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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="math_toolkit.cardinal_cubic_b"></a><a class="link" href="cardinal_cubic_b.html" title="Cardinal Cubic B-spline interpolation">Cardinal Cubic B-spline
interpolation</a>
</h2></div></div></div>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h0"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.synopsis"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.synopsis">Synopsis</a>
</h4>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">interpolators</span><span class="special">/</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">math</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">interpolators</span> <span class="special">{</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Real</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">cardinal_cubic_b_spline</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">BidiIterator</span><span class="special">></span>
<span class="identifier">cardinal_cubic_b_spline</span><span class="special">(</span><span class="identifier">BidiIterator</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">BidiIterator</span> <span class="identifier">b</span><span class="special">,</span> <span class="identifier">Real</span> <span class="identifier">left_endpoint</span><span class="special">,</span> <span class="identifier">Real</span> <span class="identifier">step_size</span><span class="special">,</span>
<span class="identifier">Real</span> <span class="identifier">left_endpoint_derivative</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>::</span><span class="identifier">quiet_NaN</span><span class="special">(),</span>
<span class="identifier">Real</span> <span class="identifier">right_endpoint_derivative</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>::</span><span class="identifier">quiet_NaN</span><span class="special">());</span>
<span class="identifier">cardinal_cubic_b_spline</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Real</span><span class="special">*</span> <span class="keyword">const</span> <span class="identifier">f</span><span class="special">,</span> <span class="identifier">size_t</span> <span class="identifier">length</span><span class="special">,</span> <span class="identifier">Real</span> <span class="identifier">left_endpoint</span><span class="special">,</span> <span class="identifier">Real</span> <span class="identifier">step_size</span><span class="special">,</span>
<span class="identifier">Real</span> <span class="identifier">left_endpoint_derivative</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>::</span><span class="identifier">quiet_NaN</span><span class="special">(),</span>
<span class="identifier">Real</span> <span class="identifier">right_endpoint_derivative</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">Real</span><span class="special">>::</span><span class="identifier">quiet_NaN</span><span class="special">());</span>
<span class="identifier">Real</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Real</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">Real</span> <span class="identifier">prime</span><span class="special">(</span><span class="identifier">Real</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="identifier">Real</span> <span class="identifier">double_prime</span><span class="special">(</span><span class="identifier">Real</span> <span class="identifier">x</span><span class="special">)</span> <span class="keyword">const</span><span class="special">;</span>
<span class="special">};</span>
<span class="special">}}}</span> <span class="comment">// namespaces</span>
</pre>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h1"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.cardinal_cubic_b_spline_interpol"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.cardinal_cubic_b_spline_interpol">Cardinal
Cubic B-Spline Interpolation</a>
</h4>
<p>
The cardinal cubic <span class="emphasis"><em>B</em></span>-spline class provided by Boost allows
fast and accurate interpolation of a function which is known at equally spaced
points. The cubic <span class="emphasis"><em>B</em></span>-spline interpolation is numerically
stable as it uses compactly supported basis functions constructed via iterative
convolution. This is to be contrasted to one-sided power function cubic spline
interpolation which is ill-conditioned as the global support of cubic polynomials
causes small changes far from the evaluation point to exert a large influence
on the calculated value.
</p>
<p>
There are many use cases for interpolating a function at equally spaced points.
One particularly important example is solving ODEs whose coefficients depend
on data determined from experiment or numerical simulation. Since most ODE
steppers are adaptive, they must be able to sample the coefficients at arbitrary
points; not just at the points we know the values of our function.
</p>
<p>
The first two arguments to the constructor are either:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
A pair of bidirectional iterators into the data, or
</li>
<li class="listitem">
A pointer to the data, and a length of the data array.
</li>
</ul></div>
<p>
These are then followed by:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The start of the functions domain,
</li>
<li class="listitem">
The step size.
