创建一个其值依赖于其他多个列的列

Question

I'm trying to create a new column ("newcol") in a dataframe ("data"), whose values will be determined by the contents of up to two other columns in the dataframe ("B_stance" and "C_stance"). The values within B_stance are either "L", "R", "U" or "N". Within C_stance they are either "L" or "R".

Please excuse the semi-logical language, but I need R code which will achieve this for the contents of newcol:

if (data$B_stance = "L" AND data$C_stance = "L") then (data$newcol = "N")
if (data$B_stance = "L" AND data$C_stance = "R") then (data$newcol = "Y")
if (data$B_stance = "R" AND data$C_stance = "R") then (data$newcol = "N")
if (data$B_stance = "R" AND data$C_stance = "L") then (data$newcol = "Y")
if (data$B_stance = "U") then (data$newcol = "N")
if (data$B_stance = "N") then (data$newcol = "N")

I've tried to see if/how "ifelse" could achieve this, but cannot find an example of how to draw from multiple column values in determining the new value.

Answer 1

It may be easier to create a key/val dataset and then do a join

keydat <- data.frame(B_stance = c('L', 'L', 'R', 'R'),
                      C_stance = c('L', 'R', 'R', 'L'),
                       newcol = c('N', 'Y', 'N', 'Y'),
                stringsAsFactors = FALSE)
library(dplyr)
left_join(data, keydat) %>%
           mutate(newcol = replace(newcol, is.na(newcol), 'N'))

Answer 2

In base R the ifelse function is most useful for these conditions. The dplyr library includesa more robust if_else function and a case_when function. The ifelse returns the second argument if the first is true and returns the third argument if the first argument is false.

data <- read.table(text="
B_stance C_stance
L R
L L
U X
R L
R R
N X
X X
", header= TRUE)


data$newcol = ifelse(data$B_stance == "L" & data$C_stance == "L", "N",
                     ifelse(data$B_stance == "L" & data$C_stance == "R", "Y",
                            ifelse(data$B_stance == "R" & data$C_stance == "R", "N",
                                   ifelse(data$B_stance == "R" & data$C_stance == "L", "Y",
                                          ifelse(data$B_stance == "U", "N",
                                                 ifelse(data$B_stance == "N", "N",
                                                        NA))))))

data

# B_stance C_stance newcol
# 1        L        R      Y
# 2        L        L      N
# 3        U        X      N
# 4        R        L      Y
# 5        R        R      N
# 6        N        X      N
# 7        X        X   <NA>

Answer 3

With dplyr you can use case_when . It's a little cleaner than nested if_else s if you have numerous conditions.

df <- data.frame(
  B_stance = c('L', 'L', 'R', 'R'),
  C_stance = c('L', 'R', 'R', 'L'),
  stringsAsFactors = FALSE
)

df %>% mutate(
  newcol = case_when(
    B_stance == 'U'                   ~ 'N',
    B_stance == 'N'                   ~ 'N',
    B_stance == 'L' & C_stance == 'L' ~ 'N',
    B_stance == 'L' & C_stance == 'R' ~ 'Y',
    B_stance == 'R' & C_stance == 'L' ~ 'Y',
    B_stance == 'R' & C_stance == 'R' ~ 'N',
    TRUE                              ~ B_stance
  )
)

#   B_stance C_stance newcol
# 1        L        L      N
# 2        L        R      Y
# 3        R        R      N
# 4        R        L      Y

Note that the conditioning within case_when is lazy; the first true statement is executed. The final TRUE ensures there's a fallback in case no statement is true.