python - 寻求反馈，无限循环

Question

我正在练习一些新获得的技能，并希望利用社区来获取有关以下代码的一些反馈。

目标:创建一个简单的程序，通过解构找零的组成部分(美元、25 美分等)来帮助想象中的收银员归还正确的找零。 我能想到的最好方法是使用带有嵌套 while 循环的 if 语句。

2 个问题:

1).尽管不再满足 runningTotal != change 条件，但 while 循环不会在脚本末尾终止。到底是为什么呢？ while 不是应该一直运行直到满足该条件并终止吗？我一定错过了一些东西...你们有没有看到我错过了什么明显的东西？

2).我仍然是一个初学者(正如你可能知道的那样)。您对下面的脚本有什么反馈意见？我做得好的事情、做得不好的事情或者只是一般性的想法。我真的很努力变得更好，所以非常感谢您的评论。谢谢!

脚本:

def changeCalc(cost,pmt):
    change = float(pmt - cost)
    print("Total Change: " + str(change))
    runningTotal = 0 #used to count up the change paid in the while loop below

    #make sure they paid enough
    if (pmt - cost) < 0:
        print("The customer needs to pay " + str(abs(change)) + " more.")
    else:
        #check to see if any change is due
        while runningTotal != change:
            #how many DOLLAR bills to return
            dollarBills = int(change - runningTotal)
            print("Number of Dollar Bills: " + str(dollarBills))

            #add to runningTotal
            runningTotal = float(runningTotal + dollarBills)
            print runningTotal

            #how many QUARTERS to return
            numOFqtrs = int((change - runningTotal)/(.25))
            print("Number of Quarters: " + str(numOFqtrs))

            #add to running total
            runningTotal = float(runningTotal + (numOFqtrs * (.25)))
            print runningTotal

            #how many DIMES
            numOFdimes = int((change - runningTotal)/(.10))
            print("Number of Dimes: " + str(numOFdimes))
            runningTotal = float(runningTotal + (numOFdimes * (.10)))

            #how many NICKELS
            print runningTotal
            numOFnickels = int((change - runningTotal)/(.05))
            print("Number of nickels: " + str(numOFnickels))
            runningTotal = float(runningTotal + (numOFnickels * (.05)))
            print runningTotal

            #how many PENNIES
            numOFpennies = int((change - runningTotal)/(.01))
            print("Number of Pennies: " + str(numOFpennies))
            runningTotal = float(runningTotal + (numOFpennies * (.01)))

            print runningTotal
            print change
            #####WHY DOES THE LOOP NOT END HERE??????????##########

            break

运行changeCalc(87.63,103.86)会导致无限循环，输出如下。

Total Change: 16.23
Number of Dollar Bills: 16
16.0
Number of Quarters: 0
16.0
Number of Dimes: 2
16.2
Number of nickels: 0
16.2
Number of Pennies: 3
16.23
16.23

Answer 1

正如其他人所指出的，您的 while 循环的问题在于浮点精度:runningTotal 非常接近，但只是不太接近与更改相同。您可以通过使用一些小的epsilon进行比较，或者使用整数或类似的东西来解决这个问题。

但是，您似乎根本不需要 while 循环，是吗？另请注意，不同类型硬币的代码都是相同的，因此您可以使用循环来迭代所有不同类型的硬币:

COINS = (("Dollars", 1.), 
         ("Quarters", .25), 
         ("Dimes",    .10), 
         ("Nickels",  .05), 
         ("Pennies",  .01))

def changeCalc(cost, pmt):
    change = pmt - cost
    print("Total Change: %.2f" % change)
    if (pmt - cost) < 0:
        print("The customer needs to pay %.2f more." % abs(change))
    else:
        runningTotal = 0
        for (name, value) in COINS:
            number = int((change - runningTotal) / value)
            if number > 0:
                print("Number of %s: %d" % (name, number))
                runningTotal += number * value
        print runningTotal, change, (runningTotal - change)