• NAME
• F95 INTERFACE
• C INTERFACE
• PURPOSE
• ARGUMENTS
• SEE ALSO

NAME

cffts - initialize the trigonometric weight and factor tables or compute the inverse Fast Fourier Transform of a complex sequence as follows. =head1 SYNOPSIS

  SUBROUTINE CFFTS( IOPT, N, SCALE, X, Y, TRIGS, IFAC, WORK, LWORK, 
 *      IERR)
  COMPLEX X(*)
  INTEGER IOPT, N, LWORK, IERR
  INTEGER IFAC(*)
  REAL SCALE
  REAL Y(*), TRIGS(*), WORK(*)

  SUBROUTINE CFFTS_64( IOPT, N, SCALE, X, Y, TRIGS, IFAC, WORK, LWORK, 
 *      IERR)
  COMPLEX X(*)
  INTEGER*8 IOPT, N, LWORK, IERR
  INTEGER*8 IFAC(*)
  REAL SCALE
  REAL Y(*), TRIGS(*), WORK(*)

F95 INTERFACE

  SUBROUTINE FFT( IOPT, N, [SCALE], X, Y, TRIGS, IFAC, WORK, [LWORK], 
 *       IERR)
  COMPLEX, DIMENSION(:) :: X
  INTEGER :: IOPT, N, LWORK, IERR
  INTEGER, DIMENSION(:) :: IFAC
  REAL :: SCALE
  REAL, DIMENSION(:) :: Y, TRIGS, WORK

  SUBROUTINE FFT_64( IOPT, N, [SCALE], X, Y, TRIGS, IFAC, WORK, [LWORK], 
 *       IERR)
  COMPLEX, DIMENSION(:) :: X
  INTEGER(8) :: IOPT, N, LWORK, IERR
  INTEGER(8), DIMENSION(:) :: IFAC
  REAL :: SCALE
  REAL, DIMENSION(:) :: Y, TRIGS, WORK

C INTERFACE

#include <sunperf.h>

void cffts(int iopt, int n, float scale, complex *x, float *y, float *trigs, int *ifac, float *work, int lwork, int *ierr);

void cffts_64(long iopt, long n, float scale, complex *x, float *y, float *trigs, long *ifac, float *work, long lwork, long *ierr);

PURPOSE

cffts initializes the trigonometric weight and factor tables or computes the inverse Fast Fourier Transform of a complex sequence as follows: .Ve

               N-1

Y(k) = scale * SUM W*X(j)

               j=0
.Ve

where

k ranges from 0 to N-1

i = sqrt(-1)

isign = 1 for inverse transform or -1 for forward transform

W = exp(isign*i*j*k*2*pi/N)

In complex-to-real transform of length N, the (N/2+1) complex input data points stored are the positive-frequency half of the spectrum of the Discrete Fourier Transform. The other half can be obtained through complex conjugation and therefore is not stored. Furthermore, due to symmetries the imaginary of the component of X(0) and X(N/2) (if N is even in the latter) is assumed to be zero and is not referenced.

ARGUMENTS

• IOPT (input)
  Integer specifying the operation to be performed:

  IOPT = 0 computes the trigonometric weight table and factor table

  IOPT = 1 computes inverse FFT

• N (input)
  Integer specifying length of the input sequence X. N is most efficient when it is a product of small primes. N > = 0. Unchanged on exit.

• SCALE (input)
  Real scalar by which transform results are scaled. Unchanged on exit.

• X (input)
  On entry, X is a complex array whose first (N/2+1) elements are the input sequence to be transformed.

• Y (output)
  Real array of dimension at least N that contains the transform results. X and Y may be the same array starting at the same memory location. Otherwise, it is assumed that there is no overlap between X and Y in memory.

• TRIGS (input/output)
  Real array of length 2*N that contains the trigonometric weights. The weights are computed when the routine is called with IOPT = 0 and they are used in subsequent calls when IOPT = 1. Unchanged on exit.

• IFAC (input/output)
  Integer array of dimension at least 128 that contains the factors of N. The factors are computed when the routine is called with IOPT = 0 and they are used in subsequent calls where IOPT = 1. Unchanged on exit.

• WORK (output)
  Real array of dimension at least N. The user can also choose to have the routine allocate its own workspace (see LWORK).

• LWORK (input)
  Integer specifying workspace size. If LWORK = 0, the routine will allocate its own workspace.

• IERR (output)
  On exit, integer IERR has one of the following values:

  0 = normal return

  -1 = IOPT is not 0 or 1

  -2 = N < 0

  -3 = (LWORK is not 0) and (LWORK is less than N)

  -4 = memory allocation for workspace failed

SEE ALSO

fft