NAG FL Interface
m01djf (realmat_​rank_​columns)

1 Purpose

m01djf ranks the columns of a matrix of real numbers in ascending or descending order.

2 Specification

Fortran Interface
Subroutine m01djf ( rm, ldm, m1, m2, n1, n2, order, irank, ifail)
Integer, Intent (In) :: ldm, m1, m2, n1, n2
Integer, Intent (Inout) :: ifail
Integer, Intent (Out) :: irank(n2)
Real (Kind=nag_wp), Intent (In) :: rm(ldm,n2)
Character (1), Intent (In) :: order
C Header Interface
#include <nag.h>
void  m01djf_ (const double rm[], const Integer *ldm, const Integer *m1, const Integer *m2, const Integer *n1, const Integer *n2, const char *order, Integer irank[], Integer *ifail, const Charlen length_order)
The routine may be called by the names m01djf or nagf_sort_realmat_rank_columns.

3 Description

m01djf ranks columns n1 to n2 of a matrix, using the data in rows m1 to m2 of those columns. The ordering is determined by first ranking the data in row m1, then ranking any tied columns according to the data in row m1+1, and so on up to row m2.
m01djf uses a variant of list-merging, as described on pages 165–166 in Knuth (1973). The routine takes advantage of natural ordering in the data, and uses a simple list insertion in a preparatory pass to generate ordered lists of length at least 10. The ranking is stable: equal columns preserve their ordering in the input data.

4 References

Knuth D E (1973) The Art of Computer Programming (Volume 3) (2nd Edition) Addison–Wesley

5 Arguments

1: rmldmn2 Real (Kind=nag_wp) array Input
On entry: rows m1 to m2 of columns n1 to n2 of rm must contain real data to be ranked.
2: ldm Integer Input
On entry: the first dimension of the array rm as declared in the (sub)program from which m01djf is called.
Constraint: ldmm2.
3: m1 Integer Input
On entry: the index of the first row of rm to be used.
Constraint: m1>0.
4: m2 Integer Input
On entry: the index of the last row of rm to be used.
Constraint: m2m1.
5: n1 Integer Input
On entry: the index of the first column of rm to be ranked.
Constraint: n1>0.
6: n2 Integer Input
On entry: the index of the last column of rm to be ranked.
Constraint: n2n1.
7: order Character(1) Input
On entry: if order='A', the columns will be ranked in ascending (i.e., nondecreasing) order.
If order='D', into descending order.
Constraint: order='A' or 'D'.
8: irankn2 Integer array Output
On exit: elements n1 to n2 of irank contain the ranks of the corresponding columns of rm. Note that the ranks are in the range n1 to n2: thus, if the ith column of rm is the first in the rank order, iranki is set to n1.
9: ifail Integer Input/Output
On entry: ifail must be set to 0, -1 or 1. If you are unfamiliar with this argument you should refer to Section 4 in the Introduction to the NAG Library FL Interface for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1 or 1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this argument, the recommended value is 0. When the value -1 or 1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
ifail=1
On entry, m1=value.
Constraint: m11.
On entry, m1=value and m2=value.
Constraint: m1m2.
On entry, m2=value.
Constraint: m21.
On entry, m2=value and ldm=value.
Constraint: m2ldm.
On entry, n1=value.
Constraint: n11.
On entry, n1=value and n2=value.
Constraint: n1n2.
On entry, n2=value.
Constraint: n21.
ifail=2
On entry, order has an illegal value: order=value.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

Not applicable.

8 Parallelism and Performance

m01djf is not threaded in any implementation.

9 Further Comments

The average time taken by the routine is approximately proportional to n×logn, where n=n2-n1+1.

10 Example

This example reads a matrix of real numbers and ranks the columns in ascending order.

10.1 Program Text

Program Text (m01djfe.f90)

10.2 Program Data

Program Data (m01djfe.d)

10.3 Program Results

Program Results (m01djfe.r)