问题
Im trying to match a specific value in one column and replace it with the corresponding value from another column (same row). This is probably very easy... I have been trying to find a solution with for loop, sub, subset, data.table but I have not succeeded. There must be a neat way of doing this.
Example data, where we aim at swapping a in the first column with the corresponding value in the second column and outputting the column again.
df <- data.frame(rbind(c('a','D'),c('H','W'),c('Q','E'),c('a','F'),c('U','P'),c('a','B')))
df$X1 <- as.character(df$X1)
df$X2 <- as.character(df$X2)
# not working
for (i in seq_along(df$X1)){
a <- df$X1[i]
b <- df$X2[i]
x <- ifelse(a[i=='a'], a[i]<-b[i], do.nothing )
print(x)
}
The output would be like this;
X1 X2
1 D a
2 H W
3 Q E
4 F a
5 U P
6 B a
(The switch isn't necessary). It's the first column Im interested in.
Any pointer would be appreciated, thanks!
回答1:
There are several alternatives. Here are three:
Most basic, using data.frames :
df[ df$X1 == "a" , "X1" ] <- df[ df$X1 == "a", "X2" ]
More Terse, using with:
df$X1 <- with( df, ifelse( X1 == "a", X2, X1 ) )
Most terse and transparent Using data.tables
library(data.table) ## >= 1.9.0
setDT(df) ## converts to data.table by reference, no need for `<-`
df[ X1 == "a", X1 := X2 ]
回答2:
Here's another approach if you have more than one condition (swap "a" for a vector of values).
> find.a <- df$X1 %in% "a"
> df[find.a, "X1"] <- df[find.a, "X2"]
> df
X1 X2
1 D D
2 3 W
3 Q E
4 F F
5 U P
6 B B
来源：https://stackoverflow.com/questions/22814515/replace-value-in-column-with-corresponding-value-from-another-column-in-same-dat