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A Tour of NTL: Examples: Polynomials </title>
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A Tour of NTL: Examples: Polynomials
</p>
</h1>
<p> <hr> <p>
NTL provides extensive support for very fast polynomial arithmetic.
In fact, this was the main motivation for creating NTL in the first place,
because existing computer algebra systems and software
libraries had very slow polynomial arithmetic.
The class <tt>ZZX</tt> represents univariate polynomials
with integer coefficients.
The following program reads a polynomial,
factors it, and prints the factorization.
<!-- STARTPLAIN
#include <NTL/ZZXFactoring.h>
using namespace std;
using namespace NTL;
int main()
{
ZZX f;
cin >> f;
Vec< Pair< ZZX, long > > factors;
ZZ c;
factor(c, factors, f);
cout << c << "\n";
cout << factors << "\n";
}
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000"><font face="monospace">
<font color="#1773cc">#include </font><font color="#4a6f8b"><NTL/ZZXFactoring.h></font><br>
<br>
<font color="#b02f60"><b>using</b></font> <font color="#008b00"><b>namespace</b></font> std;<br>
<font color="#b02f60"><b>using</b></font> <font color="#008b00"><b>namespace</b></font> NTL;<br>
<br>
<font color="#008b00"><b>int</b></font> main()<br>
{<br>
ZZX f;<br>
<br>
cin >> f;<br>
<br>
Vec< Pair< ZZX, <font color="#008b00"><b>long</b></font> > > factors;<br>
ZZ c;<br>
<br>
factor(c, factors, f);<br>
<br>
cout << c << <font color="#4a6f8b">"</font><font color="#8a2ae2">\n</font><font color="#4a6f8b">"</font>;<br>
cout << factors << <font color="#4a6f8b">"</font><font color="#8a2ae2">\n</font><font color="#4a6f8b">"</font>;<br>
}<br>
</font></font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
When this program is compiled an run on input
<pre>
[2 10 14 6]
</pre>
which represents the polynomial <tt>2 + 10*X + 14*x^2 +6*X^3</tt>,
the output is
<pre>
2
[[[1 3] 1] [[1 1] 2]]
</pre>
The first line of output is the content of the polynomial, which
is 2 in this case as each coefficient of the input polynomial
is divisible by 2.
The second line is a vector of pairs, the first member of each
pair is an irreducible factor of the input, and the second
is the exponent to which is appears in the factorization.
Thus, all of the above simply means that
<pre>
2 + 10*X + 14*x^2 +6*X^3 = 2 * (1 + 3*X) * (1 + X)^2
</pre>
<p>
Admittedly, I/O in NTL is not exactly user friendly,
but then NTL has no pretensions about being an interactive
computer algebra system: it is a library for programmers.
<p>
In this example, the type <tt>Vec< Pair< ZZX, long > ></tt>
is an NTL vector whose base type is <tt>Pair< ZZX, long ></tt>.
The type <tt>Pair< ZZX, long ></tt> is the instantiation
of a template "pair" type defined by NTL.
See <a href="pair.cpp.html"><tt>pair.txt</tt></a> for more details.
<p> <hr> <p>
Here is another example.
The following program prints out the first 100 cyclotomic polynomials.
<!-- STARTPLAIN
#include <NTL/ZZX.h>
using namespace std;
using namespace NTL;
int main()
{
Vec<ZZX> phi(INIT_SIZE, 100);
for (long i = 1; i <= 100; i++) {
ZZX t;
t = 1;
for (long j = 1; j <= i-1; j++)
if (i % j == 0)
t *= phi(j);
phi(i) = (ZZX(INIT_MONO, i) - 1)/t;
cout << phi(i) << "\n";
}
}
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000"><font face="monospace">
<br>
<font color="#1773cc">#include </font><font color="#4a6f8b"><NTL/ZZX.h></font><br>
<br>
<font color="#b02f60"><b>using</b></font> <font color="#008b00"><b>namespace</b></font> std;<br>
<font color="#b02f60"><b>using</b></font> <font color="#008b00"><b>namespace</b></font> NTL;<br>
<br>
<font color="#008b00"><b>int</b></font> main()<br>
{<br>
Vec<ZZX> phi(INIT_SIZE, <font color="#ff8b00">100</font>); <br>
<br>
<font color="#b02f60"><b>for</b></font> (<font color="#008b00"><b>long</b></font> i = <font color="#ff8b00">1</font>; i <= <font color="#ff8b00">100</font>; i++) {<br>
ZZX t;<br>
t = <font color="#ff8b00">1</font>;<br>
<br>
<font color="#b02f60"><b>for</b></font> (<font color="#008b00"><b>long</b></font> j = <font color="#ff8b00">1</font>; j <= i-<font color="#ff8b00">1</font>; j++)<br>
<font color="#b02f60"><b>if</b></font> (i % j == <font color="#ff8b00">0</font>)<br>
t *= phi(j);<br>
<br>
phi(i) = (ZZX(INIT_MONO, i) - <font color="#ff8b00">1</font>)/t; <br>
<br>
cout << phi(i) << <font color="#4a6f8b">"</font><font color="#8a2ae2">\n</font><font color="#4a6f8b">"</font>;<br>
}<br>
}<br>
</font></font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
<p>
To illustrate more of the NTL interface, let's look at alternative ways
this routine could have been written.
