kaprekar v1.0.2

Kaprekar's Routine

In number theory, Kaprekar's routine is an iterative algorithm named after its inventor, Indian mathematician D. R. Kaprekar. Each iteration starts with a number, sorts the digits into descending and ascending order, and calculates the difference between the two new numbers.

As an example, starting with the number 8991 in base 10:

9981 – 1899 = 8082

8820 – 0288 = 8532

8532 – 2358 = 6174

7641 – 1467 = 6174

6174, known as Kaprekar's constant, is a fixed point of this algorithm. The only four-digit numbers for which Kaprekar's routine does not reach 6174 are repeated digits such as 1111, which give the result 0000 after a single iteration.

This package returns the number of iterations needed to reach the Kaprekar's constant starting from a given number. Four repeated digits will returns -1.

Install

npm install kaprekar

Usage

var k = require('kaprekar');

k.kaprekar(0);      // -> -1
k.kaprekar(12345);  // -> -1
k.kaprekar(1111);   // -> -1
k.kaprekar(6174);   // -> 0
k.kaprekar(8991);   // -> 3
k.kaprekar(1);      // -> 5
k.kaprekar(21);     // -> 3