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<h1 class="title">math::interpolate(n) 1.0.2 tcllib "Tcl Math Library"</h1>
<div id="name" class="section"><h2><a name="name">Name</a></h2>
<p>math::interpolate - Interpolation routines</p>
</div>
<div id="toc" class="section"><h2><a name="toc">Table Of Contents</a></h2>
<ul class="toc">
<li class="section"><a href="#toc">Table Of Contents</a></li>
<li class="section"><a href="#synopsis">Synopsis</a></li>
<li class="section"><a href="#section1">Description</a></li>
<li class="section"><a href="#section2">PROCEDURES</a></li>
<li class="section"><a href="#section3">EXAMPLES</a></li>
<li class="section"><a href="#section4">BUGS, IDEAS, FEEDBACK</a></li>
<li class="section"><a href="#keywords">Keywords</a></li>
<li class="section"><a href="#category">Category</a></li>
<li class="section"><a href="#copyright">Copyright</a></li>
</ul>
</div>
<div id="synopsis" class="section"><h2><a name="synopsis">Synopsis</a></h2>
<div class="synopsis">
<ul class="requirements">
<li>package require <b class="pkgname">Tcl <span class="opt">?8.4?</span></b></li>
<li>package require <b class="pkgname">struct</b></li>
<li>package require <b class="pkgname">math::interpolate <span class="opt">?1.0.2?</span></b></li>
</ul>
<ul class="syntax">
<li><a href="#1"><b class="cmd">::math::interpolate::defineTable</b> <i class="arg">name</i> <i class="arg">colnames</i> <i class="arg">values</i></a></li>
<li><a href="#2"><b class="cmd">::math::interpolate::interp-1d-table</b> <i class="arg">name</i> <i class="arg">xval</i></a></li>
<li><a href="#3"><b class="cmd">::math::interpolate::interp-table</b> <i class="arg">name</i> <i class="arg">xval</i> <i class="arg">yval</i></a></li>
<li><a href="#4"><b class="cmd">::math::interpolate::interp-linear</b> <i class="arg">xyvalues</i> <i class="arg">xval</i></a></li>
<li><a href="#5"><b class="cmd">::math::interpolate::interp-lagrange</b> <i class="arg">xyvalues</i> <i class="arg">xval</i></a></li>
<li><a href="#6"><b class="cmd">::math::interpolate::prepare-cubic-splines</b> <i class="arg">xcoord</i> <i class="arg">ycoord</i></a></li>
<li><a href="#7"><b class="cmd">::math::interpolate::interp-cubic-splines</b> <i class="arg">coeffs</i> <i class="arg">x</i></a></li>
<li><a href="#8"><b class="cmd">::math::interpolate::interp-spatial</b> <i class="arg">xyvalues</i> <i class="arg">coord</i></a></li>
<li><a href="#9"><b class="cmd">::math::interpolate::interp-spatial-params</b> <i class="arg">max_search</i> <i class="arg">power</i></a></li>
<li><a href="#10"><b class="cmd">::math::interpolate::neville</b> <i class="arg">xlist</i> <i class="arg">ylist</i> <i class="arg">x</i></a></li>
</ul>
</div>
</div>
<div id="section1" class="section"><h2><a name="section1">Description</a></h2>
<p>This package implements several interpolation algorithms:</p>
<ul class="itemized">
<li><p>Interpolation into a table (one or two independent variables), this is useful
for example, if the data are static, like with tables of statistical functions.</p></li>
<li><p>Linear interpolation into a given set of data (organised as (x,y) pairs).</p></li>
<li><p>Lagrange interpolation. This is mainly of theoretical interest, because there is
no guarantee about error bounds. One possible use: if you need a line or
a parabola through given points (it will calculate the values, but not return
the coefficients).</p>
<p>A variation is Neville's method which has better behaviour and error
bounds.</p></li>
<li><p>Spatial interpolation using a straightforward distance-weight method. This procedure
allows any number of spatial dimensions and any number of dependent variables.</p></li>
<li><p>Interpolation in one dimension using cubic splines.</p></li>
</ul>
<p>This document describes the procedures and explains their usage.</p>
</div>
<div id="section2" class="section"><h2><a name="section2">PROCEDURES</a></h2>
<p>The interpolation package defines the following public procedures:</p>
<dl class="definitions">
<dt><a name="1"><b class="cmd">::math::interpolate::defineTable</b> <i class="arg">name</i> <i class="arg">colnames</i> <i class="arg">values</i></a></dt>
<dd><p>Define a table with one or two independent variables (the distinction is implicit in
the data). The procedure returns the name of the table - this name is used whenever you
want to interpolate the values. <em>Note:</em> this procedure is a convenient wrapper for the
