find function on specific row and column of an array.

Hi,

I used find function in order to find row location of an array cell. To illustrate in an example:

A=magic(10)

A =

    92    99     1     8    15    67    74    51    58    40
    98    80     7    14    16    73    55    57    64    41
     4    81    88    20    22    54    56    63    70    47
    85    87    19    21     3    60    62    69    71    28
    86    93    25     2     9    61    68    75    52    34
    17    24    76    83    90    42    49    26    33    65
    23     5    82    89    91    48    30    32    39    66
    79     6    13    95    97    29    31    38    45    72
    10    12    94    96    78    35    37    44    46    53
    11    18   100    77    84    36    43    50    27    59

find(A(4:5,4)==21)

ans =

       1

What I wanted to get is in intersection of a row window and a specific column, I wanted to find a specific input's row location in the matrix. I expected to find the row number as 4, however I get 1. Find function thinks A(4:5,4) is a new array and gives locations accordingly.

Does anyone know how to get the true row location when I specify an mxn sub-matrix in a rxs matrix where r>m, s>n?

Thx,

JD

Answer 1

[Image Analyst]

To find a submatrix in another matrix, use ismember:

m = magic(10)
subm = m(4:6, 3:8)
% c = normxcorr2(subm, m)
[ia, ib] = ismember(m, subm)

Answer 2

[Image Analyst]

Just don't specify a sub-matrix. Why are you doing that? It just ruins it and doesn't give you what you want. Just do it like this:

A=magic(10)
[rows, columns] = find(A == 21)

[Image Analyst]

John's "Answer" moved here:

It is due to a matrix like

A=

17  24  1  8  15
23  5  1  14  16
4  6  1  20  22
10  12  1  21  3
11  18  1  2  9
11  13  2  5  1
10  20  2  5  13
18  5  2  5  8
11  14  2  6  19
19  6  2  6  1
25  28  3  28  22
24  25  3  22  23
25  20  3  30  21
28  21  3  6  26
23  27  3  28  27

I want to find row number of

A(9,4)

if I know I should look for it at

A(:,3)==2 and A(:,4)

I've got to use a window of row on a specific column. I thought find() could do it.

[Image Analyst]

I don't know why you're extracting the 3rd and 4th columns. If you want to get row 9 and column 4 and find all elements in A that match the value at (9,4) then just use find exactly as I said:

[rows, columns] = find(A == A(9,4))

rows and columns will be vectors with the row and column where A = 2. So you will find the (9,4) element plus lots of other places where A = 2.

[JohnDylon]

Before code finds its location, I don't know it is at (9,4) coordinate in the array. I expect the code finds it, I just told it in order to make my problem clear.

Matrix A is like

A=

1  24  1  1  15
2  5  1  2  16
3  6  1  3  22
4  12  1  4  3
5  18  1  5  9
6  13  2  1  1
7  20  2  2       13
8  5  2  2  8
9  14  2  3  19
10  6  2  4  1
11  28  3  1  22
12  25  3  2  23
13  20  3  3  21
14  21  3  4  26
15  27  3  5  27

It has a sub matrix

A1=

6  13  2  1  1
7  20  2  2       13
8  5  2  2  8
9  14  2  3  19
10  6  2  4  1

and I wanted to find row location of A(7,4) however I didn't know it was at the 7th row at first place, I needed that info.

I guess find still can work for this specific problem with

row=find(A(:,3)==2 & A(:,4)==2, 1, 'first')

that generates row=7 and

row=find(A(:,3)==2 & A(:,4)==2, 1, 'last')

that generates row=8.

Thank you for help.

JD

[Image Analyst]

So you want to do basically a template matching. You have a known submatrix and want to find its location inside a larger matrix. You can use normxcorr2() to do that. I attach a demo.