在lapply中分配值

Question

这是一个玩具的例子,我总是写一个循环.我无法找出一个班轮.我确定我已经看过了,但它并没有坚持下去.

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]]
}

如何在一行中完成循环？这不会这样做.

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

Answer 1

丑陋,但有效:

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

请注意<<-.它无法使用<-的原因是在函数内修改了someList的副本.

这里的"丑陋"意味着你永远不应该使用这种语法,原因有两个.首先,具有副作用的功能容易出错.其次,lapply忽略返回值.这些都表明显式循环是最好的.

@thelatemail不那么难看,几乎被盗了:

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

例:

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

这是一个简单的合并,如果你smallList适当地重塑你的

# 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

或者,如果您真的热衷于在smallFrame内有效地分配,请使用 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