struct::matrix procedure. Therefore you can access the data at any location in your program.</p>
<dl class="arguments">
<dt>string <i class="arg">name</i> (in)</dt>
<dd><p>Name of the table to be created</p></dd>
<dt>list <i class="arg">colnames</i> (in)</dt>
<dd><p>List of column names</p></dd>
<dt>list <i class="arg">values</i> (in)</dt>
<dd><p>List of values (the number of elements should be a
multiple of the number of columns. See <span class="sectref"><a href="#section3">EXAMPLES</a></span> for more information on the
interpretation of the data.</p>
<p>The values must be sorted with respect to the independent variable(s).</p></dd>
</dl></dd>
<dt><a name="2"><b class="cmd">::math::interpolate::interp-1d-table</b> <i class="arg">name</i> <i class="arg">xval</i></a></dt>
<dd><p>Interpolate into the one-dimensional table "name" and return a list of values, one for
each dependent column.</p>
<dl class="arguments">
<dt>string <i class="arg">name</i> (in)</dt>
<dd><p>Name of an existing table</p></dd>
<dt>float <i class="arg">xval</i> (in)</dt>
<dd><p>Value of the independent <em>row</em> variable</p></dd>
</dl></dd>
<dt><a name="3"><b class="cmd">::math::interpolate::interp-table</b> <i class="arg">name</i> <i class="arg">xval</i> <i class="arg">yval</i></a></dt>
<dd><p>Interpolate into the two-dimensional table "name" and return the interpolated value.</p>
<dl class="arguments">
<dt>string <i class="arg">name</i> (in)</dt>
<dd><p>Name of an existing table</p></dd>
<dt>float <i class="arg">xval</i> (in)</dt>
<dd><p>Value of the independent <em>row</em> variable</p></dd>
<dt>float <i class="arg">yval</i> (in)</dt>
<dd><p>Value of the independent <em>column</em> variable</p></dd>
</dl></dd>
<dt><a name="4"><b class="cmd">::math::interpolate::interp-linear</b> <i class="arg">xyvalues</i> <i class="arg">xval</i></a></dt>
<dd><p>Interpolate linearly into the list of x,y pairs and return the interpolated value.</p>
<dl class="arguments">
<dt>list <i class="arg">xyvalues</i> (in)</dt>
<dd><p>List of pairs of (x,y) values, sorted to increasing x.
They are used as the breakpoints of a piecewise linear function.</p></dd>
<dt>float <i class="arg">xval</i> (in)</dt>
<dd><p>Value of the independent variable for which the value of y
must be computed.</p></dd>
</dl></dd>
<dt><a name="5"><b class="cmd">::math::interpolate::interp-lagrange</b> <i class="arg">xyvalues</i> <i class="arg">xval</i></a></dt>
<dd><p>Use the list of x,y pairs to construct the unique polynomial of lowest degree
that passes through all points and return the interpolated value.</p>
<dl class="arguments">
<dt>list <i class="arg">xyvalues</i> (in)</dt>
<dd><p>List of pairs of (x,y) values</p></dd>
<dt>float <i class="arg">xval</i> (in)</dt>
<dd><p>Value of the independent variable for which the value of y
must be computed.</p></dd>
</dl></dd>
<dt><a name="6"><b class="cmd">::math::interpolate::prepare-cubic-splines</b> <i class="arg">xcoord</i> <i class="arg">ycoord</i></a></dt>
<dd><p>Returns a list of coefficients for the second routine
<em>interp-cubic-splines</em> to actually interpolate.</p>
<dl class="arguments">
<dt>list <i class="arg">xcoord</i></dt>
<dd><p>List of x-coordinates for the value of the
function to be interpolated is known. The coordinates must be strictly
ascending. At least three points are required.</p></dd>
<dt>list <i class="arg">ycoord</i></dt>
<dd><p>List of y-coordinates (the values of the
function at the given x-coordinates).</p></dd>
</dl></dd>
<dt><a name="7"><b class="cmd">::math::interpolate::interp-cubic-splines</b> <i class="arg">coeffs</i> <i class="arg">x</i></a></dt>
<dd><p>Returns the interpolated value at coordinate x. The coefficients are
computed by the procedure <em>prepare-cubic-splines</em>.</p>
<dl class="arguments">
<dt>list <i class="arg">coeffs</i></dt>
<dd><p>List of coefficients as returned by
prepare-cubic-splines</p></dd>
<dt>float <i class="arg">x</i></dt>
<dd><p>x-coordinate at which to estimate the function. Must
be between the first and last x-coordinate for which values were given.</p></dd>
</dl></dd>
<dt><a name="8"><b class="cmd">::math::interpolate::interp-spatial</b> <i class="arg">xyvalues</i> <i class="arg">coord</i></a></dt>
<dd><p>Use a straightforward interpolation method with weights as function of the
inverse distance to interpolate in 2D and N-dimensional space</p>
<p>The list xyvalues is a list of lists:</p>
<pre class="example">
{ {x1 y1 z1 {v11 v12 v13 v14}}
{x2 y2 z2 {v21 v22 v23 v24}}
...
