What is the meaning of sol(31,: ,1)

What is the meaning of A(31,: ,1)? If A is a matrix. Thanks

Answer 1

[John D'Errico]

Do you understand indexing? Perhaps you really need to start with a tutorial, the Onramp tutorials may be a good place to start.

If A is a matrix,

A = magic(5)

A = 5×5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

then you can index into the matrix. A colon as an index tells MATLAB to take all of the elements in that row or column, etc.

A(3,:)

ans = 1×5

4 6 13 20 22

So A(3,:) extracts the 3rd row, with all of the elements in that row.

But all matrices in MATLAB can also be viewed as higher dimensional matrices. So a 5x5 matrix, is also a 5x5x1 matrix. So this:

A(3,:,1)

ans = 1×5

4 6 13 20 22

does exactly the same thing. Or, we might have a matrix with multiple true planes in that matrix.

A = randi(5,[3,4,2])

A = 
A(:,:,1) =

     3     2     4     2
     5     2     5     5
     3     3     3     5


A(:,:,2) =

     2     5     1     5
     4     3     2     2
     5     4     4     1

So that matrix now has 2 planes, EACH of which is a 3x4 matrix. Now

A(:,3,2)

ans = 3×1

1 2 4

That extracted the third column, of the 2nd plane of the matrix. The result is 3x1 vector.