Divide 2 columns from 2 different dataframes - r

Question

Does anybody know how to divide two columns from two different dataframes when there are multiple columns to id from?
Example:
library(dplyr)
name <- c('A','A',
'B','B')
month = c("oct 2018", "nov 2018",
"oct 2018", "nov 2018")
var1 = c("99", "99",
"99", "99")
value <- seq(1:length(month))
df1 = data.frame(name, month, var1, value)
df2 = df1
df2["var1"] = c("992", "992", "992", "992")
df2["value"] = c(2, 4, 6, 8)
df1
df2
Output
> df1
name month var1 value
1 A oct 2018 99 1
2 A nov 2018 99 2
3 B oct 2018 99 3
4 B nov 2018 99 4
> df2
name month var1 value
1 A oct 2018 992 2
2 A nov 2018 992 4
3 B oct 2018 992 6
4 B nov 2018 992 8
Does anybody know how to create a new dataframe that divides the "value"-column in df2 by the value column of df1? The method should be possible to use also when there are more columns than in the current example.

Answer 1

In base R, we can do merge
df3 <- merge(df1, df2, by = c("name", "month"))
df3$value <- df3$value.x/df3$value.y
df3
# name month var1.x value.x var1.y value.y value
#1 A nov 2018 99 2 992 4 0.5
#2 A oct 2018 99 1 992 2 0.5
#3 B nov 2018 99 4 992 8 0.5
#4 B oct 2018 99 3 992 6 0.5
You can drop value.x and value.y column if they are not needed.

Answer 2

Join the two data frames together and then perform the division and drop unwanted columns that were generated by the join (assuming you want computed value column to replace the value columns from the original data frames). Depending on what you want you may need a different *_join.
library(dplyr)
df1 %>%
inner_join(df2, by = c("name", "month")) %>%
mutate(value = value.x / value.y) %>%
select(-value.x, -value.y)
giving:
name month var1.x var1.y value
1 A oct 2018 99 992 0.5
2 A nov 2018 99 992 0.5
3 B oct 2018 99 992 0.5
4 B nov 2018 99 992 0.5

Answer 3

We can use data.table as well to do a join and create the column 'value' by dividing the 'value' column by the corresponding column in the other dataset while joining on 'name' and 'month'
library(data.table)
df3 <- copy(df1)
setDT(df3)[df2, value := value/i.value, on = .(name, month)]
df3
# name month var1 value
#1: A oct 2018 99 0.5
#2: A nov 2018 99 0.5
#3: B oct 2018 99 0.5
#4: B nov 2018 99 0.5