I have a data frame like this:
D <- data.frame(V1=c(1,2,3,4), V2=c(6,7,8,9), V3=c(3,4,5,6), sign=c("+", "-", "-", "+"))
If the character in column "sign" is "-", I would like to switch values in columns V1 and V2
My desired output would be:
V1 V2 V3 sign
1 1 6 3 +
2 7 2 4 -
3 8 3 5 -
4 4 9 6 +
I have tried for loops and if and else statements, but it is not working. Could you please tell me how I could achieve this?
Use mapply to loop through the 3 columns at once and assign the result to columns V1 and V2.
D <- data.frame(V1=c(1,2,3,4), V2=c(6,7,8,9), V3=c(3,4,5,6), sign=c("+", "-", "-", "+"))
D[c("V1", "V2")] <- t(mapply(\(x, y, s){
if(s == "-") c(y, x) else c(x, y)
}, D$V1, D$V2, D$sign))
D
#> V1 V2 V3 sign
#> 1 1 6 3 +
#> 2 7 2 4 -
#> 3 8 3 5 -
#> 4 4 9 6 +
Created on 2022-03-18 by the reprex package (v2.0.1)
Though the answer was already accepted, here is the code above as a function together with two other functions, in increasing order of speed. The original is the slowest, the 3rd is the fastest.
swapCols1 <- function(X, flag){
X[] <- t(mapply(\(x, y, s){
if(s) c(y, x) else c(x, y)
}, X[[1]], X[[2]], flag))
X
}
swapCols2 <- function(X, flag){
s <- seq.int(nrow(X))
i <- cbind(c(s, s), c(flag, !flag) + 1L)
matrix(X[i], nrow = nrow(X))
}
swapCols3 <- function(X, flag){
tmp <- X[[1]][flag]
X[[1]][flag] <- X[[2]][flag]
X[[2]][flag] <- tmp
X
}