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### IMSL_FFTINIT

IMSL_FFTINIT

The IMSL_FFTINIT function computes the parameters for a one-dimensional FFT to be used in the IMSL_FFTCOMP function with the keyword INIT_PARAMS.

The IMSL_FFTINIT function should be used when many calls are to be made to IMSL_FFTCOMP without changing the data type of the array and the length of the sequence. The default action of IMSL_FFTINIT is to compute the parameters necessary for a real FFT. If parameters for a complex FFT are needed, the keyword COMPLEX should be specified.

The IMSL_FFTINIT function is based on the routines RFFTI and RFFTI in FFTPACK, which was developed by Paul Swarztrauber at the National Center for Atmospheric Research.

## Example

In this example, two distinct, real FFTs are computed by calling IMSL_FFTINIT once, then calling IMSL_FFTCOMP twice.

`.RUN`
`n = 7`
` `
`; Define the length of the signals.`
`init_params = IMSL_FFTINIT(7)`
` `
`; Initialize the parameters by calling IMSL_FFTINIT.`
`FOR j = 0, 2 DO BEGIN`
`  p = COS(j * FINDGEN(n) * 2 * !Pi/n)`
`  q = IMSL_FFTCOMP(p, Init_Params = init_params)`
`  PM, 'p', 'q', FORMAT = '(7x, a1, 10x, a1)'`
`  FOR i = 0, n - 1 DO PM, p(i), q(i), FORMAT = '(f10.5, f10.2)'`
`ENDFOR`
`END`
`; For each pass through loop, compute a real FFT of an array of`
`; length n and output both original signal and computed FFT.`

IDL prints:

## Syntax

Result = IMSL_FFTINIT(N [, /DOUBLE] [, COMPLEX=value] [, SINE=value] [, COSINE=value])

## Return Value

A one-dimensional array of length 2N + 15 that can then be used by IMSL_FFTCOMP when the optional parameter INIT_PARAMS is specified.

## Arguments

### N

Length of the sequence to be transformed.

## Keywords

### COMPLEX (optional)

If present and nonzero, the complex transform is computed.

### COSINE (optional)

If present and nonzero, then parameters for a discrete Fourier cosine transformation are returned. See the IMSL_FFTCOMP keyword SINE.

### DOUBLE (optional)

If present and nonzero, then double precision is used.

### SINE (optional)

If present and nonzero, then parameters for a discrete Fourier cosine transformation are returned. See the IMSL_FFTCOMP keyword SINE.

## Version History

 6.4 Introduced

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