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HTML

              
                <h1>6174: Kaprekar's Constant</h1>
<p>Type in any 4 digit number that contains at least 2 different digits (leading zeros are allowed). <strong>Deviating this will cause an infinite loop. You have been warned!</strong>
<br>
It will be rearranged into descending and ascending order and the smaller number will be subtracted from the larger one.
<br> 
The subtracted difference between the 2 numbers will eventually reach 6174 </p>
<input type="text"maxlength="4"/>
<button>Go</button>
<p class="counter">It took <span id="counterSpan"></span> iterations to reach Kaprekar's constant.</p>
<div class="output"></div>
              
            
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CSS

              
                
              
            
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JS

              
                //Document ready function
$(function(){

//Hide the output until the function is run at least once
  $(".counter").hide();
  
//Set global variables so they can be set later
var ascInt = "",
  descInt = "";
   
 //The main function for Kaprekar's constant
function kC(num) {
  
  //Turn integer into array of integers so that it may be sorted
  var numArray = (""+num).split("");
  
  //Sort the array into descending order
  var descArray = numArray.sort(function(a,b){
    return b - a;
  });
  
  //Sort the array into ascending order
  //Slice creates a copy of the array so as to not alter the descending one
  var ascArray = numArray.slice().sort(function(a,b){
    return a - b;
  });
  
  //Use join to turn the arrays into strings
  //Use parse int to turn those strings to integers
  descInt = parseInt(descArray.join(''));
  ascInt  = parseInt(ascArray.join(''));
  
  //calculate and return the difference betweent the numbers
  return (descInt - ascInt);
}
 
 //Button click sets it all off!
$("button").on("click", function(){
  
  //Remove the output div from the previous calculation
  $(".output").text("");
  
  //The counter variable counts how many times the function runs
  var counter = 1,
    diff = 0,
  //get the integer from the input box
    userVal = $("input").val();
  
  //Run the function with the user val
  diff = kC(userVal);

  //Append the calculation that was run to the page to show the progress of the function
  $(".output").append(descInt + " - " + ascInt + " = " + diff + "<br>");

  //Check that the constant hasn't been reach and continue to run the function
while (diff !== 6174) {
  //Run the function again with the number it previously output
  diff = kC(diff);
  
  //Increment the counter
  counter++;
  
  //Append the subsquent calculations that occur during the while loop after the initial call of the function
  $(".output").append(descInt + " - " + ascInt + " = " + diff + "<br>");
} 
  //Show the counter div after the function has run once
  $(".counter").show();
  
  // Insert the final counter number to show how many times the iteration ran
  $("#counterSpan").text(counter);  
})  
 
});

              
            
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