我有一个包含一些数字列，一些因子列和一些字符列的数据框。我已经创建了df作为表示。当我尝试使用“聚合”将此数据框折叠为“PO.Line”列时，它会将我的所有列转换为因子。这是为什么？
作为旁注，如果我的数据框不包含Customer.Name列，则聚合似乎可以正常工作。
df
Order.Number PO.Line Unit.Selling.Price Agreement Shipped Customer.Name
1 1 A_1 10 1 One
2 3 A_1 0 NA two
3 4 A_2 15 Platinum Member NA bill
4 7 A_2 0 1 craster
5 8 B_3 12 3 marilyn
str(df)
# 'data.frame': 5 obs. of 6 variables:
# $ Order.Number : num 1 3 4 7 8
# $ PO.Line : Factor w/ 3 levels "A_1","A_2","B_3": 1 1 2 2 3
# $ Unit.Selling.Price: num 10 0 15 0 12
# $ Agreement : Factor w/ 2 levels "","Platinum Member": 1 1 2 1 1
# $ Shipped : num 1 NA NA 1 3
# $ Customer.Name : chr "One" "two" "bill" "craster" ...
str(aggregate(. ~ PO.Line, data = df, head, 1))
# 'data.frame': 3 obs. of 6 variables:
# $ PO.Line : Factor w/ 3 levels "A_1","A_2","B_3": 1 2 3
# $ Order.Number : Factor w/ 3 levels "1","7","8": 1 2 3
# $ Unit.Selling.Price: Factor w/ 3 levels "0","10","12": 2 1 3
# $ Agreement : Factor w/ 1 level "1": 1 1 1
# $ Shipped : Factor w/ 2 levels "1","3": 1 1 2
# $ Customer.Name : Factor w/ 3 levels "craster","marilyn",..: 3 1 2
df$Customer.Name <- NULL
str(aggregate(. ~ PO.Line, data = df, head, 1))
# 'data.frame': 3 obs. of 5 variables:
# $ PO.Line : Factor w/ 3 levels "A_1","A_2","B_3": 1 2 3
# $ Order.Number : num 1 7 8
# $ Unit.Selling.Price: num 10 0 12
# $ Agreement : num 1 1 1
# $ Shipped : num 1 1 3