spdiags
Extraction of diagonal/sub-or-super-diagonals of a matrix, or construction of a sparse matrix with given diagonal/sub-or-sper-diagonals.
`(D,dind)=spdiags(A)`
`(D)=spdiags(A,d)`
`(S)=spdiags(A,d,B)`
`(S)=spdiags(A,d,m,n)`
 Inputs `A` Any matrix `d` A vector of integers specifying the diagonals of `A` . `B` A matrix. `m` An integer. `n` An integer. Outputs `D` A matrix of various diagonals of A. `dind` A vector of integers. `S` A sparse matrix.

Description
`spdiags(A)` returns all the `(m+n-1)` diagonals of the `pXq` matrix `A` in the coulmns of `D` . In this context, a diagonal is one of the following:
• The main diagonal, i.e., the vector `A(i,i), 1<=i<=min(p,q).`
• The sub-diagonals, i.e., for a given `j` , the vector `A(1+j+k,j+k)` , with `0<=k<=min(p-1-j,q-j)` , identified by `(-j)` , with `1<=j<=p-1` .
• The (super-diagonals, i.e., the vector, for a given `j` , `A(j+k,1+j+k)` , with `0<=k<=min(p-j,q-1-j)` , identified by `(j)` , with `1<=j<=q-1` .
The vector `dind` is the integer vector `[-(p-1):1:q-1]` .

`spdiags(A, d)` , with each element of the integer vector `d` satisfying `-(p-1)<=d[i]<=(q-1)` , returns the matrix whose columns are the diagonals of `A` specified by the elements of `d` .

`spdiags(A,d,m,n)` returns an `mXn` sparse matrix whose diagonals specified by the vector `d` are the columns of `A` . The number of columns of `A` must be identical to the number of elements in `d` , and the elements of `d` must be in the range `[-(m-1):1:(n-1)]` . If ```m , the sub-diagonal (-N) of the resulting matrix is the part of the corrsponding column of A that begins from the row (N+1) , and all the super-diagonals of the resulting matrix are the approriate columns of A starting from the first row. Conversely, if m>=nn , the sub-diagonal (-N) of the resulting matrix is the part of the corresponding column of A that begins from the row (N+1) , and all the super-diagonals of the resulting matrix are the approriate columns of A starting from the first row. spdiags(A,d,B) returns a sparse matrix constructed from mXn matrix B , but whose diagonals, specified by the vector d , are replaced by columns of A . The number of columns of A must be identical to the number of elements in d , and the elements of d must be in the range [-(m-1):1:(n-1)] . ```