变量在while循环中不改变

Question

I am trying challenges in order to practice, learn, and better understand coding. I came across a hard and decided to try it, just to see. It has no time limit luckily. The challenge was (you can skip if you wish)

"Using the JavaScript language, have the function KaprekarsConstant(num) take the num parameter being passed which will be a 4-digit number with at least two distinct digits. Your program should perform the following routine on the number: Arrange the digits in descending order and in ascending order (adding zeroes to fit it to a 4-digit number), and subtract the smaller number from the bigger number. Then repeat the previous step. Performing this routine will always cause you to reach a fixed number: 6174. Then performing the routine on 6174 will always give you 6174 (7641 - 1467 = 6174). Your program should return the number of times this routine must be performed until 6174 is reached. For example: if num is 3524 your program should return 3 because of the following steps: (1) 5432 - 2345 = 3087, (2) 8730 - 0378 = 8352, (3) 8532 - 2358 = 6174."

(coderbyte)

After working for a long while, I had found something that I imagined would work. I could not test the loop yet though, because if I did I could not rerun the code, because the loop was incomplete and would never finish processing. After it was all finished, I tested it. And I got 0, no matter what I tried. If I entered letters I got an error, what should happen, but no matter what numbers I put, even the 4 digit one from the example.

Here is my code:

 function KaprekarsConstant(num) { var forD = []; var sNum = num.toString(); var result = 0; var chngdN = []; var trkr = 0; for (var i = 0; i < sNum.length; i ++) { forD.push(+sNum.charAt(i)); } for (var j = 0; j < sNum.length; j ++) { chngdN.push(+sNum.charAt(j)); } while(while(forD-chngdN === trkr){ forD = trkr.toString(); chngdN = trkr.toString(); forD = forD.split(""); chngdN = chngdN.split(""); forD = forD.sort(); chngdN = chngdN.sort(); forD = forD.reverse(); forD = forD.join(""); chngdN = chngdN.join(""); forD = parseFloat(forD); chngdN = parseFloat(chngdN); trkr = forD - chngdN; forD = trkr.toString(); chngdN = trkr.toString(); result = result +1; } // code goes here return result; } // keep this function call here KaprekarsConstant(readline()); 

I am just starting, so I really do not know what is wrong here. I would love it if someone could tell me what was wrong. (I know that it sounds like I'm trying to seem like I don't understand any of it, but I really just don't have a firm grasp on everything I've learned yet. I learned it all in a short time). Thank you in advance.

Answer 1

The following will solve the problem, first version was not actually an implementation of the requirements but this one is.

I used the following for this:

1. Recursion (function calling itself until desired result)

2. Array.from and Number.prototype.toString to make sure the number string is 4 characters long.

3. Ternary operator with direction that is an argument for sortNum to sort ascending or descending.

  const makeAtLeast4 = num => //number to string make sure you have a string that's 4 characters (add 0 if not) num.toString() + Array.from(//create an array from someghing new Array(//that something is a new array //the lenght of this array is 4 minus the length of the string 4-num.toString().length), ()=>"0"//for each item of the array insert the string "0" ).join("");//join this array (num 111 will be sring "1110") //create ascending or descending sorted number // if direction is 1 it's ascending const sortNum = (numString,direction) => parseInt( (direction === 1) //if direction is 1 do ? else do : ? numString.toString().split("").sort().join("") : numString.toString().split("").sort().reverse().join("") ,10//decimal number ); //take number, sort digits descending and ascending // substract ascending from descending const descending_minus_ascending = num => { const stringNum = makeAtLeast4(num); return sortNum(stringNum) - sortNum(stringNum,1) } const kaprekarsConstant = (num,times=0) => { //get result for this number const result = descending_minus_ascending(num); //if result is the same as the number, return times tried if(result === num){ return times } console.log(`Try:${times+1}, result:${result}`); //try again with the result and increase times tried by one // recursively call this function return kaprekarsConstant(result,times+1); } console.log(kaprekarsConstant(11)) 

[UPDATE]

Here is the code as a while loop, I would advice you to avoid any kind of loops until you've used recursion and array functions, these maybe take a bit longer to learn but let you write easier to understand code in the long run:

const kaprekarsConstant = (num) => {
  //get result for this number
  //  !! notice that result is not a constant but a var
  //  we will have keep re assigning it, this can be confusing
  //  in complex code and should be avoided if possible
  //  with recursion we don't need to do this
  var result = descending_minus_ascending(num);
  var times=0;
  //keep re assigning result until it's same as num
  while(result !== num){
    console.log(`Try:${times+1}, result:${result}`);
    result = descending_minus_ascending(num);
  }
  return times;
}