如何替换data.frame中的列值？

Question

I have two data.frame.

d <- data.frame(a=letters[1:5], b=c(1:5))
  a b
1 a 1
2 b 2
3 c 3
4 d 4
5 e 5

t <- data.frame(old=c('a','c','d'), new=c('aa','cc','dd'))
  old new
1   a  aa
2   c  cc
3   d  dd

And I want to replace like below.

  a b
1 aa 1
2 b  2
3 cc 3
4 dd 4
5 e  5

I'd like to use apply function. How should I do?

Answer 1

We can use join from data.table . We convert the 'data.frame' to 'data.table' ( setDT(d) ) and join with 't' `on' the the first column, assign the 'a' column with the 'new' to replace the values in 'a' from 'd' dataset.

library(data.table)#v1.9.6+
setDT(d)[t, a:= new, on=c('a'='old')][]
d
#    a b
#1: aa 1
#2:  b 2
#3: cc 3
#4: dd 4
#5:  e 5

Answer 2

Since you have factors:

levels(d$a)[match(t$old, levels(d$a))] <- as.character(t$new)
#   a b
#1 aa 1
#2  b 2
#3 cc 3
#4 dd 4
#5  e 5

This will give an error if one of your t$old is not in d$a .

Answer 3

Used merge along with ifelse

df <- merge(d, t, all.x = T, by.x = "a", by.y = "old")
df$a <- ifelse(is.na(df$new), as.character(df$a), as.character(df$new))
#Removing the "new" column
df <- df[, -3]

#a b
#1 aa 1
#2  b 2
#3 cc 3
#4 dd 4
#5  e 5