median.js (1582B)
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 erfcinv = require( '@stdlib/math/base/special/erfcinv' ); 24 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 25 var pow = require( '@stdlib/math/base/special/pow' ); 26 27 28 // VARIABLES // 29 30 var DENOM = 2.0 * pow( erfcinv( 0.5 ), 2.0 ); 31 32 33 // MAIN // 34 35 /** 36 * Returns the median for a Lévy distribution with location `mu` and scale `c`. 37 * 38 * @param {number} mu - location parameter 39 * @param {PositiveNumber} c - scale parameter 40 * @returns {number} median 41 * 42 * @example 43 * var y = median( 0.0, 1.0 ); 44 * // returns ~2.198 45 * 46 * @example 47 * var y = median( 5.0, 2.0 ); 48 * // returns ~9.396 49 * 50 * @example 51 * var y = median( NaN, 1.0 ); 52 * // returns NaN 53 * 54 * @example 55 * var y = median( 0.0, NaN ); 56 * // returns NaN 57 * 58 * @example 59 * var y = median( 0.0, 0.0 ); 60 * // returns NaN 61 */ 62 function median( mu, c ) { 63 if ( 64 isnan( mu ) || 65 isnan( c ) || 66 c <= 0.0 67 ) { 68 return NaN; 69 } 70 return mu + ( c / DENOM ); 71 } 72 73 74 // EXPORTS // 75 76 module.exports = median;