in which columns match each other?

[Md. Nazrul Islam Siddique]

I want to find the matching elements. The above example will do. Is there any other approach?

[Md. Nazrul Islam Siddique]

If I want to match two matrices for the common elements, what will be the approach?

for example:

x = [

1 2 3

4 5 6]

y = [1 5 7

2 5 8].

[Walter Roberson]

x = [
      1 2 3
        4 5 6]

x = 2×3

1 2 3 4 5 6

y = [1 5 7
      2 5 8]

y = 2×3

1 5 7 2 5 8

intersect(x, y)

ans = 3×1

1 2 5

[Star Strider]

@Walter Roberson — Thank you!

[Md. Nazrul Islam Siddique]

I want to know the total number which is repeated. For example, 1 and 2 repeated two times, 5 repeated three times. Other numbers are repeated only one. The total count is: 12.

[Star Strider]

Just sum the ‘Count’ results —

x = [1 2 3
     4 5 6];

y = [1 5 7
     2 5 8];

xy = [x; y]

xy = 4×3

1 2 3 4 5 6 1 5 7 2 5 8

xy = xy(xy~=0);
[XYunique,~,idx] = unique(xy(:));
Counts = accumarray(idx, 1);

Result = table(XYunique, Counts)

Result = 8×2 table
    XYunique    Counts
    ________    ______

       1          2   
       2          2   
       3          1   
       4          1   
       5          3   
       6          1   
       7          1   
       8          1   

TotalCounts = sum(Counts)

TotalCounts = 12

If the matrices are not conformable to concatenation (they are conformable here), then reshape both to be column vectors using the column vector operator ‘(:)’ and then vertically concatenate those. Then use unique and accumarray.

.