I have a matrix A
A =
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 i have another matrix to specify the indices
index =
1 2 3 4 Now, i have got third matrix C from A
C = A(index) C =
1 6 11 16 Problem: I am unable to understand how come i have received such a matrixC. I mean, what is logi behind it?
The logic behind it is linear indexing: When you provide a single index, Matlab moves along columns first, then along rows, then along further dimensions (according to their order).
So in your case (4 x 5 matrix) the entries of A are being accessed in the following order (each number here represents order, not the value of the entry):
1 5 9 13 17 2 6 10 14 18 3 7 11 15 19 4 8 12 16 20 Once you get used to it, you'll see linear indexing is a very powerful tool.
As an example: to obtain the maximum value in A you could just use max(A(1:20)). This could be further simplified to max(A(1:end)) or max(A(:)). Note that "A(:)" is a common Matlab idiom, used to turn any array into a column vector; which is sometimes called linearizing the array.
See also ind2sub and sub2ind, which are used to convert from linear index to standard indices and vice versa.
