ARRAYTRACE Function
Computes the trace of a matrix or a tensor contraction on a multidimensional array.
enabled.
Usage
t = ARRAYTRACE( b[, j, k] )
Input Parameters
b—Numerical n-dimensional array.
j—(Required for n>2) A positive integer (≥0) designating a dimension of b. If j is supplied then k must be supplied and distinct from j.
k—(Required for n>2) A positive integer (≥0) designating a dimension of b. If j is supplied then k must be supplied and distinct from j.
Returned Value
t—An (n–2)-dimensional array which is the tensor contraction of b over dimensions j and k. For n=2 this is the trace of the matrix b, and more generally it is the sum of all the (n–2)-D subarrays defined along the main diagonal of the j-k plane. If b is of integral data type then the data type of t is single-precision floating-point; otherwise, the data type of t is the same as that of b.
Keywords
None.
Discussion
For large or multidimensional arrays, computing traces with ARRAYTRACE is generally much faster than computing them with TOTAL and its Dimension keyword. One other difference between the two methods is that the result from TOTAL retains the contracted dimensions of length 1 (which can be eliminated with REFORM), whereas ARRAYTRACE always eliminates these degenerate dimensions from the result.
Examples
b = FINDGEN(5, 4)
PM, b
; PV-WAVE prints:
; 0.00000 5.00000 10.0000 15.0000
; 1.00000 6.00000 11.0000 16.0000
; 2.00000 7.00000 12.0000 17.0000
; 3.00000 8.00000 13.0000 18.0000
; 4.00000 9.00000 14.0000 19.0000
t = ARRAYTRACE(b)
PM, t
; PV-WAVE prints:
; 36.0000
b = FINDGEN(3, 3, 3)
PM, b
; PV-WAVE prints:
; 0.00000 3.00000 6.00000
; 1.00000 4.00000 7.00000
; 2.00000 5.00000 8.00000
; 9.00000 12.0000 15.0000
; 10.0000 13.0000 16.0000
; 11.0000 14.0000 17.0000
; 18.0000 21.0000 24.0000
; 19.0000 22.0000 25.0000
; 20.0000 23.0000 26.0000
PM, ARRAYTRACE(b, 0, 1)
; PV-WAVE prints:
; 12.0000
; 39.0000
; 66.0000
PM, ARRAYTRACE(b, 0, 2)
; PV-WAVE prints:
; 30.0000
; 39.0000
; 48.0000
PM, ARRAYTRACE(b, 1, 2)
; PV-WAVE prints:
; 36.0000
; 39.0000
; 42.0000
See Also