NAG Library Function Document

nag_make_indices (m01zac)

The m01d- functions generate ranks, and the m01e- functions require indices to be supplied to specify the re-ordering. However if it is desired simply to refer to the data in sorted order without actually re-ordering them, indices are more convenient than ranks (see Section 10). nag_make_indices (m01zac) can be used to convert ranks to indices, or indices to ranks, as the two permutations are inverses of one another.

4 References

None.

5 Arguments

1: ranks[n] – size_tInput/Output: On entry: ranks must contain a permutation of the Integers 0 to $n - 1$ .

On exit: ranks contains the inverse permutation.
2: n – size_tInput: On entry: the length of the array ranks.
3: fail – NagError *Input/Output: The NAG error argument (see Section 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_BAD_RANK: Invalid ranks vector.
Elements of ranks contain a value outside the range 0 to $n - 1$ or contain a repeated value. ranks does not contain a permutation of the Integers 0 to $n - 1$ ; on exit these elements are usually corrupted.
NE_INT_ARG_GT: On entry, $n = ⟨value⟩$ .
Constraint: $n \leq ⟨value⟩$ .
n is limited to an implementation-dependent size which is printed in the error message.
NE_INT_ARG_LT: On entry, $n = ⟨value⟩$ .
Constraint: $n \geq 0$ .

7 Accuracy

Not applicable.

8 Parallelism and Performance

Not applicable.

9 Further Comments

None.

10 Example

The example program reads a matrix of real numbers and prints its rows with the elements of the 1st column in ascending order as ranked by nag_rank_sort (m01dsc). The program first calls nag_rank_sort (m01dsc) to rank the rows, and then calls nag_make_indices (m01zac) to convert the rank vector to an index vector, which is used to refer to the rows in sorted order.

NAG Library Function Documentnag_make_indices (m01zac)

+− Contents

1 Purpose

2 Specification

3 Description