根据R数据帧中的其他列对一列执行计算

Question

I have an R data frame that looks something like this:

Company Date         Number
ACoy    2015-08-28   1000
ACoy    2015-08-29   1300 
ACoy    2015-08-30   1500
BCoy    2015-08-30   3000
CCoy    2015-08-30   2000
CCoy    2015-08-31   3000
ACoy    2015-08-31   1500
BCoy    2015-08-31   3000
CCoy    2015-09-01   3500
CCoy    2015-09-02   1000
ACoy    2015-09-02    900
CCoy    2015-09-03   2000
BCoy    2015-08-31   3000
CCoy    2015-08-31   3000

How can I perform a calculation on Number based on the value of Company, but only after a specific date?

Specifically, I am trying to get Number = Number/3 where Company == ACoy and Date > 2015-08-30

Result:

Company Date         Number
ACoy    2015-08-28   1000
ACoy    2015-08-29   1300 
ACoy    2015-08-30   1500
BCoy    2015-08-30   3000
CCoy    2015-08-30   2000
CCoy    2015-08-31   3000
ACoy    2015-08-31    500
BCoy    2015-08-31   3000
CCoy    2015-09-01   3500
CCoy    2015-09-02   1000
ACoy    2015-09-02    300
CCoy    2015-09-03   2000
BCoy    2015-08-31   3000
CCoy    2015-08-31   3000

Answer 1

This assumes that the Date column is already classed as such.

## determine which rows match the specified condition
w <- with(df, Company == "ACoy" & Date > "2015-08-30")
## replace only those 'w' values with the specified calculation
df$Number <- replace(df$Number, w, df$Number[w] / 3)
## result
df
#    Company       Date Number
# 1     ACoy 2015-08-28   1000
# 2     ACoy 2015-08-29   1300
# 3     ACoy 2015-08-30   1500
# 4     BCoy 2015-08-30   3000
# 5     CCoy 2015-08-30   2000
# 6     CCoy 2015-08-31   3000
# 7     ACoy 2015-08-31    500
# 8     BCoy 2015-08-31   3000
# 9     CCoy 2015-09-01   3500
# 10    CCoy 2015-09-02   1000
# 11    ACoy 2015-09-02    300
# 12    CCoy 2015-09-03   2000
# 13    BCoy 2015-08-31   3000
# 14    CCoy 2015-08-31   3000

Answer 2

Here is an approach using data.table . We convert the 'data.frame' to 'data.table' ( setDT(df1) ). Based on the condition in the 'i' ( Company=='ACoy' & Date > '2015-08-30' ), we assign 'Number' as the Number/3 .

library(data.table)
setDT(df1)[Company=='ACoy' & Date > '2015-08-30', Number:= Number/3]

NOTE: We assume that 'Date' column is Date class and the 'Number' is numeric class.