# nlcpy.fft.rfft

nlcpy.fft.rfft(a, n=None, axis=- 1, norm=None)[ソース]

Computes the one-dimensional discrete fourier transform for a real array.

This function computes the one-dimensional n-point discrete fourier transform (DFT) of a real-valued array by means of an efficient algorithm called the fast fourier transform (FFT).

Parameters
aarray_like

Input array.

nint,optional

Number of points along transformation axis in the input to use. If n is smaller than the length of the input, the input is cropped. If it is larger, the input is padded with zeros. If n is not given, the length of the input along the axis specified by axis is used.

axisint,optional

Axis over which to compute the FFT. If not given, the last axis is used. If axes is larger than the last axis of a, IndexError occurs.

norm{None, "ortho"},optional

Normalization mode. By default(None), the transforms are unscaled. It norm is set to "ortho", the return values will be scaled by .

Returns
outcomplex ndarray

The truncated or zero-padded input, transformed along the axis indicated by axis, or the last one if axis is not specified. If n is even, the length of the transformed axis is (n/2)+1. If n is odd, the length is (n+1)/2.

irfft

Computes the inverse of the n-point DFT for a real array.

fft

Computes the one-dimensional discrete fourier transform.

fftn

Computes the n-dimensional discrete fourier transform.

rfftn

Computes the n-dimensional discrete fourier transform for a real array.

When the DFT is computed for purely a real array, the output is Hermitian-symmetric, i.e. the negative frequency terms are just the complex conjugates of the corresponding positive-frequency terms, and the negative-frequency terms are therefore redundant. This function does not compute the negative frequency terms, and the length of the transformed axis of the output is therefore n//2 + 1.

When A = rfft(a) and fs is the sampling frequency, A contains the zero-frequency term 0*fs, which is real due to Hermitian symmetry.

If n is even, A[-1] contains the term representing both positive and negative Nyquist frequency (+fs/2 and -fs/2), and must also be purely real. If n is odd, there is no term at fs/2; A[-1] contains the largest positive frequency (fs/2*(n-1)/n), and is complex in the general case.

If the input a contains an imaginary part, it is silently discarded.

Examples

>>> import nlcpy as vp
>>> vp.fft.fft([0, 1, 0, 0])
array([ 1.+0.j,  0.-1.j, -1.+0.j,  0.+1.j])  # may vary
>>> vp.fft.rfft([0, 1, 0, 0])
array([ 1.+0.j,  0.-1.j, -1.+0.j])   # may vary


Notice how the final element of the fft() output is the complex conjugate of the second element, for a real array. For rfft, this symmetry is exploited to compute only the non-negative frequency terms.