</li>
</ul></div>
<p>
Optionally, you may provide two additional arguments to the constructor, namely
the derivative of the function at the left endpoint, and the derivative at
the right endpoint. If you do not provide these arguments, they will be estimated
using one-sided finite-difference formulas. An example of a valid call to the
constructor is
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">f</span><span class="special">{</span><span class="number">0.01</span><span class="special">,</span> <span class="special">-</span><span class="number">0.02</span><span class="special">,</span> <span class="number">0.3</span><span class="special">,</span> <span class="number">0.8</span><span class="special">,</span> <span class="number">1.9</span><span class="special">,</span> <span class="special">-</span><span class="number">8.78</span><span class="special">,</span> <span class="special">-</span><span class="number">22.6</span><span class="special">};</span>
<span class="keyword">double</span> <span class="identifier">t0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">h</span> <span class="special">=</span> <span class="number">0.01</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">interpolators</span><span class="special">::</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">spline</span><span class="special">(</span><span class="identifier">f</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">t0</span><span class="special">,</span> <span class="identifier">h</span><span class="special">);</span>
</pre>
<p>
The endpoints are estimated using a one-sided finite-difference formula. If
you know the derivative at the endpoint, you may pass it to the constructor
via
</p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">interpolators</span><span class="special">::</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">spline</span><span class="special">(</span><span class="identifier">f</span><span class="special">.</span><span class="identifier">begin</span><span class="special">(),</span> <span class="identifier">f</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">t0</span><span class="special">,</span> <span class="identifier">h</span><span class="special">,</span> <span class="identifier">a_prime</span><span class="special">,</span> <span class="identifier">b_prime</span><span class="special">);</span>
</pre>
<p>
To evaluate the interpolant at a point, we simply use
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">spline</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
and to evaluate the derivative of the interpolant we use
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">yp</span> <span class="special">=</span> <span class="identifier">spline</span><span class="special">.</span><span class="identifier">prime</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
Be aware that the accuracy guarantees on the derivative of the spline are an
order lower than the guarantees on the original function, see <a href="http://www.springer.com/us/book/9780387984087" target="_top">Numerical
Analysis, Graduate Texts in Mathematics, 181, Rainer Kress</a> for details.
</p>
<p>
The last interesting member is the second derivative, evaluated via
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">ypp</span> <span class="special">=</span> <span class="identifier">spline</span><span class="special">.</span><span class="identifier">double_prime</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
</pre>
<p>
The basis functions of the spline are cubic polynomials, so the second derivative
is simply linear interpolation. But the interpolation is not constrained by
data (you don't pass in the second derivatives), and hence is less accurate
than would be naively expected from a linear interpolator. The problem is especially
pronounced at the boundaries, where the second derivative is totally unconstrained.
Use the second derivative of the cubic B-spline interpolator only in desperation.
The quintic <span class="emphasis"><em>B</em></span>-spline interpolator is recommended for cases
where second derivatives are needed.
</p>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h2"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.complexity_and_performance"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.complexity_and_performance">Complexity
and Performance</a>
</h4>
<p>
The call to the constructor requires 𝑶(<span class="emphasis"><em>n</em></span>) operations, where
<span class="emphasis"><em>n</em></span> is the number of points to interpolate. Each call the
the interpolant is 𝑶(1) (constant time). On the author's Intel Xeon E3-1230,
this takes 21ns as long as the vector is small enough to fit in cache.
</p>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h3"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.accuracy"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.accuracy">Accuracy</a>
</h4>
<p>
Let <span class="emphasis"><em>h</em></span> be the stepsize. If <span class="emphasis"><em>f</em></span> is four-times
continuously differentiable, then the interpolant is <span class="emphasis"><em>𝑶(h<sup>4</sup>)</em></span>
accurate and the derivative is <span class="emphasis"><em>𝑶(h<sup>3</sup>)</em></span> accurate.
</p>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h4"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.testing"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.testing">Testing</a>
</h4>
<p>
Since the interpolant obeys <span class="serif_italic">s(x<sub>j</sub>) = f(x<sub>j</sub>)</span>
at all interpolation points, the tests generate random data and evaluate the
interpolant at the interpolation points, validating that equality with the
data holds.
