Computes the parameters needed for imsl_f_fft_cosine.
#include <imsl.h>
float *imsl_f_fft_cosine_init (int n)
The type double procedure is imsl_d_fft_cosine_init.
int n
(Input)
Length of the sequence to be transformed. It must be greater than
1.
A pointer to parameter vector of length (3*n + 15) that can then be used by imsl_f_fft_cosine when the optional argument IMSL_PARAMS is specified. To release this space, use free. If no solution was computed, then NULL is returned.
The function imsl_f_fft_cosine_init should be used when many calls must be made to imsl_f_fft_cosine without changing the sequence length n. The function imsl_f_fft_cosine_init is based on the routine COSTI in FFTPACK. The package FFTPACK was developed by Paul Swarztrauber at the National Center for Atmospheric Research.
This example computes three distinct sine FFTs by calling imsl_f_fft_cosine_init once, then calling imsl_f_fft_cosine three times. The internal parameter initialization in imsl_f_fft_cosine is now skipped.
#include <imsl.h>
#include <math.h>
main()
{
int n = 7;
int i, k;
float p[7];
float q[7];
float pi;
float *params;
pi =
imsl_f_constant("pi", 0);
/* Compute parameters for transform of
length n */
params =
imsl_f_fft_cosine_init (n);
/* Different
frequencies of the same
wave will be transformed */
for (k=0; k<3; k++) {
printf("\n");
/* Fill p with a pure cosine wave */
for (i=0; i<n; i++)
p[i] = cos((float)((k+1)*i)*pi/(float)(n-1));
/* Compute the transform of p */
imsl_f_fft_cosine (n, p,
IMSL_PARAMS, params,
IMSL_RETURN_USER, q,
0);
printf (" index\t p\t
q\n");
for (i=0; i<n; i++)
printf("\t%1d\t%5.2f\t%5.2f\n", i, p[i], q[i]);
}
}
index p q
0 1.00 -0.00
1 0.87 6.00
2 0.50 0.00
3 -0.00 0.00
4 -0.50 -0.00
5 -0.87 -0.00
6 -1.00 -0.00
index
p q
0 1.00 0.00
1 0.50 -0.00
2 -0.50 6.00
3 -1.00 0.00
4 -0.50 0.00
5 0.50 0.00
6 1.00 -0.00
index
p q
0 1.00 -0.00
1 -0.00 0.00
2 -1.00 -0.00
3 0.00 6.00
4 1.00 0.00
5 -0.00 -0.00
6 -1.00 0.00
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