#### Description:

The
routine uses the QUICKSORT algorithm to permute an array of pointers so that they access an
associated array of values in descending order. The “median of three” modification is included to
reduce the likelihood of encountering the worst-case behaviour of QUICKSORT.

The routine exists for types REAL (x=R), DOUBLE PRECISION (x=D), and INTEGER (x=I).

#### Invocation

CALL PDA_QSIDx( EL, X, IP )

#### Arguments

##### EL = INTEGER (Given)

The number of
pointers to be permuted.

##### X( EL ) = TYPE (Given)

The array to be sorted.

##### IP( EL ) = INTEGER
(Given and Returned)

The indices of the elements of X in sorted order (i.e. IP( 1 ) gives the index into
X of the highest value).

#### References

Sedgwick, R., 1988, Algorithms (Addison-Wesley).

#### Timing

If N elements are to be sorted, the average time goes as N*ln(N). The worst-case time
goes as N**2.

#### Copyright

Copyright (C) 1992 Science & Engineering Research Council