# Matrix Indexing

• Indexing involves picking out certain elements within a matrix. For a row or column matrix you simply have to specify the position in the unidirectional sequence. With larger matrices, there’s a need to specify the column and row of the element. Examples of these are seen below:

CHECKPOINT: This is the “end” (No, not really)

What would the following do to the matrices S and M above?

1. `P = M (2 : end, 2 : end)`
2. `J = S (4 : end-1)`
3. `T = S (end : -1 : 1)`

What is the key difference between matrices T and S?

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• Summary on the use of the colon operator, `:`
 Format Purpose `A(:, j)` Outputs the jth column of A `A(i, : )` Outputs the ith row of A `A(:, : )` Outputs a two-dimensional array. If A were a matrix, output would be A `A(j : k)` Outputs jth to kth elements of A `A(:, j : k)` Outputs all the elements  from the jth to kth columns of A `A(:)` Outputs all elements of A regarded as a single column
•  Empty square brackets `[ ]` are used to delete rows or columns in a matrix e.g. `A(:, j)` deletes the jth column of matrix A.