javascript - slice（）方法/ Javascript(slice() Methods/Javascript)

Question

 function checkCashRegister(price, cash, cid) { let amount = { "ONE HUNDRED": 100, "TWENTY": 20, "TEN": 10, "FIVE": 5, "ONE": 1, "QUARTER": 0.25, "DIME": 0.1, "NICKEL": 0.05, "PENNY": 0.01 } let copy = cid.slice(); let sum = cid.map(el => (el[1])).reduce((a, b) => (a + b)); let changeDue = cash - price; let newArr = []; let result = {}; if (sum === changeDue) { result.status = 'CLOSED'; result.change = copy; } for (let i = cid.length - 1; i >= 0; i--) { let count = 0; let unit = amount[cid[i][0]]; while (unit <= changeDue && unit <= cid[i][1]) { changeDue -= unit; cid[i][1] -= unit; count++; } newArr.push([cid[i][0], count * unit]); } return result; } 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] ])); 

Hello, I am trying to solve a problem that I should print the status and the amount of change.(您好，我正在尝试解决一个问题，我应该打印状态和更改量。)

I am in the middle of process, and I have a method related question.(我正在处理过程中，我有一个与方法有关的问题。) Since if the amount of the money cash machine has is the same as cash subtracted by price, I should print "status: close" and the money, I copied the "cid" array(which is input) with slice() method so that It does not affect the "cid" that I am going to use in the other condition where I use it to calculate the amount of change.(由于如果现金提款机的数量与现金减去价格后的数量相同，那么我应该打印“ status：close”和货币，因此我使用slice()方法复制了“ cid”数组（输入），以便它不会影响在其他情况下使用“ cid”来计算更改量的情况。) so to check if the closed status works well, I printed it, and I expected PENNY to be 0.5 but, for some reason, it has been affected by the code below if-statement(for loop part) and the PENNY part is 0.000XXX.(因此，要检查关闭状态是否工作正常，我将其打印出来，并希望PENNY为0.5，但由于某种原因，它已受到if-statement（用于循环部分）下面的代码的影响，并且PENNY部分为0.000XXX 。) so when I tried to move the return statement right below the if-statement.(因此，当我尝试将return语句移至if语句下方时。) it worked as I expected.(它按预期工作。) My question is I expected the for loop neither to affect nor to be affected because slice() methods does not alter the original arry(cid), but it did not work as I expected.(我的问题是我希望for循环既不会影响也不会受到影响，因为slice()方法不会更改原始的arry（cid），但是它没有按我预期的那样工作。) Can anyone figure out why?(谁能找出原因？)   ask by SH Kim translate from so

Answer 1

The problem with your code is that it subtracts pennies as fractional values, so when you do arithmetics with number of pennies you might end up with non-rounded pennies, I mean you can get 0.0799999 instead of 0.08, etc. This is because 0.01 is internally represented only approximately.(您的代码的问题在于它减去了便士作为分数值，因此当您对便士数进行算术时，您可能会得到非四舍五入的便士，我的意思是您可以获得0.0799999而不是0.08，等等。这是因为0.01是内部仅代表大约。)

This is not the case with integer values, which are always represented exactly.(整数值并非如此，整数值始终准确地表示出来。) So, the first thing I would do I would get rid of fractions by working with number of pennies instead of number of dollars (where 1 penny is 1, one dollar is 100, 100 dollars is 10000 etc)(因此，我要做的第一件事是通过使用便士数而不是美元数（其中1便士是1，1美元是100，100美元是10000等）来消除分数。) Consider the example:(考虑示例：) console.log(100 - (0.01 + 0.01)) // 99.98 console.log(100 - 0.01 - 0.01) // 99.97999999999999 As for the main question about why result is affected - this is because although you indeed copied the array by(关于为什么结果会受到影响的主要问题-这是因为尽管您确实通过以下方式复制了数组) let copy = cid.slice(); nevertheless, the new array has the same elements.(但是，新数组具有相同的元素。) I mean copy[1] and cid[1] both points to the same JS object, which happen to be ['PENNY', 0.5].(我的意思是copy [1]和cid [1]都指向同一个JS对象，碰巧是['PENNY'，0.5]。) And this is really the same object in absolute sense.(从绝对意义上讲，这实际上是同一对象。) See, you didn't do 'deep copy', you only did 'shallow' copy.(瞧，您没有做过“深层复制”，您只做了过“浅层”复制。) You copied the container, but inner elements are the same, they are linked to the same exact things.(您复制了容器，但是内部元素是相同的，它们链接到相同的事物。) Like this.(像这样。)