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# taper_n

Applies split-cosine-bell tapering to one or more series across the given dimension. Available in version 5.2.0 and later.

## Prototype

```	function taper_n (
x       : numeric,
p    : numeric,
option  : numeric,
dim  : integer
)

return_val [*] :  numeric
```

## Arguments

x

An array of one or more dimensions. The tapering will be performed on the dim-th dimension.

p

A scalar (0.0 ≤ p ≤ 1.0) which specifies the proportion of the series to be tapered. Typically, p = 0.1 (corresponding to 10% of the series).

option

As of NCL V6.2.1, option can be set to:

• option = 0
Results in the series being tapered to the series mean
(this is the default behavior in older versions of this function)

• option = 1
Forces the function to taper to 0.0

• option not equal to 0 or 1
Acts like option = 0

dim

A scalar integer indicating which dimension of x to do the calculation on. Dimension numbering starts at 0.

## Return value

Returns an array dimensioned the same as x.

The return type is floating point if the input is floating point, and double if the input is of type double.

## Description

This function is used prior to performing a fast fourier transform (FFT) to a series that is not cyclic. It reduces the leakage from strong spectral peaks. See chapter five of the following reference:

Fourier Analysis of Time Series
Peter Bloomfield
Wiley-Interscience, 1976

You can use taper if the dimension to do the calculation on is the rightmost dimension.

## Examples

Example 1

Let x be a 1D array of length nx (e.g. nx = 800):

```  xTaper = taper_n(x, 0.1, 0, 0)
xCoef  = ezfftf(xTaper)
```
Example 2

Define x(time, lat, lon) where ntim = 500, nlat = 64, mlon = 128:

```  xTaper = taper_n(x, 0.1, 0, 0)
xCoef  = ezfftf(xTaper)
```