为什么“if else 语句”不能正常工作？

Question

I am doing JS Algorithms and Data structures projects in freecodecamp and I am stuck at the last problem "Cash register". I already did some of the coding and my code doesn't pass all the tests.

The link to the problem: https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/javascript-algorithms-and-data-structures-projects/cash-register

My code doesn't pass the 3 and 5 number test. In number 3, the output is almost the same, but the last part ["PENNY",0.03], which doesn't match with the output. In number 5, the totalCid and cashback variables are equal but it doesn't return "CLOSED". If those are equal it should return the status "CLOSED".

 function checkCashRegister(price, cash, cid) { var cashback = (cash - price) var returnMoney = { status: "", change: [] } var totalCid = cid.map(v => { return v[1] }).reduce((a, b) => { return a + b }).toFixed(2) console.log(cashback) console.log(totalCid) if (totalCid === cashback) { returnMoney.status = "CLOSED" returnMoney.change = cid return returnMoney } else if (totalCid < cashback) { returnMoney.status = "INSUFFICIENT_FUND" returnMoney.change = [] return returnMoney } else { var denomValue = [{ bill: "ONE HUNDRED", val: 100 }, { bill: "TWENTY", val: 20 }, { bill: "TEN", val: 10 }, { bill: "FIVE", val: 5 }, { bill: "ONE", val: 1 }, { bill: "QUARTER", val: 0.25 }, { bill: "DIME", val: 0.10 }, { bill: "NICKEL", val: 0.05 }, { bill: "PENNY", val: 0.01 } ] var newCidArray = []; var currVal = 0; for (let i = 0; i < denomValue.length; i++) { while (cashback >= denomValue[i].val && currVal < cid[8 - i][1]) { cashback -= denomValue[i].val.toFixed(2); currVal += denomValue[i].val; } if (currVal > 0) { newCidArray.push([denomValue[i].bill, currVal]); } currVal = 0; // reset current value (currVal) to zero after the loop has completed } returnMoney.status = "OPEN" returnMoney.change = newCidArray return returnMoney } } console.log(checkCashRegister(19.5, 20, [ ["PENNY", 0.5], ["NICKEL", 0], ["DIME", 0], ["QUARTER", 0], ["ONE", 0], ["FIVE", 0], ["TEN", 0], ["TWENTY", 0], ["ONE HUNDRED", 0] ]))

Answer 1

You had a total of 3 problems in your code.

The first has been pointed out by @trincot: toFixed() returns a string, which you cannot compare with === to a number.

The second is that even if the cash in the drawer is more than the expected change, you can be in an INSUFFICIENT_FUNDS situation because you cannot split the bills (notes, in British English) and coins you have. This means that at the end of your algorithm you must check whether the change sums up.

The third, and major one, comes from working with floating point numbers. If you click on "Run code snippet under your code" you can see that the change would be 0.49000000000000027 dollars in pennies.

The only safe way out of the swamp of floating point is to convert everything to integers. So what I did, apart from fixing the first two problems, is convert dollars to pennies in your script. I tried not to alter your script too much, nonetheless.

function checkCashRegister(price, cash, cid) {
  // P stands for pennies
  var cidP = cid.map(v => [v[0], Math.round(v[1] * 100)])
  var priceP = Math.round(price * 100)
  var cashP  = Math.round(cash * 100)
  var cashbackP = (cashP - priceP)
  var returnMoney = {
    status: "",
    change: []
  }
  var totalCidP = cidP.map(v => {
      return v[1]
    })
    .reduce((a, b) => {
      return a + b
    })
  if (totalCidP === cashbackP) {
    returnMoney.status = "CLOSED"
    returnMoney.change = cid
    return returnMoney
  } else if (totalCidP < cashbackP) {
    returnMoney.status = "INSUFFICIENT_FUNDS"
    returnMoney.change = []
    return returnMoney
  } else {
    var denomValueP = [
      {bill: "ONE HUNDRED", val: 10000},
      {bill: "TWENTY", val: 2000},
      {bill: "TEN", val: 1000},
      {bill: "FIVE", val: 500},
      {bill: "ONE", val: 100},
      {bill: "QUARTER", val: 25},
      {bill: "DIME", val: 10},
      {bill: "NICKEL", val: 5},
      {bill: "PENNY", val: 1}
    ]
    var changeArray = [];
    for (let i = 0; i < denomValueP.length; i++) {
      var currValP = 0;
      while (cashbackP >= denomValueP[i].val && currValP < cidP[8 - i][1]) {
        cashbackP -= denomValueP[i].val;
        currValP  += denomValueP[i].val;
      }
      if (currValP > 0) {
        changeArray.push([denomValueP[i].bill, currValP / 100]);
      }
    }
    if (cashbackP === 0) {
      returnMoney.status = "OPEN"
      returnMoney.change = changeArray
    } else {
      returnMoney.status = "INSUFFICIENT_FUNDS"
      returnMoney.change = []
    }
    return returnMoney
  }
}

As to why those Math.round() are needed, try this:

4567.89 * 100 === 456789  // => false

EDIT

You asked for clarification about the last if-else in the code. The fourth paragraph in freecodecamp says:

Return {status: "INSUFFICIENT_FUNDS", change: []} if cash-in-drawer is less than the change due, or if you cannot return the exact change .

If you got to the end of the algorithm and cashbackP , which you have been decreasing for every bill or coin you added to the change, has not gone to zero, this means that the change is not complete, and you are in the situation I emphasized.