Variable as a column name in data frame

Question

Is there any way to use string stored in variable as a column name in a new data frame? The expected result should be:

col.name <- 'col1' df <- data.frame(col.name=1:4) print(df)  # Real output   col.name 1        1 2        2 3        3 4        4  # Expected output   col1 1    1 2    2 3    3 4    4 

I'm aware that I can create data frame and then use names() to rename column or use df[, col.name] for existing object, but I'd like to know if there is any other solution which could be used during creating data frame.

Answer 1

You cannot pass a variable into the name of an argument like that.

Instead what you can do is:

df <- data.frame(placeholder_name = 1:4) names(df)[names(df) == "placeholder_name"] <- col.name 

or use the default name of "V1":

df <- data.frame(1:4) names(df)[names(df) == "V1"] <- col.name 

or assign by position:

df <- data.frame(1:4) names(df)[1] <- col.name 

or if you only have one column just replace the entire names attribute:

df <- data.frame(1:4) names(df) <- col.name 

There's also the set_names function in the magrittr package that you can use to do this last solution in one step:

library(magrittr) df <- set_names(data.frame(1:4), col.name) 

But set_names is just an alias for:

df <- `names<-`(data.frame(1:4), col.name) 

which is part of base R. Figuring out why this expression works and makes sense will be a good exercise.

Answer 2

In addition to ssdecontrol's answer, there is a second option.

You're looking for mget. First assign the name to the variable, then the value to the variable that you have previously assigned. After that, mget will evaluate the string and pass it to data.frame.

assign(col.name, "col1") assign(paste(col.name), 1:4)  df <- data.frame(mget(col.name)) print(df)   col1 1    1 2    2 3    3 4    4