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

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

John D'Errico on 16 Jan 2022
Edited: John D'Errico on 16 Jan 2022
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.