logpdf.js (1535B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 'use strict'; 20 21 // MODULES // 22 23 var gammaLogPDF = require( './../../../../../base/dists/gamma/logpdf' ); 24 25 26 // MAIN // 27 28 /** 29 * Evaluates the natural logarithm of the probability density function (PDF) for a chi-squared distribution with degrees of freedom `k` at a value `x`. 30 * 31 * @param {number} x - input value 32 * @param {NonNegativeNumber} k - degrees of freedom 33 * @returns {number} evaluated logPDF 34 * 35 * @example 36 * var y = logpdf( 0.3, 4.0 ); 37 * // returns ~-2.74 38 * 39 * @example 40 * var y = logpdf( 0.7, 0.7 ); 41 * // returns ~-1.295 42 * 43 * @example 44 * var y = logpdf( -1.0, 0.5 ); 45 * // returns -Infinity 46 * 47 * @example 48 * var y = logpdf( 0.0, NaN ); 49 * // returns NaN 50 * 51 * @example 52 * var y = logpdf( NaN, 2.0 ); 53 * // returns NaN 54 * 55 * @example 56 * // Negative degrees of freedom: 57 * var y = logpdf( 2.0, -1.0 ); 58 * // returns NaN 59 */ 60 function logpdf( x, k ) { 61 return gammaLogPDF( x, k/2.0, 0.5 ); 62 } 63 64 65 // EXPORTS // 66 67 module.exports = logpdf;