logit.js (1607B)
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 isProbability = require( './../../../../base/assert/is-probability' ); 24 var isnan = require( './../../../../base/assert/is-nan' ); 25 var ln = require( './../../../../base/special/ln' ); 26 var PINF = require( '@stdlib/constants/float64/pinf' ); 27 var NINF = require( '@stdlib/constants/float64/ninf' ); 28 29 30 // MAIN // 31 32 /** 33 * Evaluates the logit function. 34 * 35 * @param {Probability} p - input value 36 * @returns {number} function value 37 * 38 * @example 39 * var y = logit( 0.2 ); 40 * // returns ~-1.386 41 * 42 * @example 43 * var y = logit( 0.9 ); 44 * // returns ~2.197 45 * 46 * @example 47 * var y = logit( -4.0 ); 48 * // returns NaN 49 * 50 * @example 51 * var y = logit( 1.5 ); 52 * // returns NaN 53 * 54 * @example 55 * var y = logit( NaN ); 56 * // returns NaN 57 */ 58 function logit( p ) { 59 if ( isnan( p ) ) { 60 return p; 61 } 62 if ( !isProbability( p ) ) { 63 return NaN; 64 } 65 if ( p === 0.0 ) { 66 return NINF; 67 } 68 if ( p === 1.0 ) { 69 return PINF; 70 } 71 return ln( p / ( 1.0-p ) ); 72 } 73 74 75 // EXPORTS // 76 77 module.exports = logit;