What does L == 2 mean in MATLAB?

Question

    BW = logical([1 1 1 0 0 0 0 0
                  1 1 1 0 1 1 0 0
                  1 1 1 0 1 1 0 0
                  1 1 1 0 0 0 1 0
                  1 1 1 0 0 0 1 0
                  1 1 1 0 0 0 1 0
                  1 1 1 0 0 1 1 0
                  1 1 1 0 0 0 0 0]);
    L = bwlabel(BW,4);
    [r,c] = find(L == 2);

How can a matrix been compared with scalar?

Answer 1

The find(L==2) will return all the rows and columns of elements that are equal to 2.

So, if you get [r]=[1 2 3 5 6] and [c]=[1 2 3 5 6], that means that the rows and columns of the elements that are equal to 2 is {1,1}, {2,2}, {3,3} and so on.

The official matlab explanation is here. You can also google the term "matlab find"

Answer 2

see this, it will usefull for you

http://www.mathworks.com/access/helpdesk/help/toolbox/images/bwlabel.html

Examples

Label components using 4-connected objects. Notice objects 2 and 3; with 8-connected labeling, bwlabel would consider these a single object rather than two separate objects.

BW = logical ([1 1 1 0 0 0 0 0 1 1 1 0 1 1 0 0 1 1 1 0 1 1 0 0 1 1 1 0 0 0 1 0 1 1 1 0 0 0 1 0 1 1 1 0 0 0 1 0 1 1 1 0 0 1 1 0 1 1 1 0 0 0 0 0]);

L = bwlabel(BW,4)

L =

 1     1     1     0     0     0     0     0
 1     1     1     0     2     2     0     0
 1     1     1     0     2     2     0     0
 1     1     1     0     0     0     3     0
 1     1     1     0     0     0     3     0
 1     1     1     0     0     0     3     0
 1     1     1     0     0     3     3     0
 1     1     1     0     0     0     0     0

[r, c] = find(L==2); rc = [r c]

rc =

 2     5
 3     5
 2     6
 3     6

Answer 3

Afair comparing a matrix with an scalar results in element-wise comparison. I.e. each element of the matrix is compared with the scalar. The result is a matrix with 1 in all positions for which the comparison returned true and 0 in all other positions.

find on the other hand returns all indices for which the argument is non-zero.

Answer 4

"How can a matrix been compared with scalar?": try doc eq at the matlab prompt and it says:

A == B compares each element ... for equality ... Each input of the expression can be an array or a scalar value.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

Answer 5

Maybe a visual example might help.

>> b=[1 2 3;2 3 1;3 1 2]

b =

     1     2     3
     2     3     1
     3     1     2



>> b==2

ans =

     0     1     0
     1     0     0
     0     0     1



>> b==3

ans =

     0     0     1
     0     1     0
     1     0     0

A logical-class matrix of the same size as the matrix being compared is returned.

>> find(ans)

ans =

     2
     4
     9

find then returns the linear address of the non-zero elements.

Answer 6

Another explanation:

Say,

A = [1 2 3 
     4 5 6
     7 8 9]

then when you say:

A == 5;

what MATLAB sees (the actual implementation is for more different than this, but the logic stays the same):

A == B*5;

where,

B = [1 1 1
     1 1 1
     1 1 1];

You can extend this for an arbitrary A matrix.

Same for addition.