我需要找到每个城市的前十名客户，按不同产品和不同时间段的还款额计算。该功能应能够获取产品（金/银/铂）和时间段（每年或每月），并且应该返回前10位客户。
head(Repayment,20)
Customer Month Amount Limit Product City
1 A1 2004-01-12 495414.75 500000 Gold BANGALORE
2 A1 2004-01-03 245899.02 500000 Gold BANGALORE
3 A1 2004-01-15 259490.06 500000 Gold BANGALORE
4 A1 2004-01-25 437555.12 500000 Gold BANGALORE
5 A1 2005-01-17 165972.88 500000 Gold BANGALORE
6 A1 2005-02-23 365366.62 500000 Gold BANGALORE
7 A1 2005-02-01 14473.41 500000 Gold BANGALORE
8 A8 2004-02-05 100002.00 100002 Silver PATNA
9 A9 2005-02-28 100003.00 100003 Silver CHENNAI
10 A10 2005-02-16 48759.19 500000 Gold CALCUTTA
11 A11 2005-02-23 208317.94 500000 Gold COCHIN
12 A12 2005-02-03 150281.43 500000 Gold BOMBAY
13 A13 2006-02-04 339078.03 500000 Gold BANGALORE
14 A14 2006-03-25 137834.50 500000 Gold BANGALORE
15 A15 2006-03-31 437119.51 500000 Gold CALCUTTA
16 A16 2006-03-23 103923.70 500000 Gold COCHIN
17 A17 2004-03-19 408466.94 500000 Gold BOMBAY
18 A18 2006-03-05 100000.00 100000 Silver BANGALORE
19 A19 2005-04-04 10000.00 10000 Platinum BANGALORE
20 A20 2006-04-30 10001.00 10001 Platinum CALCUTTA
>
我的代码尝试：我对此感到有些困惑。我尝试了if语句是否在函数内部。
if(time_period== "monthly"){
top1 <- group_by(Repayment, product=product_name, month(Month, Customer, City)) %>%
summarise(repay = Sum(Amount))
top1 <- arrange(top1, desc(repay))
return top1[1:10,1]
} else {
top2 <- group_by(Repayment, product=product_name, year(Month), Customer, City) %>%
summarise(repay = Sum(Amount))
top2 <- arrange(top2, desc(repay))
return top2[1:10,1]
}