Rename multiple dataframe columns, referenced by current names

Question

I want to rename some random columns of a large data frame and I want to use the current column names, not the indexes. Column indexes might change if I add or remove columns to the data, so I figure using the existing column names is a more stable solution. This is what I have now:

mydf = merge(df.1, df.2) colnames(mydf)[which(colnames(mydf) == "MyName.1")] = "MyNewName" 

Can I simplify this code, either the original merge() call or just the second line? "MyName.1" is actually the result of an xts merge of two different xts objects.

Answer 1

The trouble with changing column names of a data.frame is that, almost unbelievably, the entire data.frame is copied. Even when it's in .GlobalEnv and no other variable points to it.

The data.table package has a setnames() function which changes column names by reference without copying the whole dataset. data.table is different in that it doesn't copy-on-write, which can be very important for large datasets. (You did say your data set was large.). Simply provide the old and the new names:

require(data.table) setnames(DT,"MyName.1", "MyNewName") # or more explicit: setnames(DT, old = "MyName.1", new = "MyNewName") ?setnames 

Answer 2

names(mydf)[names(mydf) == "MyName.1"] = "MyNewName" # 13 characters shorter.  

Although, you may want to replace a vector eventually. In that case, use %in% instead of == and set MyName.1 as a vector of equal length to MyNewName