</p>
<p>
In addition, constant, linear, and quadratic functions are interpolated to
ensure that the interpolant behaves as expected.
</p>
<h4>
<a name="math_toolkit.cardinal_cubic_b.h5"></a>
<span class="phrase"><a name="math_toolkit.cardinal_cubic_b.example"></a></span><a class="link" href="cardinal_cubic_b.html#math_toolkit.cardinal_cubic_b.example">Example</a>
</h4>
<p>
This example demonstrates how to use the cubic b spline interpolator for regularly
spaced data.
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">math</span><span class="special">/</span><span class="identifier">interpolators</span><span class="special">/</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">// We begin with an array of samples:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v</span><span class="special">(</span><span class="number">500</span><span class="special">);</span>
<span class="comment">// And decide on a stepsize:</span>
<span class="keyword">double</span> <span class="identifier">step</span> <span class="special">=</span> <span class="number">0.01</span><span class="special">;</span>
<span class="comment">// Initialize the vector with a function we'd like to interpolate:</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="identifier">v</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">i</span><span class="special">*</span><span class="identifier">step</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">// We could define an arbitrary start time, but for now we'll just use 0:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">interpolators</span><span class="special">::</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">spline</span><span class="special">(</span><span class="identifier">v</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">size</span><span class="special">(),</span> <span class="number">0</span> <span class="comment">/* start time */</span><span class="special">,</span> <span class="identifier">step</span><span class="special">);</span>
<span class="comment">// Now we can evaluate the spline wherever we please.</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">mt19937</span> <span class="identifier">gen</span><span class="special">;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">random</span><span class="special">::</span><span class="identifier">uniform_real_distribution</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">absissa</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">v</span><span class="special">.</span><span class="identifier">size</span><span class="special">()*</span><span class="identifier">step</span><span class="special">);</span>
<span class="keyword">for</span> <span class="special">(</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">10</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">absissa</span><span class="special">(</span><span class="identifier">gen</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"sin("</span> <span class="special"><<</span> <span class="identifier">x</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">sin</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="string">", spline interpolation gives "</span> <span class="special"><<</span> <span class="identifier">spline</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"cos("</span> <span class="special"><<</span> <span class="identifier">x</span> <span class="special"><<</span> <span class="string">") = "</span> <span class="special"><<</span> <span class="identifier">cos</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="string">", spline derivative interpolation gives "</span> <span class="special"><<</span> <span class="identifier">spline</span><span class="special">.</span><span class="identifier">prime</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">// The next example is less trivial:</span>
<span class="comment">// We will try to figure out when the population of the United States crossed 100 million.</span>
<span class="comment">// Since the census is taken every 10 years, the data is equally spaced, so we can use the cubic b spline.</span>
<span class="comment">// Data taken from https://en.wikipedia.org/wiki/United_States_Census</span>
<span class="comment">// We'll start at the year 1860:</span>
<span class="keyword">double</span> <span class="identifier">t0</span> <span class="special">=</span> <span class="number">1860</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">time_step</span> <span class="special">=</span> <span class="number">10</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">population</span><span class="special">{</span><span class="number">31443321</span><span class="special">,</span> <span class="comment">/* 1860 */</span>
<span class="number">39818449</span><span class="special">,</span> <span class="comment">/* 1870 */</span>
<span class="number">50189209</span><span class="special">,</span> <span class="comment">/* 1880 */</span>
<span class="number">62947714</span><span class="special">,</span> <span class="comment">/* 1890 */</span>
<span class="number">76212168</span><span class="special">,</span> <span class="comment">/* 1900 */</span>
<span class="number">92228496</span><span class="special">,</span> <span class="comment">/* 1910 */</span>
<span class="number">106021537</span><span class="special">,</span> <span class="comment">/* 1920 */</span>
<span class="number">122775046</span><span class="special">,</span> <span class="comment">/* 1930 */</span>
