Mega Code Archive
Returns n! Shorthand for n Factorial, the product of the numbers 1, ,n as a double
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import java.io.File;
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*
*/
public class Main {
/**
* Returns n!. Shorthand for <code>n</code> <a
* href="http://mathworld.wolfram.com/Factorial.html"> Factorial</a>, the
* product of the numbers <code>1,...,n</code> as a <code>double</code>.
* <p>
* <Strong>Preconditions</strong>:
* <ul>
* <li> <code>n >= 0</code> (otherwise
* <code>IllegalArgumentException</code> is thrown)</li>
* <li> The result is small enough to fit into a <code>double</code>. The
* largest value of <code>n</code> for which <code>n!</code> <
* Double.MAX_VALUE</code> is 170. If the computed value exceeds
* Double.MAX_VALUE, Double.POSITIVE_INFINITY is returned</li>
* </ul>
* </p>
*
* @param n argument
* @return <code>n!</code>
* @throws IllegalArgumentException if n < 0
*/
public static double factorialDouble(final int n) {
if (n < 0) {
throw new IllegalArgumentException("must have n >= 0 for n!");
}
return Math.floor(Math.exp(factorialLog(n)) + 0.5);
}
/**
* Returns the natural logarithm of n!.
* <p>
* <Strong>Preconditions</strong>:
* <ul>
* <li> <code>n >= 0</code> (otherwise
* <code>IllegalArgumentException</code> is thrown)</li>
* </ul></p>
*
* @param n argument
* @return <code>n!</code>
* @throws IllegalArgumentException if preconditions are not met.
*/
public static double factorialLog(final int n) {
if (n < 0) {
throw new IllegalArgumentException("must have n > 0 for n!");
}
double logSum = 0;
for (int i = 2; i <= n; i++) {
logSum += Math.log((double)i);
}
return logSum;
}
}
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