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## Math::Libgsl::Interpolation zef:FRITH last updated on 2023-01-03

098e8a44ceb93495cfe061c2fb56661f4986449a/

# NAME

Math::Libgsl::Interpolation - An interface to libgsl, the Gnu Scientific Library - Interpolation

# SYNOPSIS

```use Math::Libgsl::Constants;
use Math::Libgsl::Interpolation;

my constant \N = 4;
my Num @x = 0e0, .1e0, .27e0, .3e0;
my Num @y = .15e0, .7e0, -.1e0, .15e0;

my \$s = Math::Libgsl::Interpolation::OneD.new(:type(CSPLINE_PERIODIC), :size(N), :spline).init(@x, @y);

for ^100 -> \$i {
my Num \$xi = (1 - \$i / 100) * @x[0] + (\$i / 100) * @x[N - 1];
my Num \$yi = \$s.eval: \$xi;
printf "%g %g\n", \$xi, \$yi;
}
```

# DESCRIPTION

Math::Libgsl::Interpolation is an interface to the interpolation functions of libgsl, the Gnu Scientific Library. This module exports two classes:

• Math::Libgsl::Interpolation::OneD

• Math::Libgsl::Interpolation::TwoD

## Math::Libgsl::Interpolation::OneD

### new(Int :\$type!, Int :\$size!)

This multi method constructor requires two simple or named arguments, the type of interpolation and the size of the array of data points.

All the following methods throw on error if they return self, otherwise they fail on error.

### init(Num() @xarray where ([<] @xarray), Num() @yarray --> Math::Libgsl::Interpolation::OneD)

This method initializes the interpolation internal data using the X and Y coordinate arrays. It must be called each time one wants to use the object to evaluate interpolation points on another data set. The X array has to be strictly ordered, with increasing x values. This method returns self, so it may be chained.

### min-size(--> UInt)

This method returns the minimum number of points required by the interpolation object.

### name(--> Str)

This method returns the name of the interpolation type.

### find(Num() @xarray, Num() \$x --> UInt)

This method performs a lookup action on the data array @xarray and returns the index i such that @xarray[i] <= \$x < @xarray[i+1].

### reset(--> Math::Libgsl::Interpolation::OneD)

This method reinitializes the internal data structure and deletes the cache. It should be used when switching to a new dataset. This method returns self, so it may be chained.

### eval(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax --> Num)

This method returns the interpolated value of y for a given point x, which is inside the range of the x data set.

### eval-deriv(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax --> Num)

This method returns the derivative of an interpolated function for a given point x, which is inside the range of the x data set.

### eval-deriv2(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax --> Num)

This method returns the second derivative of an interpolated function for a given point x, which is inside the range of the x data set.

### eval-integ(Num() \$a where \$!spline.interp.xmin ≤ *, Num() \$b where { \$b ≤ \$!spline.interp.xmax && \$b ≥ \$a } --> Num)

This method returns the numerical integral result of an interpolated function over the range [\$a, \$b], two points inside the x data set.

## Math::Libgsl::Interpolation::TwoD

### new(Int :\$type!, Int :\$xsize!, Int :\$ysize!)

This multi method constructor requires three simple or named arguments, the type of interpolation and the size of the arrays of the x and y data points.

### init(Num() @xarray where ([<] @xarray), Num() @yarray where ([<] @yarray), Num() @zarray where *.elems == @xarray.elems * @yarray.elems --> Math::Libgsl::Interpolation::TwoD)

This method initializes the interpolation internal data using the X, Y, and Z coordinate arrays. It must be called each time one wants to use the object to evaluate interpolation points on another data set. The X and Y arrays have to be strictly ordered, with increasing values. The Z array, which represents a grid, must have a dimension of size xsize * ysize. The position of grid point (x, y) is given by y * xsize + x (see also the zidx method). This method returns self, so it may be chained.

### zidx(UInt \$x, UInt \$y --> UInt)

This method returns the index of the Z array element corresponding to grid coordinates (x, y).

### min-size(--> UInt)

This method returns the minimum number of points required by the interpolation object.

### name(--> Str)

This method returns the name of the interpolation type.

### eval(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of z for a given point (x, y), which is inside the range of the x and y data sets.

### extrap(Num() \$x, Num() \$y --> Num)

This method returns the interpolated value of z for a given point (x, y), with no bounds checking, so when the point (x, y) is outside the range, an extrapolation is performed.

### deriv-x(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of ∂z/∂x for a given point (x, y), which is inside the range of the x and y data sets.

### deriv-y(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of ∂z/∂y for a given point (x, y), which is inside the range of the x and y data sets.

### deriv-xx(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of ∂²z/∂²x for a given point (x, y), which is inside the range of the x and y data sets.

### deriv-yy(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of ∂²z/∂²y for a given point (x, y), which is inside the range of the x and y data sets.

### deriv-xy(Num() \$x where \$!spline.interp.xmin ≤ * ≤ \$!spline.interp.xmax, Num() \$y where \$!spline.interp.ymin ≤ * ≤ \$!spline.interp.ymax --> Num)

This method returns the interpolated value of ∂²z/∂x∂y for a given point (x, y), which is inside the range of the x and y data sets.

## Accessory subroutines

### bsearch(Num() @xarray, Num() \$x, UInt \$idx-lo = 0, UInt \$idx-hi = @xarray.elems - 1 --> UInt)

This sub returns the index i of the array @xarray such that @xarray[i] <= \$x < @xarray[i+1]. The index is searched for in the range [\$idx-lo, \$idx-hi].

### min-size2d(Int \$type --> UInt)

These subs return the minimum number of points required by the interpolation object, without the need to create an object. min-size is an alias for min-size1d.

# C Library Documentation

For more details on libgsl see https://www.gnu.org/software/gsl/. The excellent C Library manual is available here https://www.gnu.org/software/gsl/doc/html/index.html, or here https://www.gnu.org/software/gsl/doc/latex/gsl-ref.pdf in PDF format.

# Prerequisites

This module requires the libgsl library to be installed. Please follow the instructions below based on your platform:

## Debian Linux and Ubuntu 20.04+

``````sudo apt install libgsl23 libgsl-dev libgslcblas0
``````

That command will install libgslcblas0 as well, since it's used by the GSL.

## Ubuntu 18.04

libgsl23 and libgslcblas0 have a missing symbol on Ubuntu 18.04. I solved the issue installing the Debian Buster version of those three libraries:

# Installation

To install it using zef (a module management tool):

``````\$ zef install Math::Libgsl::Interpolation
``````

# AUTHOR

Fernando Santagata [email protected]