Transfer values from one dataframe to another

Question

I have two data frames. The first looks like this:

value <- seq(1, 100, length.out=20)
df1 <- data.frame(id=as.character(1:20), 
                  value=value, 
                  stringsAsFactors=F)

I have a second dataframe that looks like this

df2 <- data.frame(id=as.character(c(1:5, 21:23)),
                  v2=NA, 
                  stringsAsFactors=F)

I need the values transferring from df1 to df2, but only where df1$id == df2$id. So the dataframe I need is:

df2Needed <- data.frame(id=as.character(c(1:5, 21:23)),
                      v2=c(value[1:5], NA, NA, NA),
                      stringsAsFactors=F)

Is there a way to do this?

Answer 1

Using data.table:

require(data.table)
dt1 <- data.table(df1, key="id")
dt2 <- data.table(df2)

dt1[dt2$id, value]

#    id     value
# 1:  1  1.000000
# 2:  2  6.210526
# 3:  3 11.421053
# 4:  4 16.631579
# 5:  5 21.842105
# 6: 21        NA
# 7: 22        NA
# 8: 23        NA

or using base merge as @TheodoreLytras mentioned under comment:

# you don't need to have `v2` column in df2
merge(df2, df1, by="id", all.x=T, sort=F)

#   id v2     value
# 1  1 NA  1.000000
# 2  2 NA  6.210526
# 3  3 NA 11.421053
# 4  4 NA 16.631579
# 5  5 NA 21.842105
# 6 21 NA        NA
# 7 22 NA        NA
# 8 23 NA        NA

Answer 2

One way to do this using merge():

df2Needed <- merge(df2,df1,by="id",all.x=TRUE, sort=FALSE)
df2Needed <- df2Needed[,c("id","value")]
colNames(df2Needed) <- c("id","v2")

and another (more elegant, I think) using match():

df2Needed <- df2
df2Needed$v2 <- df1$value[match(df2$id, df1$id)]