Replace value in data frame based on conditional (`if`) operator in R

Question

In the R data frame encoded below, I would like to replace all the times when B appears with B

 junk <- data.frame(x <- rep(LETTERS[1:4], 3), y <- letters[1:12]) colnames(junk) <- c("nm", "val") 

this provides:

  nm val 1 A a 2 B b 3 C c 4 D d 5 A e 6 B f 7 C g 8 D h 9 A i 10 B j 11 C k 12 D l 

My initial attempt was to use the for and if :

 for(i in junk$nm) if(i %in% "B") junk$nm <- "b" 

but as I am sure you can see, this replaces ALL junk$nm values ​​with B I can understand why this is done, but I can not get it to replace only those cases of garbage $ nm, where the original value was B

Thank.

NOTE. I managed to solve the problem with gsub , but in the interest of learning R, I would still like to know how to get my original approach to work (if possible)

Answer 1

It is easier to convert nm to characters and then make changes:

 junk$nm <- as.character(junk$nm) junk$nm[junk$nm == "B"] <- "b" 

EDIT: And if you really need to support nm as factors, add this to the end:

 junk$nm <- as.factor(junk$nm) 

Answer 2

another useful way to replace values

  library(plyr) revalue(junk$nm, c("B"="b")) 

Answer 3

Short answer:

 junk$nm[junk$nm %in% "B"] <- "b" 

Take a look at Index vectors in the R Introduction (if you haven't read it yet).

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EDIT. As noted in the comments, this solution works for character vectors, so you lose your data.

The best way for the factor is to change the level:

 levels(junk$nm)[levels(junk$nm)=="B"] <- "b" 

Answer 4

Since the data you show are factors, this makes things a bit more complicated. @diliop Answers a problem by converting a character variable to nm . To return to the initial factors, one more step is necessary.

An alternative is to manage factor levels in place.

 > lev <- with(junk, levels(nm)) > lev[lev == "B"] <- "b" > junk2 <- within(junk, levels(nm) <- lev) > junk2 nm val 1 A a 2 bb 3 C c 4 D d 5 A e 6 bf 7 C g 8 D h 9 A i 10 bj 11 C k 12 D l 

It's pretty simple, and I often forget that there is a replacement function for levels() .

Edit: As @Seth noted in the comments, this can be done in single-line mode without loss of clarity:

 within(junk, levels(nm)[levels(nm) == "B"] <- "b") 

Answer 5

The easiest way to do this in one command is to use the which command, and you also do not need to change factors to a character by doing this:

 junk$nm[which(junk$nm=="B")]<-"b" 

Answer 6

You created a factor variable in nm , so you need to either not do this or add an extra layer to the factor attributes. You should also avoid using <- in data.frame () arguments

Option 1:

 junk <- data.frame(x = rep(LETTERS[1:4], 3), y =letters[1:12], stringsAsFactors=FALSE) junk$nm[junk$nm == "B"] <- "b" 

Option 2:

 levels(junk$nm) <- c(levels(junk$nm), "b") junk$nm[junk$nm == "B"] <- "b" junk