Assigning values within lapply

Question

This is a toy example of something I always end up writing a loop for. I'm having trouble figuring out the one liner. I'm sure I've seen it, but it didn't stick.

smallFrame <- data.frame(colA = c('A', 'B', 'C' ,'D'), colB = c(1,1,1,1))
someList <- list(A=20, B=30, C=40, D=50)
for(letter in names(someList)){
  smallFrame[smallFrame$colA==letter, 'newColumn'] <- someList[[letter]]
}

how do I do the loop in one line? This won't do it.

lapply(names(someList), function(x) {smallFrame[smallFrame$colA==x, 'newColumn'] <- someList[[x]]})

Answer 1

Ugly, but works:

lapply(names(someList), function(x) {smallFrame[smallFrame$colA==x, 'newColumn'] <<- someList[[x]]})

Note the <<-. The reason that it won't work with <- is that a copy of someList is modified within the function.

Take "Ugly" here to mean that you should never use this syntax, for two reasons. First, functions with side effects are prone to error. Second, the return value of lapply is ignored. Either of these suggest that an explicit loop is best.

Less ugly, and almost stolen from @thelatemail:

smallFrame$newColumn <- unlist(someList[match(smallFrame$colA, names(someList))])

Example:

smallFrame <- data.frame(colA = c('A', 'B', 'C' ,'D', 'A'), colB = c(1,1,1,1,1))
> smallFrame

> smallFrame
  colA colB
1    A    1
2    B    1
3    C    1
4    D    1
5    A    1

smallFrame$newColumn <- unlist(someList[match(smallFrame$colA, names(someList))])

> smallFrame
  colA colB newColumn
1    A    1        20
2    B    1        30
3    C    1        40
4    D    1        50
5    A    1        20

Answer 2

This is a simple merge, if you reshape your smallList appropriately

# reshape2 for melt.
library(reshape2)
# slightly expanded version with duplicate colA == A
smallFrame <- data.frame(colA = c('A', 'A', 'B', 'C' ,'D'), colB = c(1,2,1,1,1))
someList <- list(A=20, B=30, C=40, D=50)
merge(smallFrame, melt(someList), by.x = 'colA', by.y = 'L1')

  colA colB value
1    A    1    20
2    A    2    20
3    B    1    30
4    C    1    40
5    D    1    50

or, if you are really keen on assigning within smallFrame and efficiently, use data.table

library(data.table)
smallDT <- data.table(smallFrame, key = 'colA')
someDT <- data.table(melt(someList), key = 'L1')

# left join smallDT and someDT, assigning the `value` column by reference
# within smallDT as the column `newColumn`
smallDT[someDT, newColumn := value]




smallDT
   colA colB newColumn
1:    A    1        20
2:    A    2        20
3:    B    1        30
4:    C    1        40
5:    D    1        50