IDL Reference Guide: Procedures and Functions

DIAG_MATRIX

The DIAG_MATRIX function constructs a diagonal matrix from an input vector, or if given a matrix, then DIAG_MATRIX will extract a diagonal vector.

Syntax

Result = DIAG_MATRIX(A [, Diag] )

Return Value

Arguments

A

Either an n-element input vector to convert to a diagonal matrix, or a n-by-m input array to extract a diagonal. A may be any numeric type.

Diag

An optional argument that specifies the subdiagonal (Diag < 0) or superdiagonal (Diag > 0) to fill or extract. The default is Diag=0 which puts or extracts the values along the diagonal. If A is a vector with the m elements, then the result is an n-by-n array, where n = m + ABS(Diag). If A is an array, then the result is a vector whose length depends upon the number of elements remaining along the subdiagonal or superdiagonal.

Keywords

None.

Examples

Create a tridiagonal matrix and extract the diagonal using the following program:

PRO ExDiagMatrix  
; Convert three input vectors to a tridiagonal matrix:  
diag = [1, -2, 3, -4]  
sub = [5, 10, 15]  
super = [3, 6, 9]  
array = DIAG_MATRIX(diag) + $  
DIAG_MATRIX(super, 1) + DIAG_MATRIX(sub, -1)  
PRINT, 'DIAG_MATRIX array:'  
PRINT, array  
  
; Extract the diagonal:  
PRINT, 'DIAG_MATRIX diagonal:'  
PRINT, DIAG_MATRIX(array)  
END  

When this program is compiled and run, IDL prints:

DIAG_MATRIX array:  
1       3       0       0  
5      -2       6       0  
0      10       3       9  
0       0      15      -4  
DIAG_MATRIX diagonal:  
1      -2       3      -4