<span class="number">132164569</span><span class="special">,</span> <span class="comment">/* 1940 */</span>
<span class="number">150697361</span><span class="special">,</span> <span class="comment">/* 1950 */</span>
<span class="number">179323175</span><span class="special">};/*</span> <span class="number">1960</span> <span class="special">*/</span>
<span class="comment">// An eyeball estimate indicates that the population crossed 100 million around 1915.</span>
<span class="comment">// Let's see what interpolation says:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">interpolators</span><span class="special">::</span><span class="identifier">cardinal_cubic_b_spline</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">population</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span class="identifier">population</span><span class="special">.</span><span class="identifier">size</span><span class="special">(),</span> <span class="identifier">t0</span><span class="special">,</span> <span class="identifier">time_step</span><span class="special">);</span>
<span class="comment">// Now create a function which has a zero at p = 100,000,000:</span>
<span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="special">[=](</span><span class="keyword">double</span> <span class="identifier">t</span><span class="special">){</span> <span class="keyword">return</span> <span class="identifier">p</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span> <span class="special">-</span> <span class="number">100000000</span><span class="special">;</span> <span class="special">};</span>
<span class="comment">// Boost includes a bisection algorithm, which is robust, though not as fast as some others</span>
<span class="comment">// we provide, but let's try that first. We need a termination condition for it, which</span>
<span class="comment">// takes the two endpoints of the range and returns either true (stop) or false (keep going),</span>
<span class="comment">// we could use a predefined one such as boost::math::tools::eps_tolerance<double>, but that</span>
<span class="comment">// won't stop until we have full double precision which is overkill, since we just need the</span>
<span class="comment">// endpoint to yield the same month. While we're at it, we'll keep track of the number of</span>
<span class="comment">// iterations required too, though this is strictly optional:</span>
<span class="keyword">auto</span> <span class="identifier">termination</span> <span class="special">=</span> <span class="special">[](</span><span class="keyword">double</span> <span class="identifier">left</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">right</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">double</span> <span class="identifier">left_month</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">round</span><span class="special">((</span><span class="identifier">left</span> <span class="special">-</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">left</span><span class="special">))</span> <span class="special">*</span> <span class="number">12</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
<span class="keyword">double</span> <span class="identifier">right_month</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">round</span><span class="special">((</span><span class="identifier">right</span> <span class="special">-</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">right</span><span class="special">))</span> <span class="special">*</span> <span class="number">12</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
<span class="keyword">return</span> <span class="special">(</span><span class="identifier">left_month</span> <span class="special">==</span> <span class="identifier">right_month</span><span class="special">)</span> <span class="special">&&</span> <span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">left</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">right</span><span class="special">));</span>
<span class="special">};</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">uintmax_t</span> <span class="identifier">iterations</span> <span class="special">=</span> <span class="number">1000</span><span class="special">;</span>
<span class="keyword">auto</span> <span class="identifier">result</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">::</span><span class="identifier">bisect</span><span class="special">(</span><span class="identifier">f</span><span class="special">,</span> <span class="number">1910.0</span><span class="special">,</span> <span class="number">1920.0</span><span class="special">,</span> <span class="identifier">termination</span><span class="special">,</span> <span class="identifier">iterations</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">time</span> <span class="special">=</span> <span class="identifier">result</span><span class="special">.</span><span class="identifier">first</span><span class="special">;</span> <span class="comment">// termination condition ensures that both endpoints yield the same result</span>
<span class="keyword">auto</span> <span class="identifier">month</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">round</span><span class="special">((</span><span class="identifier">time</span> <span class="special">-</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">time</span><span class="special">))*</span><span class="number">12</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