}
</pre>
<p>The last element of each inner list is either a single number or a list in itself.
In the latter case the return value is a list with the same number of elements.</p>
<p>The method is influenced by the search radius and the power of the inverse distance</p>
<dl class="arguments">
<dt>list <i class="arg">xyvalues</i> (in)</dt>
<dd><p>List of lists, each sublist being a list of coordinates and
of dependent values.</p></dd>
<dt>list <i class="arg">coord</i> (in)</dt>
<dd><p>List of coordinates for which the values must be calculated</p></dd>
</dl></dd>
<dt><a name="9"><b class="cmd">::math::interpolate::interp-spatial-params</b> <i class="arg">max_search</i> <i class="arg">power</i></a></dt>
<dd><p>Set the parameters for spatial interpolation</p>
<dl class="arguments">
<dt>float <i class="arg">max_search</i> (in)</dt>
<dd><p>Search radius (data points further than this are ignored)</p></dd>
<dt>integer <i class="arg">power</i> (in)</dt>
<dd><p>Power for the distance (either 1 or 2; defaults to 2)</p></dd>
</dl></dd>
<dt><a name="10"><b class="cmd">::math::interpolate::neville</b> <i class="arg">xlist</i> <i class="arg">ylist</i> <i class="arg">x</i></a></dt>
<dd><p>Interpolates between the tabulated values of a function
whose abscissae are <i class="arg">xlist</i>
and whose ordinates are <i class="arg">ylist</i> to produce an estimate for the value
of the function at <i class="arg">x</i>. The result is a two-element list; the first
element is the function's estimated value, and the second is an estimate
of the absolute error of the result. Neville's algorithm for polynomial
interpolation is used. Note that a large table of values will use an
interpolating polynomial of high degree, which is likely to result in
numerical instabilities; one is better off using only a few tabulated
values near the desired abscissa.</p></dd>
</dl>
</div>
<div id="section3" class="section"><h2><a name="section3">EXAMPLES</a></h2>
<p><em>TODO</em>
Example of using the cubic splines:</p>
<p>Suppose the following values are given:</p>
<pre class="example">
x y
0.1 1.0
0.3 2.1
0.4 2.2
0.8 4.11
1.0 4.12
</pre>
<p>Then to estimate the values at 0.1, 0.2, 0.3, ... 1.0, you can use:</p>
<pre class="example">
set coeffs [::math::interpolate::prepare-cubic-splines {0.1 0.3 0.4 0.8 1.0} {1.0 2.1 2.2 4.11 4.12}]
foreach x {0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0} {
puts "$x: [::math::interpolate::interp-cubic-splines $coeffs $x]"
}
</pre>
<p>to get the following output:</p>
<pre class="example">
0.1: 1.0
0.2: 1.68044117647
0.3: 2.1
0.4: 2.2
0.5: 3.11221507353
0.6: 4.25242647059
0.7: 5.41804227941
0.8: 4.11
0.9: 3.95675857843
1.0: 4.12
</pre>
<p>As you can see, the values at the abscissae are reproduced perfectly.</p>
</div>
<div id="section4" class="section"><h2><a name="section4">BUGS, IDEAS, FEEDBACK</a></h2>
<p>This document, and the package it describes, will undoubtedly contain
bugs and other problems.
Please report such in the category <em>math :: interpolate</em> of the
<a href="http://sourceforge.net/tracker/?group_id=12883">Tcllib SF Trackers</a>.
Please also report any ideas for enhancements you may have for either
package and/or documentation.</p>
</div>
<div id="keywords" class="section"><h2><a name="keywords">Keywords</a></h2>
<p><a href="../../../../index.html#key778">interpolation</a>, <a href="../../../../index.html#key67">math</a>, <a href="../../../../index.html#key779">spatial interpolation</a></p>
</div>
<div id="category" class="section"><h2><a name="category">Category</a></h2>
<p>Mathematics</p>
</div>
<div id="copyright" class="section"><h2><a name="copyright">Copyright</a></h2>
<p>Copyright © 2004 Arjen Markus <[email protected]><br>
Copyright © 2004 Kevn B. Kenny <[email protected]></p>
</div>
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