refroll refroll - 3 months ago 8
R Question

For each row extract the value in the column name that match another value in the cell

I have a question which can be easily solved with a for-loop. However, since I have hundred-thousands rows in a dataframe, this would take very long computational time, and thus I am looking for a quick and smart solution.

For each row in my dataframe, I would like to paste the value of the cell whose column name matches the one from the first column (INDEX)

The dataframe looks like this

> mydata
INDEX 1 2 3 4 5 6
1 2 18.9 9.5 22.6 4.7 16.2 7.4
2 2 18.9 9.5 22.6 4.7 16.2 7.4
3 2 18.9 9.5 22.6 4.7 16.2 7.4
4 4 18.9 9.5 22.6 4.7 16.2 7.4
5 4 18.9 9.5 22.6 4.7 16.2 7.4
6 5 18.9 9.5 22.6 4.7 16.2 7.4


Here's the code for reproducing it:

mydata <- data.frame(INDEX=c(2,2,2,4,4,5), ONE=(rep(18.9,6)), TWO=(rep(9.5,6)),
THREE=(rep(22.6,6)), FOUR=(rep(4.7,6)), FIVE=(rep(16.2,6)), SIX=(rep(7.4,6)))
colnames(mydata) <- c("INDEX",1,2,3,4,5,6)


And this is the new dataframe with the newly calculated variable:

> new_mydf
INDEX 1 2 3 4 5 6 VARIABLE
3 2 18.9 9.5 22.6 4.7 16.2 7.4 9.5
2 2 18.9 9.5 22.6 4.7 16.2 7.4 9.5
1 2 18.9 9.5 22.6 4.7 16.2 7.4 9.5
5 4 18.9 9.5 22.6 4.7 16.2 7.4 4.7
4 4 18.9 9.5 22.6 4.7 16.2 7.4 4.7
6 5 18.9 9.5 22.6 4.7 16.2 7.4 16.2


I solved it using the for-loop here below, but, as I wrote above, I am looking for a more straightforward solution (maybe using packages like dplyr, or other functions?), as the loop is to slow for my extended dataset

id = mydata$INDEX
new_mydf <- data.frame()
for (i in 1:length(id)) {
mydata_row <- mydata[i,]
value <- mydata_row$INDEX
mydata_row["VARIABLE"] <- mydata_row[,names(mydata_row) == value]
new_mydf <- rbind(mydata_row,new_mydf)
}
new_mydf <- new_mydf[ order(new_mydf[,1]), ]

Answer

Based on your loop, this use of apply with an anonymous function may be faster (with your mydata initial definition) :

mydata$VARIABLE<-apply(mydata, 1, function(x) { x[names(x)==x[names(x)=="INDEX"]] })

Edit : And it works even with INDEX in characters :

mydata <- data.frame(INDEX=c("B","B","B","D","D","E"), "A"=(rep(18.9,6)), "B"=(rep(9.5,6)), 
                 "C"=(rep(22.6,6)), "D"=(rep(4.7,6)), "E"=(rep(16.2,6)), "F"=(rep(7.4,6)))

mydata$VARIABLE<-apply(mydata, 1, function(x) { x[names(x)==x[names(x)=="INDEX"]] })

> mydata INDEX A B C D E F VARIABLE 1 B 18.9 9.5 22.6 4.7 16.2 7.4 9.5 2 B 18.9 9.5 22.6 4.7 16.2 7.4 9.5 3 B 18.9 9.5 22.6 4.7 16.2 7.4 9.5 4 D 18.9 9.5 22.6 4.7 16.2 7.4 4.7 5 D 18.9 9.5 22.6 4.7 16.2 7.4 4.7 6 E 18.9 9.5 22.6 4.7 16.2 7.4 16.2

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