R：将数据框x中的行名与数据框y中的列名匹配，以使两个数据框共享的列中的给定变量值

Question

Say i have two data frames which looks like the ones below

> df1
        date firm1 firm2 firm3
1 01-01-2017     1     2    3
2 01-02-2017     4     5    6
3 01-03-2017     7     8    9

> df2
            date
firm1 01-02-2017
firm2 01-01-2017
firm3 01-03-2017

Is it possible to extract a new data frame with the values from df1 where the column name and date value of df1 matches the row name and date of df2?

The data frame is was hoping to get would look like:

      Value
firm1     2
firm2     4
firm3     9

Any suggestions would be greatly appreciated!

Answer 1

We can use row/column indexing to extract the values from 'df1' and create a data.frame

df3 <- data.frame(Value = df1[-1][cbind(1:nrow(df1), match(df2$date, df1$date))])
row.names(df3) <- row.names(df2)
df3
#      Value
#firm1     2
#firm2     4
#firm3     9

Answer 2

library(reshape2)
#regenerating the initial datasets
date <- c("01-01-2017","01-02-2017","01-03-2017")
firm1 <- c(1,4,7)
firm2 <- c(2,5,8)
firm3 <- c(3,6,9)
df1 <- data.frame(date,firm1,firm2,firm3)

df1
        date  firm1  firm2  firm3
1: 01-01-2017     1     2     3
2: 01-02-2017     4     5     6
3: 01-03-2017     7     8     9

variable <- c("firm1","firm2","firm3")
date <- c("01-02-2017","01-01-2017","01-03-2017")
df2 <- data.frame(date,variable)

df2
      date      variable
1: 01-01-2017    firm1
2: 01-02-2017    firm2
3: 01-03-2017    firm3

#changing the format from wide to long

df1b <- melt(df1,id.vars = "date")
df1b
       date variable value
1: 01-01-2017    firm1     1
2: 01-02-2017    firm1     4
3: 01-03-2017    firm1     7
4: 01-01-2017    firm2     2
5: 01-02-2017    firm2     5
6: 01-03-2017    firm2     8
7: 01-01-2017    firm3     3
8: 01-02-2017    firm3     6
9: 01-03-2017    firm3     9

res <- merge(df2,df1b,by=c("date","variable"))

res  
    date variable value
1: 01-01-2017    firm2     2
2: 01-02-2017    firm1     4
3: 01-03-2017    firm3     9