mean.js (1591B)
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 isnan = require( '@stdlib/math/base/assert/is-nan' ); 24 25 26 // MAIN // 27 28 /** 29 * Returns the expected value of a negative binomial distribution. 30 * 31 * @param {PositiveNumber} r - number of failures until experiment is stopped 32 * @param {Probability} p - success probability 33 * @returns {NonNegativeNumber} expected value 34 * 35 * @example 36 * var v = mean( 100, 0.2 ); 37 * // returns 400.0 38 * 39 * @example 40 * var v = mean( 20, 0.5 ); 41 * // returns 20.0 42 * 43 * @example 44 * var v = mean( 10.3, 0.8 ); 45 * // returns ~2.575 46 * 47 * @example 48 * var v = mean( -2, 0.5 ); 49 * // returns NaN 50 * 51 * @example 52 * var v = mean( 20, 1.1 ); 53 * // returns NaN 54 * 55 * @example 56 * var v = mean( 20, NaN ); 57 * // returns NaN 58 * 59 * @example 60 * var v = mean( NaN, 0.5 ); 61 * // returns NaN 62 */ 63 function mean( r, p ) { 64 if ( 65 isnan( r ) || 66 isnan( p ) 67 ) { 68 return NaN; 69 } 70 if ( 71 r <= 0.0 || 72 p < 0.0 || 73 p > 1.0 74 ) { 75 return NaN; 76 } 77 return ( (1.0-p) * r ) / p; 78 } 79 80 81 // EXPORTS // 82 83 module.exports = mean;