<span class="keyword">auto</span> <span class="identifier">year</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">time</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"The population of the United States surpassed 100 million on the "</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">month</span> <span class="special"><<</span> <span class="string">"th month of "</span> <span class="special"><<</span> <span class="identifier">year</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Found in "</span> <span class="special"><<</span> <span class="identifier">iterations</span> <span class="special"><<</span> <span class="string">" iterations"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="comment">// Since the cubic B spline offers the first derivative, we could equally have used Newton iterations,</span>
<span class="comment">// this takes "number of bits correct" as a termination condition - 20 should be plenty for what we need,</span>
<span class="comment">// and once again, we track how many iterations are taken:</span>
<span class="keyword">auto</span> <span class="identifier">f_n</span> <span class="special">=</span> <span class="special">[=](</span><span class="keyword">double</span> <span class="identifier">t</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="identifier">p</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span> <span class="special">-</span> <span class="number">100000000</span><span class="special">,</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">prime</span><span class="special">(</span><span class="identifier">t</span><span class="special">));</span> <span class="special">};</span>
<span class="identifier">iterations</span> <span class="special">=</span> <span class="number">1000</span><span class="special">;</span>
<span class="identifier">time</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">math</span><span class="special">::</span><span class="identifier">tools</span><span class="special">::</span><span class="identifier">newton_raphson_iterate</span><span class="special">(</span><span class="identifier">f_n</span><span class="special">,</span> <span class="number">1910.0</span><span class="special">,</span> <span class="number">1900.0</span><span class="special">,</span> <span class="number">2000.0</span><span class="special">,</span> <span class="number">20</span><span class="special">,</span> <span class="identifier">iterations</span><span class="special">);</span>
<span class="identifier">month</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">round</span><span class="special">((</span><span class="identifier">time</span> <span class="special">-</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">time</span><span class="special">))*</span><span class="number">12</span> <span class="special">+</span> <span class="number">1</span><span class="special">);</span>
<span class="identifier">year</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span><span class="special">(</span><span class="identifier">time</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"The population of the United States surpassed 100 million on the "</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">month</span> <span class="special"><<</span> <span class="string">"th month of "</span> <span class="special"><<</span> <span class="identifier">year</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"Found in "</span> <span class="special"><<</span> <span class="identifier">iterations</span> <span class="special"><<</span> <span class="string">" iterations"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<pre class="programlisting"><span class="identifier">Program</span> <span class="identifier">output</span> <span class="identifier">is</span><span class="special">:</span>
</pre>
<pre class="programlisting">sin(4.07362) = -0.802829, spline interpolation gives - 0.802829
cos(4.07362) = -0.596209, spline derivative interpolation gives - 0.596209
sin(0.677385) = 0.626758, spline interpolation gives 0.626758
cos(0.677385) = 0.779214, spline derivative interpolation gives 0.779214
sin(4.52896) = -0.983224, spline interpolation gives - 0.983224
cos(4.52896) = -0.182402, spline derivative interpolation gives - 0.182402
sin(4.17504) = -0.85907, spline interpolation gives - 0.85907
cos(4.17504) = -0.511858, spline derivative interpolation gives - 0.511858
sin(0.634934) = 0.593124, spline interpolation gives 0.593124
cos(0.634934) = 0.805111, spline derivative interpolation gives 0.805111
sin(4.84434) = -0.991307, spline interpolation gives - 0.991307
cos(4.84434) = 0.131567, spline derivative interpolation gives 0.131567
sin(4.56688) = -0.989432, spline interpolation gives - 0.989432
cos(4.56688) = -0.144997, spline derivative interpolation gives - 0.144997
sin(1.10517) = 0.893541, spline interpolation gives 0.893541
cos(1.10517) = 0.448982, spline derivative interpolation gives 0.448982
sin(3.1618) = -0.0202022, spline interpolation gives - 0.0202022
cos(3.1618) = -0.999796, spline derivative interpolation gives - 0.999796
sin(1.54084) = 0.999551, spline interpolation gives 0.999551
cos(1.54084) = 0.0299566, spline derivative interpolation gives 0.0299566
The population of the United States surpassed 100 million on the 11th month of 1915
Found in 12 iterations
The population of the United States surpassed 100 million on the 11th month of 1915
Found in 3 iterations
</pre>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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