<p>
First, instead of
<!-- STARTPLAIN
Vec<ZZX> phi(INIT_SIZE, 100);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
Vec<ZZX> phi(INIT_SIZE, <font color="#ff8c00">100</font>); <br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
one can write
<!-- STARTPLAIN
Vec<ZZX> phi;
phi.SetLength(100);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
Vec<ZZX> phi;<br>
phi.SetLength(<font color="#ff8c00">100</font>);<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
<p>
Second,
instead of
<!-- STARTPLAIN
t *= phi(j);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
t *= phi(j);<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
one can write this as
<!-- STARTPLAIN
mul(t, t, phi(j));
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
mul(t, t, phi(j));<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
or
<!-- STARTPLAIN
t = t * phi(j);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
t = t * phi(j);<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
Also, one can write <tt>phi[j-1]</tt> in place of <tt>phi(j)</tt>.
<p>
Third, instead of
<!-- STARTPLAIN
phi(i) = (ZZX(INIT_MONO, i) - 1)/t;
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
phi(i) = (ZZX(INIT_MONO, i) - <font color="#ff8c00">1</font>)/t; <br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
one can write
<!-- STARTPLAIN
ZZX t1;
SetCoeff(t1, i);
SetCoeff(t1, 0, -1);
div(phi(i), t1, t);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
ZZX t1;<br>
SetCoeff(t1, i);<br>
SetCoeff(t1, <font color="#ff8c00">0</font>, -<font color="#ff8c00">1</font>);<br>
div(phi(i), t1, t);<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
Alternatively, one could directly access the coefficient vector as follows:
<!-- STARTPLAIN
ZZX t1;
t1.SetLength(i+1); // all vector elements are initialized to zero
t1[i] = 1;
t1[0] = -1;
t1.normalize(); // not necessary here, but good practice in general
div(phi(i), t1, t);
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
ZZX t1;<br>
t1.SetLength(i+<font color="#ff8c00">1</font>); <font color="#0000ee"><i>// all vector elements are initialized to zero</i></font><br>
t1[i] = <font color="#ff8c00">1</font>;<br>
t1[<font color="#ff8c00">0</font>] = -<font color="#ff8c00">1</font>;<br>
t1.normalize(); <font color="#0000ee"><i>// not necessary here, but good practice in general</i></font><br>
div(phi(i), t1, t);<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
Generally, you can freely access the coefficient vector
of a polynomial, as above.
However, after fiddling with this vector, you should "normalize"
the polynomial, so that the leading coefficient is non-zero:
this is an invariant which all routines that work with polynomials
expect to hold.
Of course, if you can avoid directly accessing the
coefficient vector, you should do so.
You can always use the <tt>SetCoeff</tt> routine above to set or
change coefficients, and you can always read the value of a coefficient
using the routine <tt>coeff</tt>, e.g.,
<!-- STARTPLAIN
... f[i] == 1 ...
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
... f[i] == <font color="#ff8c00">1</font> ...<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
is equivalent to
<!-- STARTPLAIN
... coeff(f, i) == 1 ...
ENDPLAIN -->
<!-- STARTPRETTY {{{ -->
<p><p><table cellPadding=10px><tr><td><font color="#000000">
<font face="monospace">
... coeff(f, i) == <font color="#ff8c00">1</font> ...<br>
</font>
</font></td></tr></table><p><p>
<!-- }}} ENDPRETTY -->
except that in the latter case, a read-only reference to zero is returned
if the index <tt>i</tt> is out of range.
There are also special-purpose read-only access routines <tt>LeadCoeff(f)</tt>
and <tt>ConstTerm(f)</tt>.
<p>
NTL provides a full compliment of arithmetic operations for polynomials
over the integers, in both operator and procedural form.
All of the basic operations support a "promotion logic" similar
to that for <tt>ZZ</tt>, except that inputs of <i>both</i> types
<tt>long</tt> and <tt>ZZ</tt> are promoted to <tt>ZZX</tt>.
See <a href="ZZX.cpp.html"><tt>ZZX.txt</tt></a> for details,
and see <a href="ZZXFactoring.cpp.html"><tt>ZZXFactoring.txt</tt></a> for details
on the polynomial factoring routines.
<p>
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