FreeMat: DIAG Diagonal Matrix Construction/Extraction

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DIAG Diagonal Matrix Construction/Extraction

Section: Array Generation and Manipulations

# Usage

The `diag` function is used to either construct a diagonal matrix from a vector, or return the diagonal elements of a matrix as a vector. The general syntax for its use is

      y = diag(x,n)

If `x` is a matrix, then `y` returns the `n`-th diagonal. If `n` is omitted, it is assumed to be zero. Conversely, if `x` is a vector, then `y` is a matrix with `x` set to the `n`-th diagonal.

# Examples

Here is an example of `diag` being used to extract a diagonal from a matrix.

    --> A = int32(10*rand(4,5))

    A =
      5  8  8  3  6
      4  8  4  3  7
      9  5  8  4  2
      1  0 10  0  4

    --> diag(A)

    ans =
     5
     8
     8
     0

    --> diag(A,1)

    ans =
     8
     4
     4
     4

Here is an example of the second form of `diag`, being used to construct a diagonal matrix.

    --> x = int32(10*rand(1,3))

    x =
     6 3 9

    --> diag(x)

    ans =
     6 0 0
     0 3 0
     0 0 9

    --> diag(x,-1)

    ans =
     0 0 0 0
     6 0 0 0
     0 3 0 0
     0 0 9 0