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Contents → Standard User Functions → fft.4

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Fft.4 does a fast Fourier transform of the current data.

fn fft.4
fn fft.4 options
or
fn . .

With no arguments, you are prompted for the type of transform and type of data in which you are interested. If you are supplying complex input data or asking for complex output data, the real part is stored in the x data column and the imaginary part is stored in the s (error-bar) data column. You also can supply options on the command line. Valid options are:

.   :   use same options as last time   +f or -f   :   forward (or reverse) transform   +r or -r   :   real (or complex) input data   +o or -o   :   real (or complex) output   +a or -a   :   take absolute value (or don't)   +s or -s   :   take absolute value squared (or don't)   +m or -m   :   sort and merge the data (or don't)   +i or -i   :   interpolate input data (or don't)   n=#   :   set number of points   f=#   :   set from value for interpolation   t=#   :   set to value for interpolation

The default options are:

+fro  -asmi  n=npts f=first_x t=last_x

Here npts, first_x and last_x refer to the number of points and the range of the current data. Interpolation, if used, is by a cubic spline. (See spline.4 ). Naturally, points in x must be equally spaced for the transform to make sense.

Transforming more points than the number of in-core points, as set in the $CPLOTHOME/cplot_config or $HOME/.cplot_init file is extremely slow because of the large number of file accesses required.

When entering options interactively, it is possible to turn on a verbose mode that will show the status of the transform as it works through the data.

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