freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/rosetta-code/gamma-function.english.md

title	id	challengeType
Gamma function	5a23c84252665b21eecc7e76	5

Description

Implement one algorithm (or more) to compute the Gamma ($\Gamma$) function (in the real field only). The Gamma function can be defined as:

$\Gamma(x) = \displaystyle\int_0^\infty t^{x-1}e^{-t} dt$

Instructions

Tests

tests:
  - text: <code>gamma</code> should be a function.
    testString: 'assert(typeof gamma=="function","<code>gamma</code> should be a function.")'
  - text: '<code>gamma("+tests[0]+")</code> should return a number.'
    testString: 'assert(typeof gamma(tests[0])=="number","<code>gamma("+tests[0]+")</code> should return a number.")'
  - text: '<code>gamma("+tests[0]+")</code> should return <code>"+results[0]+"</code>.'
    testString: 'assert.equal(gamma(tests[0]),results[0],"<code>gamma("+tests[0]+")</code> should return <code>"+results[0]+"</code>.")'
  - text: '<code>gamma("+tests[1]+")</code> should return <code>"+results[1]+"</code>.'
    testString: 'assert.equal(gamma(tests[1]),results[1],"<code>gamma("+tests[1]+")</code> should return <code>"+results[1]+"</code>.")'
  - text: '<code>gamma("+tests[2]+")</code> should return <code>"+results[2]+"</code>.'
    testString: 'assert.equal(gamma(tests[2]),results[2],"<code>gamma("+tests[2]+")</code> should return <code>"+results[2]+"</code>.")'
  - text: '<code>gamma("+tests[3]+")</code> should return <code>"+results[3]+"</code>.'
    testString: 'assert.equal(gamma(tests[3]),results[3],"<code>gamma("+tests[3]+")</code> should return <code>"+results[3]+"</code>.")'
  - text: '<code>gamma("+tests[4]+")</code> should return <code>"+results[4]+"</code>.'
    testString: 'assert.equal(gamma(tests[4]),results[4],"<code>gamma("+tests[4]+")</code> should return <code>"+results[4]+"</code>.")'

Challenge Seed

function gamma (x) {
  // Good luck!
}

After Test

console.info('after the test');

Solution

function gamma(x) {
  var p = [0.99999999999980993, 676.5203681218851, -1259.1392167224028,
  771.32342877765313, -176.61502916214059, 12.507343278686905,
  -0.13857109526572012, 9.9843695780195716e-6, 1.5056327351493116e-7
  ];

  var g = 7;
  if (x < 0.5) {
    return Math.PI / (Math.sin(Math.PI * x) * gamma(1 - x));
  }

  x -= 1;
  var a = p[0];
  var t = x + g + 0.5;
  for (var i = 1; i < p.length; i++) {
  a += p[i] / (x + i);
  }

  var result=Math.sqrt(2 * Math.PI) * Math.pow(t, x + 0.5) * Math.exp(-t) * a;

  return result;
}