pdf.js (2136B)
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 * Evaluates the probability density function (PDF) for a triangular distribution with lower limit `a` and upper limit `b` and mode `c` at a value `x`. 30 * 31 * @param {number} x - input value 32 * @param {number} a - lower limit 33 * @param {number} b - upper limit 34 * @param {number} c - mode 35 * @returns {number} evaluated PDF 36 * 37 * @example 38 * var y = pdf( 0.5, -1.0, 1.0, 0.0 ); 39 * // returns 0.5 40 * 41 * @example 42 * var y = pdf( 0.5, -1.0, 1.0, 0.5 ); 43 * // returns 1.0 44 * 45 * @example 46 * var y = pdf( -10.0, -20.0, 0.0, -2.0 ); 47 * // returns ~0.056 48 * 49 * @example 50 * var y = pdf( -2.0, -1.0, 1.0, 0.0 ); 51 * // returns 0.0 52 * 53 * @example 54 * var y = pdf( NaN, 0.0, 1.0, 0.5 ); 55 * // returns NaN 56 * 57 * @example 58 * var y = pdf( 0.0, NaN, 1.0, 0.5 ); 59 * // returns NaN 60 * 61 * @example 62 * var y = pdf( 0.0, 0.0, NaN, 0.5 ); 63 * // returns NaN 64 * 65 * @example 66 * var y = pdf( 2.0, 1.0, 0.0, NaN ); 67 * // returns NaN 68 * 69 * @example 70 * var y = pdf( 2.0, 1.0, 0.0, 1.5 ); 71 * // returns NaN 72 */ 73 function pdf( x, a, b, c ) { 74 if ( 75 isnan( x ) || 76 isnan( a ) || 77 isnan( b ) || 78 isnan( c ) || 79 a > c || 80 c > b 81 ) { 82 return NaN; 83 } 84 if ( x < a ) { 85 return 0.0; 86 } 87 // Case: x >= a 88 if ( x < c ) { 89 return ( 2.0 * ( x - a ) ) / ( ( b - a ) * ( c - a ) ); 90 } 91 if ( x === c ) { 92 return 2.0 / ( b - a ); 93 } 94 // Case: x > c 95 if ( x <= b ) { 96 return ( 2.0 * ( b - x ) ) / ( ( b - a ) * ( b - c ) ); 97 } 98 // Case: x > b 99 return 0.0; 100 } 101 102 103 // EXPORTS // 104 105 module.exports = pdf;