NAG Library Routine Document
C06PRF computes the discrete Fourier transforms of sequences, each containing complex data values.
||M, N, IFAIL
complex data values
, C06PRF simultaneously calculates the (forward
) discrete Fourier transforms of all the sequences defined by
(Note the scale factor
in this definition.) The minus sign is taken in the argument of the exponential within the summation when the forward transform is required, and the plus sign is taken when the backward transform is required.
A call of C06PRF with followed by a call with will restore the original data.
The routine uses a variant of the fast Fourier transform (FFT) algorithm (see Brigham (1974)
) known as the Stockham self-sorting algorithm, which is described in Temperton (1983)
. Special code is provided for the factors
Brigham E O (1974) The Fast Fourier Transform Prentice–Hall
Temperton C (1983) Self-sorting mixed-radix fast Fourier transforms J. Comput. Phys. 52 1–23
- 1: DIRECT – CHARACTER(1)Input
: if the forward transform as defined in Section 3
is to be computed, then DIRECT
must be set equal to 'F'.
If the backward transform is to be computed then DIRECT
must be set equal to 'B'.
- 2: M – INTEGERInput
On entry: , the number of sequences to be transformed.
- 3: N – INTEGERInput
On entry: , the number of complex values in each sequence.
- 4: X() – COMPLEX (KIND=nag_wp) arrayInput/Output
: the complex data must be stored in X
as if in a two-dimensional array of dimension
; each of the
sequences is stored in a row
of each array.
In other words, if the elements of the
th sequence to be transformed are denoted by
On exit: is overwritten by the complex transforms.
- 5: WORK() – COMPLEX (KIND=nag_wp) arrayWorkspace
the dimension of the array WORK
must be at least
The workspace requirements as documented for C06PRF may be an overestimate in some implementations.
: the real part of
contains the minimum workspace required for the current values of M
with this implementation.
- 6: IFAIL – INTEGERInput/Output
must be set to
. If you are unfamiliar with this parameter you should refer to Section 3.3
in the Essential Introduction for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value
is recommended. If the output of error messages is undesirable, then the value
is recommended. Otherwise, if you are not familiar with this parameter, the recommended value is
. When the value is used it is essential to test the value of IFAIL on exit.
unless the routine detects an error or a warning has been flagged (see Section 6
6 Error Indicators and Warnings
If on entry
, explanatory error messages are output on the current error message unit (as defined by X04AAF
Errors or warnings detected by the routine:
|On entry,|| or .|
|On entry,||N has more than prime factors.|
An unexpected error has occurred in an internal call. Check all subroutine calls and array dimensions. Seek expert help.
Some indication of accuracy can be obtained by performing a subsequent inverse transform and comparing the results with the original sequence (in exact arithmetic they would be identical).
The time taken by C06PRF is approximately proportional to , but also depends on the factors of . C06PRF is fastest if the only prime factors of are , and , and is particularly slow if is a large prime, or has large prime factors.
This example reads in sequences of complex data values and prints their discrete Fourier transforms (as computed by C06PRF with ). Inverse transforms are then calculated using C06PRF with and printed out, showing that the original sequences are restored.
9.1 Program Text
Program Text (c06prfe.f90)
9.2 Program Data
Program Data (c06prfe.d)
9.3 Program Results
Program Results (c06prfe.r)