在for循环中计算累积总和 - Python

Question

编辑：以下是我根据收到的反馈和答案修改后的代码。

这个问题源于我在学习Python和计算机科学时，使用麻省理工学院的开放课程资料时遇到的一个问题。--可以查看我之前的问题--

我正在使用以下代码来制作每月付款和其他事项的列表。不过在循环结束时，我需要给出一个累计总额，显示到目前为止支付的总金额。

原始代码

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)


    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

    if month in xrange(12, 12+1):
        print 'RESULTS'
        print 'Total amount paid: '
        print 'Remaining balance: %.2f' % (remainingBalance,)

问题是我还没弄明白如何保持已支付金额的累计总额。我尝试过添加 totalPaid = round(interestPaid + principalPaid, 2)，但这只计算了单个月的总额，我似乎无法让它保持每个月的值，然后在最后把所有的值加起来打印出来。

另外，我知道最终的金额应该是1131.12。

我找到了很多在已知每个值的情况下通过列表来做这个的例子，但我似乎无法正确地推导出这个方法。

修正后的代码

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))
totalPaid = 0

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)
    totalPaid += round(minPayment, 2)

    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

    if month in xrange(12, 12+1):
        print 'RESULTS'
        print 'Total amount paid: %.2f' % (totalPaid,)
        print 'Remaining balance: %.2f' % (remainingBalance,)

Answer 1

这个回答对我有帮助：

首先，创建一个派生值：

df.loc[0, 'C'] = df.loc[0, 'D']

然后遍历剩下的行，填充计算出的值：

for i in range(1, len(df)):
    df.loc[i, 'C'] = df.loc[i-1, 'C'] * df.loc[i, 'A'] + df.loc[i, 'B']

索引日期	A	B	C	D
2015/01/31	10	10	10	10
2015/02/01	2	3	23	22
2015/02/02	10	60	290	280

Answer 2

你总是只付最低金额吗？那就直接用minPayment，不用再去算那些复杂的数学了。你可以一直累加总金额，然后在循环结束后再打印出来。

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))
paid = 0

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)
    paid += minPayment

    print  # Make the output easier to read.
    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

print
print 'RESULTS'
print 'Total amount paid:', paid
print 'Remaining balance: %.2f' % (remainingBalance,)

另外注意到range只有一个值，所以你只需要检查一下month是否等于12，但在这里其实没必要这么做。

Answer 3

在你开始循环之前，先初始化一个变量，用来累加值：

total_paid = 0

然后，在循环的主体部分，把合适的数值加到这个变量上。你可以使用 += 这个操作符来给已有的变量加值，比如：

total_paid += 1

这其实是 total_paid = total_paid + 1 的简写。你不想在每次循环时给 total_paid 赋一个新值，而是想在它原有的基础上加上新的值。

我不太清楚你具体的问题是什么，但这就是在循环中累加值的一般方法。