Exact Change, a strange thing happening

1

Tell us what’s happening:

Hi There! I am getting a very strange behaviour from this piece of code. I don’t really need help solving the problem, i’ll figure that out in a different way. It’s just that when I run this code, there seems to be an error when substracting somewhere in the while loop. And I don’t get it.

Maybe someone here is willing to try and figure out what happens. I’m just really curious where it goes wrong.
Below is my code and below that is part of the console log.

Thanks in advance for any help!

Your code so far

function checkCashRegister(price, cash, cid) {
  console.log("start");
  var cidTotal = 0;
  var changeTotal = cash - price;
  var change = 0;
  var length = cid.length;
  var nMoney = length-1;
  
  
  cid.forEach(function(value){
    switch(value[0]){
      case "PENNY":
        value[0] = 0.01;
        break;
      case "NICKEL":
        value[0] = 0.05;
        break;
      case "DIME":
        value[0] = 0.1;
        break;
      case "QUARTER":
        value[0] = 0.25;
        break;
      case "ONE":
        value[0] = 1;
        break;
      case "FIVE":
        value[0] = 5;
        break;
      case "TEN":
        value[0] = 10;
        break;
      case "TWENTY":
        value[0] = 20;
        break;
      case "ONE HUNDRED":
        value[0] = 100;
        break;
    }
  });
  
  console.log(cid[0]);
  console.log(cidTotal);
  
  while(changeTotal > 0 && nMoney > -1){
    console.log(cid[nMoney][1]);
    if((cid[nMoney][1]) > 0){
      console.log("changeTotal: " + changeTotal);
      console.log("cid[nMoney][0]: " + cid[nMoney][0]);
      console.log("cid[nMoney][1]: " + cid[nMoney][1]);
      changeTotal -= cid[nMoney][0];
      cid[nMoney][1] -= cid[nMoney][0];
    } else {
      nMoney--;
    }
  }

  return cid;
}

checkCashRegister(19.50, 20.00, [["PENNY", 0.50], ["NICKEL", 0], ["DIME", 0], ["QUARTER", 0], ["ONE", 0], ["FIVE", 0], ["TEN", 0], ["TWENTY", 0], ["ONE HUNDRED", 0]]);


20:25:41.796 VM460:47 0.5
20:25:41.796 VM460:49 changeTotal: 0.5
20:25:41.796 VM460:50 cid[nMoney][0]: 0.01
20:25:41.797 VM460:51 cid[nMoney][1]: 0.5
20:25:41.797 VM460:47 0.49
20:25:41.797 VM460:49 changeTotal: 0.49
20:25:41.797 VM460:50 cid[nMoney][0]: 0.01
20:25:41.797 VM460:51 cid[nMoney][1]: 0.49
20:25:41.797 VM460:47 0.48
20:25:41.797 VM460:49 changeTotal: 0.48
20:25:41.797 VM460:50 cid[nMoney][0]: 0.01
20:25:41.797 VM460:51 cid[nMoney][1]: 0.48
20:25:41.798 VM460:47 0.47
20:25:41.798 VM460:49 changeTotal: 0.47
20:25:41.798 VM460:50 cid[nMoney][0]: 0.01
20:25:41.798 VM460:51 cid[nMoney][1]: 0.47
20:25:41.798 VM460:47 0.45999999999999996
20:25:41.798 VM460:49 changeTotal: 0.45999999999999996
20:25:41.798 VM460:50 cid[nMoney][0]: 0.01
20:25:41.798 VM460:51 cid[nMoney][1]: 0.45999999999999996
20:25:41.798 VM460:47 0.44999999999999996
20:25:41.798 VM460:49 changeTotal: 0.44999999999999996
20:25:41.799 VM460:50 cid[nMoney][0]: 0.01
20:25:41.799 VM460:51 cid[nMoney][1]: 0.44999999999999996

Your browser information:

Your Browser User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36.

Link to the challenge:
https://www.freecodecamp.org/challenges/exact-change

2

Same as the following:

var a = 0.2;
var b = 0.1;
var c = a + b;
console.log(c); // 0.30000000000000004

It is because of the way floating point numbers are represented in computers. Use the forum search feature at the top right and search for “floating point” and you will see many topics with detailed answers of what and why this is happening to your code.

1 Like
3

Thank you very much Randell. Glad to know what principle is causing this!