如何在python中的for循環中更改變量？

Question

我試圖用python編寫一個程序，計算信用卡上的余額。 它是針對MIT開放式課件“計算機科學與程序設計導論” 。 我正在做第一個問題 。

該程序必須詢問用戶初始變量：初始余額，年利率和每月最低還款額。 這是我的代碼。

initialOutstandingBalance= float(raw_input('What is the outstanding balance on your  
card?'))
annualInterestRate=float(raw_input('What is the annual interest rate expressed as a   
decimal?'))
minimumMonthlyPaymentRate=float(raw_input('What is the minimum monthly payment rate on
your card expressed as a decimal?'))

for month in range(1,13):
    print("Month: "+ str(month))
    minimumMonthlyPayment=float(minimumMonthlyPaymentRate*initialOutstandingBalance)
    interestPaid=float((annualInterestRate)/(12*initialOutstandingBalance))
    principalPaid=float(minimumMonthlyPayment-interestPaid)
    newBalance=float(initialOutstandingBalance-principalPaid)
    print("Minimum monthly payment: $"+str(minimumMonthlyPayment))
    print("Principle paid: $"+str(principalPaid))
    print("Remaining Balance: $"+str(newBalance))

如何獲取剩余余額以正確更新？ 我不知道如何在每個月底更新余額。 到目前為止，每個月對於最低每月還款額，已付本金和剩余余額返回相同的值。

Answer 1

您在整個循環中使用相同的initialOutstandingBalance變量，並且從不對其進行更改。 相反，您應該跟蹤當前余額。 這將等於循環開始時的初始未償余額，但是會隨着循環的進行而變化。

您也不需要繼續調用float 。

current_balance = initialOutstandingBalance
for month in range(1,13):
    print("Month: "+ str(month))
    minimumMonthlyPayment = minimumMonthlyPaymentRate * current_balance
    # this calculation is almost certainly wrong, but is preserved from your starting code
    interestPaid = annualInterestRate / (12*current_balance)
    principalPaid = minimumMonthlyPayment - interestPaid
    current_balance = current_balance - principalPaid
    print("Minimum monthly payment: $"+str(minimumMonthlyPayment))
    print("Principle paid: $"+str(principalPaid))
    print("Remaining Balance: $"+str(current_balance))

Answer 2

您希望將變量newBalance保留在循環之外，否則將在每次迭代時將其重新分配。 同樣，您也不想將利率除以余額的12倍，而是將其除以12，然后再將商乘以余額。 最后，如上所述，您不需要所有的float 。

這應該工作：

newBalance = initialOutstandingBalance

for month in range(1,13):
    print("Month: " + str(month))

    minimumMonthlyPayment = minimumMonthlyPaymentRate * newBalance
    interestPaid = annualInterestRate / 12 * newBalance
    principalPaid = minimumMonthlyPayment - interestPaid
    newBalance -= principalPaid

    print("Minimum monthly payment: $" + str(minimumMonthlyPayment))
    print("Principle paid: $" +  str(principalPaid))
    print("Remaining Balance: $" + str(newBalance))